Weather Theory

Atmosphere & Stability

The Earth's atmosphere is a dynamic envelope of gases that surrounds our planet, extending from the surface to thousands of kilometers into space. Comprising a complex mixture of gases, the atmosphere plays a crucial role in shaping our planet's climate, weather patterns, and the behavior of aircraft in flight. At its most fundamental level, the atmosphere consists mainly of nitrogen (about 78%) and oxygen (about 21%), with trace amounts of other gases such as argon, carbon dioxide, and water vapor. This composition provides the necessary components for life and serves as the medium through which weather systems and atmospheric phenomena occur.

The atmosphere is divided into distinct layers based on variations in temperature and composition, each with its unique characteristics. [Figure 1] The troposphere, the lowest layer, extends from the Earth's surface up to about 10-15 kilometers and is where most weather phenomena occur. Above the troposphere lies the stratosphere, characterized by a temperature inversion where temperatures increase with altitude due to the presence of the ozone layer. Beyond the stratosphere are the mesosphere, thermosphere, and exosphere, each with its own distinct temperature and composition profiles. The circulation of the atmosphere is driven by the unequal heating of the Earth's surface by the sun, resulting in the formation of large-scale wind patterns and weather systems. The Coriolis force, a result of the Earth's rotation, influences the direction of winds and ocean currents, causing them to deflect to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This phenomenon plays a crucial role in shaping global wind patterns, such as the trade winds, prevailing westerlies, and polar easterlies.

Figure 1 - Layers of the Atmosphere (FAA)

Atmospheric pressure, the force exerted by the weight of the air above a given area, decreases with altitude due to the decreasing density of air molecules. This decrease in pressure affects aircraft performance and behavior, with lower air pressures at higher altitudes resulting in reduced lift and engine performance. Pilots must account for these altitude-induced changes in atmospheric pressure when planning flights and adjusting aircraft performance parameters such as engine power settings and airspeed. Atmospheric stability refers to the state of the atmosphere's vertical motion and its resistance to vertical disturbances. Inversions, a key aspect of atmospheric stability, occur when the normal decrease in temperature with altitude is reversed, leading to a layer of warm air overlying cooler air below. These inversions can form near the Earth's surface due to radiative cooling at night or in stable weather conditions, trapping pollutants and moisture near the ground and leading to the formation of fog and haze. Conversely, temperature inversions in the upper atmosphere, such as those found in the stratosphere, can inhibit vertical convection and limit the development of thunderstorms and convective weather systems. Moisture plays a significant role in atmospheric stability - changes in humidity levels influences the formation of clouds, precipitation, and weather patterns.

Figure 2 - High level clouds

Warm, moist air tends to be less stable than dry air due to its lower density and increased buoyancy. As moist air rises, it cools and condenses, releasing latent heat energy and enhancing vertical instability. Conversely, dry air tends to inhibit convective processes and promote stable atmospheric conditions, limiting the development of thunderstorms and convective weather phenomena.

Figure 3 - Microburst Diagram

Low-level wind shear, characterized by sudden changes in wind speed and direction over short distances, can also affect atmospheric stability and aircraft operations. Wind shear can occur near the Earth's surface due to frictional or mechanical effects, temperature gradients, or terrain features, creating hazardous conditions for takeoff and landing. Microbursts, [Figure 3] intense downdrafts associated with thunderstorms, are a particularly dangerous form of low-level wind shear that can cause sudden changes in airspeed and altitude, posing significant challenges for aircraft during approach and departure.

Furthermore, fog, [Figure 4] a type of low-lying cloud composed of water droplets suspended in the air, can significantly reduce visibility and affect atmospheric stability. Fog forms when moist air near the ground cools to its dew point temperature, causing water vapor to condense into tiny droplets. Different types of fog, such as radiation fog, advection fog, and upslope fog, occur under their own unique meteorological conditions and geographical locations, further influencing atmospheric stability and aviation operations. Pilots must be aware of these atmospheric stability factors and their effects on weather conditions and aircraft performance to ensure safe and efficient flight operations in diverse and challenging environments. Fog poses significant dangers to aviators due to its adverse effects on visibility and spatial orientation, making it challenging for pilots to navigate and maintain situational awareness. One of the most immediate dangers of fog is reduced visibility, often limiting visibility to just a few meters or even less in dense fog conditions. This limited visibility can obscure visual references such as terrain features, runway markings, and other aircraft, increasing the risk of mid-air collisions, runway incursions, and controlled flight into terrain (CFIT) accidents. Furthermore, fog can distort depth perception and spatial orientation, leading to disorientation and loss of situational awareness among pilots. In foggy conditions, pilots may struggle to accurately judge distances and altitudes, making it difficult to maintain proper aircraft control and execute safe maneuvers. This lack of spatial awareness can increase the likelihood of spatial disorientation incidents, where pilots lose track of their aircraft's attitude and orientation relative to the ground and surrounding objects.

Figure 4 - Low-level fog in the morning

Heat & Temperature

Temperature variations affect air density, pressure, and stability, impacting aircraft lift, engine performance, and maneuverability. As air is heated, its molecules gain kinetic energy and move more rapidly, causing the air to expand and become less dense. Conversely, cooling air causes molecules to slow down, leading to contraction and increased air density. These temperature-induced changes in air density and pressure can significantly affect aircraft performance, particularly with regards to distances during takeoff, climb, and landing phases of flight. Temperature inversions occur when the normal decrease in temperature with altitude is reversed, leading to layers of warm air overlying cooler air below. These inversions can trap pollutants and moisture near the ground, leading to the formation of fog, haze, and smog. Additionally, temperature inversions can inhibit vertical convection and limit the development of thunderstorms and convective weather systems, affecting aircraft operations and route planning. Furthermore, temperature differences between air masses can lead to the formation of weather fronts and associated weather phenomena, such as thunderstorms, squall lines, and frontal clouds. Temperature change can impact aircraft engine performance and fuel efficiency. Engine performance is highly dependent on the temperature and density of the air entering the engine's intake. In warmer temperatures, air density decreases, resulting in reduced engine power output and thrust.

Figure 5 - A US Army HC-130P during a simulated rescue in Djibouti

Temperature will have a significant impact on aircraft performance in both hot and cold weather operations, influencing various aspects of flight, including takeoff, climb, cruise, and landing. In hot weather conditions, high temperatures can lead to reduced air density, resulting in decreased engine power output and thrust. As air density decreases, the engine receives less oxygen per unit volume, leading to a reduction in combustion efficiency and overall engine performance. Consequently, aircraft may experience longer takeoff distances, reduced climb rates, and decreased maneuverability during hot weather operations, necessitating adjustments to performance calculations and operational procedures to ensure safe flight. High temperatures can also affect aircraft lift and aerodynamic performance. With reduced air density at higher temperatures, aircraft wings generate less lift for a given airspeed, requiring higher takeoff speeds and longer runways to achieve liftoff. Additionally, high temperatures can increase the risk of wingtip vortices and lift-induced drag, particularly during takeoff and landing phases, leading to decreased aircraft stability and control.

Figure 6 - A Japanese Hokkaido Air System Aircraft in a snowstorm

Colder than standard temperatures can lead to altimeter readings indicating higher than true altitude. This discrepancy occurs because colder temperatures result in higher air density, causing the pressure levels in the atmosphere to be closer together compared to standard conditions. Since altimeters measure air pressure to determine altitude, a denser air mass at colder temperatures would register a higher pressure, leading the altimeter to indicate a higher altitude than the actual true altitude. Pilots must be aware of this effect, known as cold temperature error, particularly during cold weather operations. To compensate for cold temperature errors, pilots can adjust their altimeters by setting them to a colder temperature setting or applying corrections provided in aircraft manuals or operational guidelines.

Conversely, in cold weather operations, [Figure 6] low temperatures can also impact aircraft performance and systems, requiring special considerations and precautions. Cold temperatures can lead to increased air density, enhancing engine performance and thrust output. However, extreme cold can also cause icing on aircraft surfaces and engine components, leading to reduced aerodynamic efficiency, increased drag, and impaired engine performance. Pilots must be vigilant for signs of icing and utilize anti-icing systems and procedures to prevent ice accumulation and maintain safe flight operations. Additionally, cold temperatures can affect aircraft systems, ground-based navigation systems, and avionics, leading to issues such as reduced battery performance, instrument malfunction, and decreased aircraft reliability. Pilots operating in cold weather conditions must be prepared for these challenges and take appropriate measures to ensure the safety and efficiency of flight operations.

Warmer than standard temperatures can result in altimeter readings indicating lower than true altitude. Warmer temperatures cause air density to decrease, resulting in lower pressure levels in the atmosphere compared to standard conditions. As a result, altimeters would register a lower pressure, leading to an indicated altitude that is lower than the actual true altitude. This effect, known as warm temperature error, can lead to inaccuracies in altitude readings, particularly during hot weather operations. Pilots must account for warm temperature errors and apply appropriate corrections to ensure accurate altitude readings and safe flight operations. Additionally, aircraft systems and avionics may incorporate temperature compensation features to automatically adjust altimeter readings based on ambient temperature conditions. [Figure 7]

Figure 7 -Temperature Change's Effect on Altimeter Readings

Global Weather Patterns

Global weather patterns are complex and dynamic systems influenced by various atmospheric and oceanic factors, including temperature gradients, air pressure systems, ocean currents, and the Earth's rotation. These patterns give rise to distinct weather phenomena such as winds, precipitation, and storm systems, which play critical roles in shaping regional climates and weather conditions around the world. One of the primary drivers of global weather patterns is the uneven heating of the Earth's surface by the sun, resulting in temperature variations between the equator and the poles. Global weather patterns are significantly influenced by large-scale atmospheric circulation cells, [Figure 8] such as the Hadley, Ferrel, and Polar cells, which play vital roles in redistributing heat and moisture across the Earth's surface. These circulation cells are driven by temperature gradients resulting from the uneven heating of the Earth's surface by the sun.

The Hadley cell, for example, forms near the equator, where intense solar heating causes warm, moist air to rise, creating a low-pressure zone. As this air rises, it cools, condenses, and releases moisture, leading to the formation of tropical rainforests and precipitation along the equator. At higher altitudes, the now-dry, cooler air moves poleward, descending in the subtropical regions to form high-pressure zones and creating arid, desert climates in regions such as the Sahara and the Australian Outback. These descending air masses then circulate back towards the equator, completing the Hadley cell's cycle. Similarly, the Ferrel cell operates between the Hadley and Polar cells, spanning mid-latitudes where polar and tropical air masses meet. In this cell, warm, moist air from the Hadley cell and cold, dry air from the Polar cell converge, creating zones of low and high pressure, respectively. As a result, the Ferrel cell is characterized by westerly winds that blow from west to east, driving weather systems and storm tracks across the mid-latitudes. The Polar cell, located near the poles, involves the circulation of cold, dense air sinking at the poles and spreading equatorward at the surface. These cells interact with each other and with other atmospheric and oceanic processes to produce the diverse weather patterns observed across the globe, including prevailing wind patterns, jet streams, and the formation of weather fronts and storm systems.

Figure 8 - Hadley Cell on Globe

Furthermore, large-scale atmospheric pressure systems, such as high-pressure and low-pressure systems, [Figure 9] exert significant control over global weather patterns. High-pressure systems, also known as anticyclones, are characterized by descending air masses, clear skies, and stable weather conditions. In contrast, low-pressure systems, or cyclones, are associated with rising air masses, cloud formation, and potentially severe weather phenomena such as thunderstorms, hurricanes, and typhoons. These pressure systems interact with temperature gradients, wind patterns, and ocean currents to produce a wide range of weather conditions observed across different regions of the world. Natural climate phenomena such as El Niño and La Niña events can significantly influence global weather patterns, leading to shifts in temperature, precipitation, and atmospheric circulation patterns around the world. El Niño events occur when sea surface temperatures in the central and eastern Pacific Ocean become anomalously warm, leading to changes in atmospheric circulation and weather patterns worldwide. In contrast, La Niña events involve cooler-than-average sea surface temperatures in the same region, resulting in contrasting atmospheric and oceanic conditions. These climate phenomena have far-reaching impacts on weather, agriculture, ecosystems, and human societies, highlighting the interconnected nature of global weather patterns and the importance of understanding their underlying mechanisms for predicting and mitigating their effects.

Figure 9 - Prog Chart showing large-scale atmospheric pressure systems & fronts

Wind & Atmospheric Currents

Wind patterns are influenced by various factors, including global circulation cells, local topography, and the distribution of land and water masses. Near the Earth's surface, wind flows from areas of high pressure to areas of low pressure, following the direction of isobars, [Figure 10] - lines connecting points of equal atmospheric pressure on weather maps. The pressure gradient force, resulting from differences in atmospheric pressure, acts as the primary driver of wind, accelerating air from regions of higher pressure to lower pressure. Convective currents, another important factor in wind and atmospheric circulation, occur when uneven heating of the Earth's surface causes air to rise or sink. In areas where the surface is heated, such as over land surfaces or warm ocean currents, warm air rises, creating low-pressure zones. Conversely, in regions where the surface is cooled, such as over cooler ocean currents or at higher latitudes, cold air sinks, resulting in high-pressure zones. These convective currents contribute to the formation of local wind patterns, such as sea breezes, mountain breezes, and valley breezes, which are driven by temperature differentials between land and water or between different elevations.

Wind pressure gradients, another essential aspect of atmospheric dynamics, refer to the rate of change of atmospheric pressure over a given distance. Wind flows from regions of higher pressure to lower pressure, following the direction of steeper pressure gradients. The magnitude of the pressure gradient determines the speed and strength of the wind, with steeper gradients producing stronger winds and weaker gradients resulting in lighter winds. For instance, in regions where there is a large pressure difference over a relatively short distance, such as near the center of a high-pressure system or a low-pressure system, wind speeds tend to be higher due to the steep pressure gradient. Conversely, in areas where the pressure difference is more gradual, such as between high and low-pressure systems, wind speeds are typically lower due to the weaker pressure gradient.

Figure 10 - Isobar diagram of various lines of constant pressure

Additionally, the interaction between wind pressure gradients and the Coriolis effect plays a crucial role in shaping wind patterns and atmospheric circulation. In regions where the pressure gradient force and the Coriolis effect are in balance, winds flow parallel to isobars, resulting in geostrophic wind patterns. Geostrophic winds are common in the upper atmosphere and are characterized by smooth, straight-line flow with minimal curvature. However, near the Earth's surface, friction between the air and the Earth's surface disrupts the balance between the pressure gradient force and the Coriolis effect, leading to the formation of surface winds that follow more curved paths. Understanding the complex interplay between wind pressure gradients, the Coriolis effect, and isobars is essential for predicting wind patterns, interpreting meteorological data, and navigating atmospheric conditions in various geographical regions.

Figure 11 - A windsock can be used from both the ground or the air to determine wind magnitude & direction while near the airport environment

Air Masses & Fronts

Air masses are large volumes of air that have uniform temperature, humidity, and stability characteristics throughout their extent. These air masses form over source regions, where they acquire their characteristic properties from surface conditions such as land or water surfaces, and prevailing atmospheric circulation patterns. For example, maritime air masses form over oceans and acquire high humidity and relatively stable conditions, while continental air masses originate over land and tend to be drier and more variable in temperature and stability. Additionally, polar air masses form in high-latitude regions and are characterized by cold temperatures, while tropical air masses form in low-latitude regions and are associated with warm temperatures. The properties of air masses are influenced by factors such as solar radiation, surface heating and cooling, and atmospheric circulation patterns, which determine their stability and moisture content.

When air masses of different characteristics come into contact with each other, they often do not mix readily due to differences in temperature, density, and moisture content. Instead, they tend to remain distinct and form boundaries known as fronts. A front is a transition zone or boundary between two air masses with contrasting characteristics, such as temperature, humidity, and stability. When air masses of different properties come into contact with each other, they often do not mix readily due to differences in density and moisture content. Instead, they remain distinct and form a front, where the leading edge of one air mass displaces or undercuts the other. Fronts can be classified into several types based on the characteristics of the air masses involved and the direction of movement.

Fronts

There are several types of fronts, each with its unique characteristics and associated weather phenomena. [Figure 12] The most common types of fronts include:

Cold Front: A cold front occurs when a cold air mass advances and replaces a warmer air mass, forcing the warm air to rise rapidly. This rapid lifting of warm air leads to the formation of cumulonimbus clouds, producing intense precipitation, thunderstorms, and sometimes severe weather such as tornadoes and hailstorms along the front.

Warm Front: A warm front forms when a warm air mass advances and overrides a retreating cold air mass, gradually lifting the cooler air. Warm fronts typically produce widespread stratiform clouds, light to moderate precipitation, and steady rain or drizzle as the warm air ascends and cools gradually along the frontal boundary.

Stationary Front: A stationary front occurs when two air masses with different characteristics meet but neither advances significantly, resulting in a nearly stationary boundary between them. Stationary fronts can lead to prolonged periods of unsettled weather, with alternating bands of clouds, precipitation, and variable winds along the frontal boundary.

Occluded Front: An occluded front develops when a fast-moving cold front overtakes a slow-moving warm front, lifting the warm air mass aloft and trapping it between two cooler air masses. Occluded fronts often produce complex weather patterns, including widespread precipitation and a mix of convective and stratiform clouds, as the contrasting air masses interact along the frontal zone.

Dry Line: A dry line is a boundary separating warm, moist air from hot, dry air, typically found in the central United States during the spring and summer months. Dry lines can trigger severe thunderstorms and supercells, leading to the development of intense thunderstorms, hail, and tornadoes along the boundary between the two air masses. [Figure 13]

Figure 12 - Symbology for various fronts on weather charts

In addition to frontal boundaries, air masses can also interact with each other along convergence zones, where air converges from different directions and rises. Convergence zones can occur along the boundaries of air masses with different characteristics, such as the intertropical convergence zone (ITCZ) near the equator, where trade winds from the Northern and Southern Hemispheres converge and ascend. These convergence zones can be associated with widespread cloudiness, thunderstorms, and heavy rainfall as warm, moist air rises and cools, leading to the formation of convective clouds and precipitation. Overall, the interaction between air masses is a fundamental process in meteorology, shaping weather patterns and atmospheric conditions across the globe and constantly influencing global flight planning. [Figure 13]

Figure 13 - Fronts, Dry Lines, & Pressure Systems in North America

Clouds, Moisture, & Precipitation

Moisture in the atmosphere plays a crucial role in the formation of clouds and precipitation, with water vapor serving as the primary source of atmospheric moisture. The dew point, the temperature at which air becomes saturated with moisture and dew begins to form, is a key indicator of atmospheric humidity. When air reaches its dew point temperature, it becomes saturated, and any further cooling results in condensation of water vapor into liquid droplets. These tiny droplets serve as cloud condensation nuclei, providing surfaces for water vapor to condense upon and forming the basis for cloud formation.

Clouds are visible aggregates of suspended water droplets or ice crystals in the atmosphere, resulting from the condensation of water vapor around cloud condensation nuclei. Clouds come in various shapes, sizes, and altitudes, ranging from low-level stratus clouds to towering cumulonimbus clouds. Cloud development occurs through processes such as adiabatic cooling, lifting of air masses, and convergence of air currents. Adiabatic cooling occurs when air rises and expands due to lower atmospheric pressure at higher altitudes, leading to a decrease in temperature and the formation of clouds. Air can be lifted by various mechanisms, including orographic lifting, where air is forced to rise over elevated terrain features such as mountains, and convective lifting, where localized heating of the Earth's surface leads to the vertical ascent of air parcels. Convergence of air currents, where air masses with different properties converge and rise, can also trigger cloud formation. Understanding the mechanisms of cloud development, moisture, and precipitation is essential for meteorologists to forecast weather patterns and for individuals to prepare for and respond to changing atmospheric conditions.

Clouds

Clouds come in various forms, each with distinct characteristics determined by their altitude, appearance, and the atmospheric conditions that give rise to them. [Figure 14]

Low-level clouds typically form below 6,500 feet (2,000 meters) and include stratus, cumulus, and stratocumulus clouds. Stratus clouds are low, uniform layers with a flat base and a grayish appearance, often bringing overcast skies and light precipitation. Cumulus clouds are individual, puffy clouds with a flat base and a towering appearance, often associated with fair weather but capable of producing localized showers. Stratocumulus clouds are low, lumpy clouds that often cover the sky but do not typically produce precipitation, contributing to partly cloudy conditions.

Mid-level clouds form between 6,500 and 20,000 feet (2,000 to 6,000 meters) and include altocumulus and altostratus clouds. Altocumulus clouds are mid-level, layered clouds with rounded masses or rolls, often occurring in groups or lines and indicating instability in the atmosphere. Altostratus clouds are mid-level, uniform sheets that cover the sky and often produce diffused sunlight or halo phenomena, indicating the presence of thin ice crystals.

High-level clouds form above 20,000 feet (6,000 meters) and include cirrus, cirrostratus, and cirrocumulus clouds. Cirrus clouds are high, wispy clouds composed of ice crystals, often indicating fair weather but also signaling the approach of a warm front. Cirrostratus clouds are high, thin sheets that cover the sky and often produce halo phenomena or give the sky a milky appearance. Cirrocumulus clouds are high, patchy clouds with small, rounded masses or rolls, often indicating instability in the upper atmosphere. These various types of clouds at different altitudes provide valuable information to meteorologists about atmospheric conditions, helping them forecast weather patterns and predict changes in the weather.

Figure 14 - Cloud types & altitudes diagram

Humidity refers to the amount of water vapor present in the air, which plays a crucial role in shaping weather patterns and atmospheric conditions. Relative humidity is a measure of the moisture content of the air relative to its capacity to hold moisture at a given temperature. It is expressed as a percentage and indicates how close the air is to saturation. When the relative humidity is 100%, the air is fully saturated with moisture, and any further increase in moisture content will lead to condensation. Relative humidity varies with temperature, as warmer air can hold more moisture than cooler air. High relative humidity levels often result in muggy or oppressive conditions, while low relative humidity levels can lead to dry or arid conditions. Water saturation occurs when the air is fully saturated with moisture, and the relative humidity reaches 100%. At this point, the air cannot hold any more moisture, and any additional water vapor will condense into liquid droplets or ice crystals. Cloud condensation nuclei (CCN) are tiny particles suspended in the atmosphere that serve as surfaces for water vapor to condense upon and form cloud droplets. Common CCN include dust particles, pollen, salt particles, and aerosols emitted by human activities. These particles provide nucleation sites for water vapor to condense, allowing cloud droplets to form and grow. Without CCN, clouds would not be able to form, and precipitation would not occur. Precipitation is any form of water, liquid or solid, that falls from the atmosphere and reaches the Earth's surface. The most common types of precipitation include rain, snow, sleet, and hail. Rain occurs when water droplets in clouds coalesce and become large enough to fall to the ground. Snow forms when water vapor in clouds condenses directly into ice crystals, which then aggregate into snowflakes and fall to the ground. Sleet is a mixture of rain and snow that occurs when partially melted snowflakes refreeze before reaching the ground. Hail forms in thunderstorms when strong updrafts carry raindrops into extremely cold regions of the atmosphere, where they freeze into ice pellets and grow larger as they are cycled through the storm cloud. Eventually, the hailstones become too heavy to be supported by the updrafts and fall to the ground as hail. These various forms of precipitation play essential roles in replenishing the Earth's water supply and shaping the planet's climate and ecosystems.

Figure 15 - Radar image depicting 7-day Observed Precipitation in Houston, TX in April 2016

Mountain Weather

Mountain weather is characterized by unique and dynamic atmospheric conditions influenced by factors such as topography, air mass interactions, and local wind patterns. As air ascends a mountain slope, it undergoes adiabatic cooling due to the decrease in atmospheric pressure with altitude. This cooling process can lead to the condensation of water vapor and the formation of clouds and precipitation. Orographic lifting, [Figure 16] where air is forced to rise over elevated terrain features such as mountains, plays a crucial role in mountain weather patterns, leading to enhanced cloud development and precipitation on windward slopes. The interaction between orographic lifting and prevailing wind patterns can result in the formation of distinct weather phenomena, such as mountain waves, lenticular clouds, and foehn winds. Mountain waves are oscillations in the atmosphere that occur downstream of mountain ranges, resulting from the interaction between stable air flowing over the mountains and turbulent eddies forming in the lee of the mountains. [Figure 17] These waves can extend for hundreds of kilometers downstream and affect aircraft operations, causing turbulence and vertical motion.

Rain Shadow Effect due to Orographic Lift.png

Figure 16 - Rain shadow effect caused by orographic lift

Foehn winds, also known as Chinook winds in North America, are warm, dry winds that descend on the leeward side of mountain ranges after crossing over the mountains. As air descends the leeward slope, it undergoes adiabatic compression, leading to warming and drying of the air mass. Foehn winds can produce dramatic changes in weather conditions, such as rapid temperature increases, evaporation of snow cover, and increased fire danger due to dry conditions.

Figure 17 - Windward side of mountain range (left) and associated turbulence created on leeward side of mountain range (right)

Mountain weather can be particularly challenging for aviation due to the presence of severe hazards like lenticular clouds and mountain wave turbulence. Lenticular clouds, [Figure 19] also known as standing wave clouds, form downwind of mountain ranges where moist air flows over the terrain and encounters obstructions such as peaks and ridges. As the air is forced to rise over the terrain, it cools adiabatically, leading to the condensation of water vapor and the formation of lens-shaped clouds. Lenticular clouds are often stationary or slow-moving and can be mistaken for UFOs due to their unusual shape and appearance. While visually striking, lenticular clouds are indicative of very strong winds and turbulent conditions aloft, making them hazardous for all aircraft. Pilots encountering lenticular clouds should exercise caution and be prepared for the possibility of severe turbulence and wind shear. Mountain wave turbulence is another significant hazard associated with mountainous terrain, occurring when stable air flows over a mountain range and is disturbed by the terrain's irregularities. As the air encounters obstacles such as peaks and ridges, it is forced to oscillate vertically, generating waves that propagate downstream. These waves can extend for hundreds of miles downwind of the mountains and can produce severe turbulence and updrafts and downdrafts capable of exceeding aircraft performance limits. Mountain wave turbulence is particularly dangerous for light aircraft and gliders, which may be unable to withstand the intense forces exerted by the turbulent airflow. Pilots operating in mountainous regions should be aware of the potential for mountain wave turbulence and take appropriate precautions or mountain flight training to avoid or mitigate its effects.

Figure 18 - NPS Aviation assisting in an Alaksa backcountry cleanup in 2015

Figure 19 - Lenticular clouds over Mt. Rainier, Washington, USA

The dangers associated with lenticular clouds and mountain wave turbulence underscore the importance of thorough flight planning and situational awareness when flying in mountainous terrain. Pilots should carefully review weather forecasts and advisories, paying close attention to wind speed and direction, temperature inversions, and the presence of lenticular clouds and other atmospheric phenomena indicative of turbulent conditions. Additionally, pilots should maintain a vigilant lookout for visual cues such as rotor clouds and wave structures that may indicate the presence of mountain wave turbulence. By understanding the unique challenges posed by mountain weather and employing effective risk management strategies, pilots can safely navigate through mountainous terrain and mitigate the risks associated with lenticular clouds and mountain wave turbulence.

Figure 20 - Mountain wave turbulence can be particularly dangerous to Light Sport Aircraft (LSA)

Weather Hazards

Turbulence & Wind Shear

Turbulence is a common atmospheric phenomenon encountered during flight that results from irregular airflow and changes in wind speed and direction. It can occur at various altitudes and is classified into different levels based on its intensity and impact on aircraft stability and passenger comfort.

Light turbulence - often referred to as "chop," is characterized by slight, rhythmic bumps or jolts that may cause a slight discomfort for passengers but generally do not pose a significant risk to aircraft safety.

Moderate turbulence - more pronounced, with occasional bumps or jolts that may cause loose objects to move and can lead to brief changes in altitude.

Severe turbulence - the most intense level, characterized by abrupt and unpredictable movements that can result in large changes in altitude and pose a significant risk to aircraft and passenger safety.

Figure 21 - Shelf Clouds with extensive vertical development indicate unstable air and turbulence

To assess turbulence conditions while en-route, pilots can rely on PIREPs (Pilot Weather Reports) submitted by other pilots who have encountered turbulence during flight. PIREPs provide real-time observations of turbulence intensity, altitude, location, and duration, allowing pilots to anticipate and avoid areas of turbulence along their flight route. Pilots can use PIREPs to adjust their altitude, change their route, or request deviations from air traffic control to mitigate the effects of turbulence and ensure the safety and comfort of their passengers. Additionally, pilots can use onboard weather radar systems to detect and avoid convective activity and turbulent areas, providing additional situational awareness and guidance during flight operations. Turbulence can occur in various weather conditions and geographic regions, including clear air turbulence (CAT) at high altitudes, mountain wave turbulence near mountainous terrain, and convective turbulence associated with thunderstorms and convective weather systems. [Figure 21] Pilots use weather forecasts, weather radar imagery, and graphical weather products to identify areas of potential turbulence along their planned route of flight and adjust their flight plan accordingly. By staying informed about current weather conditions and utilizing available resources such as PIREPs and onboard weather radar, pilots can effectively manage turbulence and ensure a smooth and safe flight for passengers and crew.

Wind shear is a rapid change in wind speed and/or direction over a short distance in the atmosphere, presenting significant hazards to aircraft during takeoff, landing, and flight. Pilots can encounter wind shear in various weather conditions and geographic locations, including near thunderstorms, frontal boundaries, mountainous terrain, and microbursts. Wind shear, like turbulence, can also occur in clear air turbulence (CAT) at high altitudes, where it may be undetectable by conventional weather radar and pose a hazard to aircraft stability and control. To mitigate the risks associated with wind shear, certain advanced aircraft are equipped with wind shear warning systems designed to detect and alert pilots of potential wind shear conditions during flight operations. These systems use onboard sensors, such as radar, lidar, or microphones, to detect changes in airspeed, altitude, and aircraft performance indicative of wind shear. When wind shear is detected, the warning system provides visual and auditory alerts to the flight crew, prompting them to take corrective action to maintain aircraft control and stability. Additionally, air traffic controllers may provide wind shear advisories to pilots during critical phases of flight, such as takeoff and landing, to help them avoid hazardous wind shear conditions and ensure the safety of the flight.

Figure 22 - Wind shear is most hazardous to aircraft during takeoff & landing procedures

Icing

Icing is a critical weather hazard in aviation that poses significant hazards to aircraft operations, particularly during flight in cold and moist atmospheric conditions. It occurs when supercooled liquid droplets in clouds freeze upon contact with an aircraft's surfaces, leading to the formation of ice accretions that can adversely affect aerodynamic performance, increase weight, and disrupt critical systems. Understanding the types and characteristics of icing is essential for pilots to recognize and mitigate its effects effectively, ensuring the safety of flight operations.

Icing typically first develops on the leading edges of an aircraft, [Figure 23] including wings, tail surfaces, and engine air intakes, where airflow and moisture content are conducive to ice accretion. Probes and sensors, such as pitot tubes, static ports, and angle-of-attack vanes, are also extremely vulnerable to icing, potentially leading to inaccurate airspeed, altitude, and angle-of-attack indications. Pilots must remain vigilant for signs of icing during flight, monitoring air temperature, dew point spread, cloud types, and precipitation intensity to identify conditions conducive to icing formation and take appropriate precautions.

To obtain information about icing conditions en route, pilots rely on Pilot Weather Reports (PIREPs) submitted by other pilots who have encountered icing during flight. PIREPs provide valuable real-time observations of icing intensity, altitude, location, and duration, allowing pilots to anticipate and avoid areas of icing along their flight route. Pilots can access PIREP icing alerts through various sources, including flight service stations, air traffic control, and aviation weather applications, enabling them to make informed decisions and adjust their flight plans to minimize the risk of encountering hazardous icing conditions. By staying informed about current weather information and utilizing PIREPs, pilots can effectively manage the risks associated with icing and ensure the safety of their flight operations.

Figure 23 - A Beech C99 airplane was substantially damaged during a hard landing following an instrument approach in instrument meteorological conditions at Kearney Municipal Airport (EAR), Kearney, Nebraska. Ice accumulation found on wing leading edge after the incident

Types of Aircraft Icing

Structural icing occurs when supercooled liquid droplets freeze upon impact with an aircraft's structure, such as wings, fuselage, or control surfaces, forming ice accretions that can adversely affect aerodynamic performance and increase aircraft weight. This type of icing is most common in clouds containing liquid water droplets at temperatures below freezing.
Induction icing affects engine performance by forming ice deposits on engine air intakes, fuel lines, or other components, disrupting airflow and reducing engine efficiency. Induction icing is particularly prevalent in carbureted piston-engine aircraft and can lead to engine power loss or failure if not mitigated promptly.
Rime icing occurs when small supercooled water droplets freeze quickly upon contact with an aircraft surface, forming a rough, milky-white ice layer. Rime ice accretions typically occur in stratiform clouds with small droplets and temperatures slightly below freezing, producing a denser and more adherent ice layer compared to clear ice.
Clear icing occurs when larger supercooled liquid droplets slowly freeze upon impact with an aircraft surface, resulting in a smooth, clear ice layer. Clear ice accretions often occur in cumuliform clouds with larger droplets and temperatures slightly below freezing, posing a greater hazard to aircraft performance due to their denser and more aerodynamically significant ice buildup.
Mixed icing combines characteristics of both rime and clear icing, producing a mixture of rough and smooth ice accretions with varying densities and aerodynamic effects. Mixed icing occurs in clouds with a wide range of droplet sizes and temperatures near the freezing point, making it unpredictable and challenging for pilots to detect and mitigate effectively. Pilots must exercise caution when encountering mixed icing conditions to ensure aircraft safety and performance.

FIgure 24 - Base support and maintenance personnel swiftly moved into action after several inches of snow made an unauthorized landing at NATO Air Base Geilenkirchen to clear the snow off the flightline and de-ice the jet

Induction icing poses a significant risk to aircraft equipped with carbureted piston engines, as it can disrupt airflow and fuel delivery, leading to engine power loss or failure. Induction icing occurs when supercooled liquid droplets in the air are drawn into the engine's carburetor and freeze upon contact with its internal components, including the venturi and throttle valve. As ice accumulates within the carburetor, it restricts airflow and disrupts the fuel-air mixture, resulting in a loss of engine power or rough engine operation. Induction icing is most likely to occur during flight in conditions conducive to carburetor icing, such as high humidity, temperatures near or below freezing, and visible moisture in the air, including clouds, rain, or fog.

To mitigate the risks associated with induction icing, pilots must remain vigilant for signs of carburetor icing during flight and take proactive measures to prevent ice accumulation. This includes regularly monitoring engine performance and observing changes in engine RPM, manifold pressure, and exhaust gas temperature, which may indicate the presence of icing. Pilots can also use carburetor heat, a feature available in many carbureted aircraft, to prevent or remove ice buildup by directing warm air into the carburetor intake, melting any ice formations and restoring normal engine operation. By understanding the factors contributing to induction icing and employing appropriate preventive measures, pilots can effectively manage the risks associated with this type of icing and ensure the continued safe operation of their aircraft's engine.

Encountering aircraft icing during flight requires immediate action and careful decision-making by pilots to ensure the safety of the aircraft and its occupants. If ice accumulation [Figure 25] is observed or suspected, pilots should communicate promptly with air traffic control (ATC) to report the icing conditions and request assistance or clearance to deviate from the planned route to avoid further exposure to icing hazards. Pilots should provide detailed information to ATC regarding the severity and location of the icing, as well as any changes in aircraft performance or handling characteristics experienced as a result of the icing encounter.

In situations where aircraft icing becomes severe or uncontrollable, pilots may consider turning around to exit the icing conditions and return to areas with known icing or warmer temperatures. Climbing or descending to different altitudes may also be effective in escaping icing conditions, as changing altitude can alter the temperature and moisture levels of the surrounding air, reducing the likelihood of ice formation on the aircraft. Pilots should be aware of the freezing level and temperature gradients at various altitudes to make informed decisions about the most suitable altitude for avoiding icing.

Figure 25 - ice formation on the spinner of an Advanced Air Mobility proprotor model tested in the Icing Research Tunnel

Figure 26 - light ice formation on the de-icing boots of a Beechcraft King Air

Aircraft equipped with de-ice and anti-ice systems [Figure 26] offer additional protection against icing conditions by actively removing or preventing ice accumulation on critical surfaces, such as wings, tail surfaces, and engine inlets. Pilots should activate these systems as soon as icing is encountered to minimize ice buildup and maintain aircraft performance. De-ice systems typically use pneumatic boots, heated surfaces, or fluid sprays [Figure 24] to remove ice, while anti-ice systems use heated elements or chemical agents to prevent ice formation. Pilots should be familiar with the operation of these systems and follow manufacturer-recommended procedures for activating and monitoring them during flight to ensure their effectiveness in mitigating the effects of aircraft icing.

Low Visibility & Fog

Figure 27 - aircraft landing in Hamburg in fog

Low visibility presents significant hazards to aviation safety, particularly when encountered in the form of fog, low-IFR ceilings, and clouds during night-time conditions. Fog is a meteorological phenomenon characterized by water droplets suspended in the air near the ground, reducing visibility to less than one kilometer. Fog poses a hazard to aviation by limiting visibility and making it difficult for pilots to visually navigate, maintain situational awareness, and detect other aircraft or obstacles in the vicinity. In low visibility conditions, pilots may experience spatial disorientation and difficulty discerning horizon references, increasing the risk of controlled flight into terrain (CFIT) accidents and collisions with terrain or obstacles.

Low ceilings, often associated with overcast or broken cloud layers, restrict vertical visibility and can create hazardous conditions for aircraft operations, especially during takeoff, approach, and landing. Instrument approach procedures require a higher level of skill and proficiency when being conducted to absolute minimums in these low visibility conditions.

Pilots may encounter difficulties in visually acquiring the runway environment and establishing a stable approach profile, increasing the risk of runway incursions, missed approaches, and controlled flight into obstacles or terrain. Low ceilings also limit the effectiveness of visual navigation aids, such as visual checkpoints and landmarks, making it challenging for pilots to maintain accurate position awareness and navigate safely in instrument meteorological conditions (IMC).

Night-time low visibility operations [Figure 28] pose additional challenges for pilots due to reduced ambient lighting and diminished visual cues, making it more difficult to perceive and assess distances, altitudes, and relative motion. Night-time flying exacerbates the effects of reduced visibility, increasing the risk of spatial disorientation, runway incursions, and controlled flight into terrain. Pilots must rely heavily on aircraft instruments, navigation aids, and cockpit lighting to maintain situational awareness and execute safe flight operations during night-time low visibility conditions. VFR into IMC (Visual Flight Rules into Instrument Meteorological Conditions) hazards occur when pilots inadvertently enter instrument meteorological conditions while operating under visual flight rules (VFR), typically due to deteriorating weather conditions or poor pre-flight planning. Pilots encountering VFR into IMC conditions may lose visual references and become disoriented, leading to loss of aircraft control and spatial disorientation. Inadvertent VFR into IMC encounters pose a significant risk of controlled flight into terrain, mid-air collisions, and other accidents, highlighting the importance of thorough pre-flight planning, weather monitoring, and adherence to instrument flight rules (IFR) procedures when operating in marginal weather conditions. Pilots should exercise caution and be prepared to transition to instrument flight and rely on cockpit instrumentation to maintain aircraft control and navigate safely in IMC.

Figure 28 - fog layers at night creates low visibility and illusions to pilots

Fog

Radiation fog: Radiation fog forms during the night or early morning when the ground loses heat through radiation, cooling the air near the surface to its dew point temperature, resulting in condensation and fog formation. This type of fog is common in low-lying areas with clear skies and light winds, where radiational cooling is most effective.
Advection fog: Advection fog occurs when warm, moist air moves horizontally over a colder surface, causing the air to cool to its dew point temperature and condense into fog. Advection fog is often associated with the movement of warm, moist air masses over cooler ocean waters or cold land surfaces, resulting in widespread fog formation along coastlines and over large bodies of water.
Upslope fog: Upslope fog forms when moist air is forced to rise along elevated terrain, such as mountains or hillsides, where it cools and condenses into fog. As air ascends the slope, it undergoes adiabatic cooling, reaching its dew point temperature and producing fog. Upslope fog is common in mountainous regions with persistent onshore flow and can persist for extended periods, especially during stable atmospheric conditions.
Evaporation (mixing) fog: Evaporation fog, also known as mixing fog, occurs when cold, dry air moves over warmer water or moist surfaces, causing the air to become saturated and fog to form. This type of fog is often observed over bodies of water, such as lakes, rivers, or wetlands, where cold air masses interact with warmer water temperatures, leading to rapid evaporation and fog formation. Evaporation fog can develop rapidly and dissipate once the temperature and moisture gradient between the air and surface diminishes.
Steam fog: Steam fog, also known as sea smoke or frost smoke, forms when cold air moves over relatively warm water surfaces, causing rapid evaporation and condensation into fog. This type of fog is commonly observed during cold winter mornings over open water bodies, such as lakes, rivers, or coastal areas, where the temperature contrast between the air and water is significant. Steam fog often appears as wispy plumes or columns rising from the water surface and can dissipate quickly as the air warms or becomes more saturated.

Convective Hazards

Convective hazards, also referred to simply as thunderstorms, pose significant risks to aviation safety due to their dynamic and unpredictable nature. Understanding the formation, characteristics, and hazards associated with thunderstorms is crucial for pilots and aviation professionals to effectively mitigate risks and ensure safe flight operations. Thunderstorms develop through a complex interplay of atmospheric instability, moisture, and lifting mechanisms. Typically, warm, moist air rises rapidly in an unstable atmosphere, forming cumulus clouds [Figure 29] that eventually evolve into towering cumulonimbus clouds—the hallmark of thunderstorms. As air parcels ascend, they cool and condense, releasing latent heat and further fueling the updrafts. This process of convection continues to intensify, leading to the formation of thunderstorms characterized by strong updrafts, downdrafts, lightning, heavy rain, hail, [Figure 30] and strong winds.

Figure 30 - medium to large sized hail from a tornado

Figure 29 - thunderstorm clouds as seen from the ground level

Thunderstorms pose a myriad of hazards to aviation, including turbulence, icing, lightning strikes, hail, microbursts, and strong winds. Turbulence within and around thunderstorms can be severe, causing sudden and unpredictable aircraft movements that can lead to passenger discomfort and potential injuries. Icing can occur at high altitudes within thunderstorm clouds, posing a risk to aircraft systems and performance. Lightning strikes are not only a threat to aircraft integrity but can also disrupt avionics and communication systems. Hail within thunderstorms can damage aircraft structures, engines, and windshields. Microbursts, sudden downdrafts of air near the surface, pose a significant hazard during takeoff and landing, potentially leading to loss of control or reduced lift.

Figure 31 - Doppler radar reflectivity loop showing severe thunderstorms over New England, USA, on July 3, 1997.

General aviation aircraft should maintain a safe distance from thunderstorms, typically a minimum of 20 nautical miles laterally and 2,000 feet vertically. However, given the unpredictable nature of thunderstorms and their associated hazards, it is often advisable for pilots to deviate even further from convective activity to ensure safety. Pilots should actively monitor weather reports, radar imagery, and convective outlooks to identify areas of thunderstorm activity and plan routes to avoid these hazards. When encountering thunderstorms en route, pilots should prioritize safety over schedule and be prepared to divert, delay, or cancel flights if necessary to avoid convective hazards. Utilizing onboard weather radar, satellite imagery, and automated weather briefing systems can enhance situational awareness and aid pilots in making informed decisions during flight operations.

Figure 32 - Stages of a thunderstorm and associated altitudes

Thunderstorm Stages

Cumulus Stage: The cumulus stage marks the initial phase of thunderstorm development, typically starting near the surface and extending upward into the atmosphere. During this stage, warm, moist air rises rapidly, forming cumulus clouds with rapidly growing vertical development. Updrafts play a crucial role in initiating cloud formation by lifting air parcels to the condensation level where cloud droplets form. Cumulus clouds often appear as towering columns or cauliflower-shaped formations, indicating potential thunderstorm development. Aviation-wise, pilots may observe cumulus clouds as an early warning sign of convective activity and potential thunderstorm formation, prompting them to exercise caution and monitor weather conditions closely.

Mature Stage: The mature stage represents the peak intensity of a thunderstorm, extending from near the surface to higher altitudes, sometimes reaching the tropopause. During this stage, the thunderstorm experiences strong updrafts and downdrafts, contributing to its vigor. Updrafts continue to feed the storm by lifting moisture-laden air to higher altitudes, while downdrafts transport cool, dense air downward. Cumulonimbus clouds, with their characteristic anvil-shaped or mushroom-like appearance, dominate the sky. Severe weather phenomena occur, including lightning, heavy rain, hail, strong winds, and turbulence, posing significant hazards to aircraft operating in the vicinity.

Dissipating Stage: In the dissipating stage, the thunderstorm weakens as updrafts diminish, and downdrafts become more dominant. This stage spans from higher altitudes down to the surface. Downdrafts inhibit further updraft development, leading to the dissipation of convective activity. Cloud formations lose their vertical development, and precipitation diminishes. While weather conditions gradually improve as the storm weakens, residual hazards such as turbulence and gusty winds may persist, especially near the dissipating thunderstorm's periphery. Pilots should remain vigilant and prepared for changing conditions as the storm dissipates.