Aviation Weather. The Earth s Atmosphere. Atmospheric Layers. Unequal Heating

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1 Aviation Weather The Earth s Atmosphere The atmosphere fluctuates continuously in pressure, temperature, and humidity. The standard atmosphere is an average condition of these fluctuations. Related: Appendix: ICAO Standard Atmosphere Atmospheric Layers Troposphere: The lowest layer of the atmosphere. It is characterized by an overall temperature decrease with altitude. Virtually all weather takes place in the troposphere. Troposphere comes from the word trope, meaning, turn or change. There are often very strong vertical air motions in this layer. The height of the troposphere varies by latitude and with the season. Typical ranges of the troposphere are from the surface to 20,000 MSL at the poles and from the surface to 65,000 MSL at the equator. Tropopause: The boundary level between the troposphere and the stratosphere. Temperature in the tropopause remains relatively constant at -57 C (-69 F). Stratosphere: In this layer, air moves horizontally in strata or layers. Temperature changes slowly at first and then increases with altitude to near 0 C. Temperature is highest at the top of the stratosphere due to absorption of solar radiation by the ozone. Stratopause: The stratopause marks the boundary between the stratosphere and the mesosphere and begins at an altitude of about 160,000 MSL. Temperature reaches a maximum value at this altitude. Mesosphere: In this layer, temperature begins to decrease once again with altitude. It extends to a height of slightly more than 280,000 MSL, where the mesopause begins. Mesopause: The boundary between the mesosphere and the thermosphere. The coldest temperatures are found in the mesopause level. Above the mesopause, temperature stops decreasing and begins to increase with altitude. Thermosphere: In this layer, temperatures generally increase with height in the thermosphere, but the meaning of temperature at this altitude is unclear due to the limited number of air molecules. Ozonosphere (Ozone Layer): This layer is found in the lower stratosphere and is characterized by a relatively high concentration of ozone (O 3 ). The maximum concentrations of O 3 occur near 80,000 MSL. The ozone layer is important to living organisms because ozone filters out most of the sun s harmful ultraviolet (UV) radiation. Ionosphere: A thick layer of ions and free electrons that extends from the lower mesosphere upward through the thermosphere. AM radio waves are reflected or absorbed by different sublayers of the ionosphere. Radio communication can be influenced by variations in these lower sublayers at sunrise and sunset and during periods of high solar activity. Unequal Heating Every physical process of weather is accompanied by or is a result of, unequal heating (heat exchange) of the Earth s surface. Factors that affect heating: Angle of the Sun s Rays Striking the Earth: Any point not directly under the sun produces less heat absorption of over a given surface area. This is the most significant factor of unequal heating. Types of Cloud Cover: Low stratified clouds influence heating more than cumulus clouds. The Surface Being Heated: Deserts change temperature more rapidly than an area of forest. The Seasons: Summer produces a greater temperature variation than winter does. Version: Page 1 of 18

2 Radiation Processes As solar energy enters the atmosphere, nearly atmospheric gases and clouds absorb 20%. In addition, another 30% is lost due to the reflection and scattering by clouds and other particles, and by reflection from the Earth s surface. The reflection of rays is known as the albedo of the Earth. The Earth absorbs all of the remaining radiation. The Earth s surface is the primary energy source for the atmosphere because it absorbs solar radiation. Energy can be transferred from the Earth s surface to the atmosphere by terrestrial radiation, conduction/advection, evaporation, and transpiration (the loss of water vapor from plants). Terrestrial Radiation Terrestrial radiation is the constant infrared radiation loss of the Earth s surface. During the day, the receipt of solar radiation minimizes this loss. At night, there is no solar radiation. The Earth s surface cools significantly. The daily variation in temperature at the Earth s surface that produces day-to-night changes in wind, ceiling, and visibility is called diurnal variation. Greenhouse Effect Radiation from the sun is in the visible light spectrum, but terrestrial radiation is in the infrared spectrum. Atmospheric gases absorb infrared radiation much better than solar radiation. The greenhouse effect is the capture of terrestrial radiation by certain atmospheric gases called greenhouse gases. Greenhouse gases are present due to natural and human-made pollutants such as carbon dioxide, methane, and chlorofluorocarbons. The concern over global warming is based upon measured increases in greenhouse gases. Atmospheric Composition Permanent Gases Nitrogen is an inert gas that is not used directly by man for life processes; however, many compounds containing nitrogen are essential to all living matter. Oxygen is an essential substance for most living processes. The rate at which oxygen can be absorbed into the blood depends upon the oxygen pressure. Greater pressure pushes the oxygen from the lung alveoli into the bloodstream. As the pressure is reduced, less oxygen is forced into and absorbed by the blood. Variable Gases Gas Symbol Percent by Volume Nitrogen N² Oxygen O² Argon Ar 0.93 Neon Ne Helium He Hydrogen H² Xenon Xe Carbon dioxide is utilized by plants during photosynthesis. It also helps control breathing. A high level of carbon dioxide in the blood triggers rapid, deep breathing to expel it. This also promotes oxygen intake for active cells to use. A low carbon dioxide level causes more relaxed breathing resulting in less oxygen intake. Ozone is a form of oxygen. Most of the atmosphere s ozone is formed by the interaction of oxygen and the sun s rays near the top of the stratosphere in an area called the ozone layer. Carbon monoxide is a colorless, odorless gas produced by incomplete combustion of hydrocarbon fuel. It is a form of smoke that can lead to carbon monoxide poisoning. Page 2 of 18 My CFI Book

3 Gas Symbol Percent by Volume Water Vapor H²0 0 to 4 Carbon Dioxide CO² Methane CH Ozone O³ Carbon Monoxide trace Pressure Pressure is the measure of the force with which air molecules push on a surface. Pressure is exerted equally in all directions. A one square inch column of air stretching from sea level into space (about 50 miles high) weighs 14.7 pounds. Gravity causes atmospheric pressure to decrease with altitude. Fewer gas molecules are present to push on an object as it travels further away from the Earth s surface. In the lower troposphere, pressure decreases steadily at about 1 of mercury (Hg) for each 1,000. Air pressure is also affected by air temperature. Unequal heating of the Earth is the main reason for changes in altimeter settings between reporting stations. Measuring Pressure Atmospheric pressure (i.e., barometric pressure) can be measured with a barometer containing mercury. The weight of the atmosphere pushes down on the mercury in the reservoir of a barometer, which causes mercury to rise in the column. At sea level, mercury is forced up into the column approximately Therefore, it is said that barometric pressure is Hg at sea level. The unit of force nearly universally used to represent atmospheric pressure in meteorology is the hectopascal (hpa). A hectopascal is a metric (SI) unit that expresses force in newtons per square meter. 1,013.2 hpa is equal to 14.7 psi. The symbol for the pressure ratio is the Greek letter delta (δ). P 0 is the standard sea level static pressure (2116 psf or 14.7 psi). δ = (P P 0 ) Isobars Meteorologists plot pressure readings on maps and connect points of equal pressure with lines of isobars. These maps show pressure gradients. The closer the pressure gradients are together, the more active the weather. Temperature In aviation, temperature is normally measured in degrees Celsius. The standard temperature at sea level is 15 C (59 F). The symbol for absolute temperature is T, and the symbol for sea level standard temperature is T 0. The temperature ratio is the Greek letter theta (θ). θ = (T T 0 ) Temperature Lapse Rates Temperature generally decreases with altitude in the troposphere, but not as steadily as air pressure does. The rate at which temperature decreases is referred to as its lapse rate. The lapse rate depends upon the moisture content of the air. Moist air cools at a slower rate than dry air. The dry adiabatic lapse rate (unsaturated air) is 3 C (5.4 F) per 1,000 feet. The moist adiabatic lapse rate varies from 1.1 C to 2.8 C (2 F to 5 F) per 1,000 feet. As air ascends through the atmosphere, the average rate of temperature change (i.e., the standards lapse rate) is 2 C (3.5 F) per 1,000. Version: Page 3 of 18

4 Temperature Inversions Temperature sometimes increases with altitude. This is known as a temperature inversion. The most frequent type of ground- or surface-based temperature inversion is one that is produced on clear, cool nights, with calm or light wind. A stable layer of air would be associated with a temperature inversion. Density Density refers to the mass of the air per unit of volume. Increasing the temperature of dry air under constant pressure will decrease its density. Conversely, a decrease in temperature increases density. Thus, the density of air varies inversely with temperature. In the atmosphere, both temperature and pressure decrease with altitude and have different effects upon density. The fairly rapid drop in pressure is usually the dominating effect. Hence, density decreases with altitude. The density of the air is the most critical property of the air in the study of aerodynamics. As air density decreases, aircraft performance also decreases. The symbol for density is the Greek letter rho (ρ). ρ = (Mass Unit Volume) The symbol for the density ratio is sigma (σ). P 0 is the standard sea level density ( slugs/ft.3) Related: Appendix: Density Altitude Chart Viscosity σ = (r P 0 ) Viscosity is the internal friction of a fluid caused by molecular attraction that makes it resist its tendency to flow. Viscosity of air is important in aviation because of its effects on a region of airflow close to the surface of an airfoil called the boundary layer. The Ideal Gas Law Gases obey a physical principle known as the ideal gas law. The gas law can be written as an equation that states that the ratio of pressure to the product of density and temperature is always equal. When pressure changes, either the density or the temperature ( or both) must also change. Moisture in the Air Air has invisible water vapor in it. When conditions are favorable, the water vapor will change into clouds, fog, dew, frost, rain, or snow. Water vapor is lighter than air. Consequently, as the water content of the air increases, it becomes less dense. A decrease in air density increases density altitude and decreases aircraft performance. It is usually not considered an important factor in calculating density altitude and aircraft performance, but it does contribute. Note: Water vapor allows solar energy to enter the atmosphere, but blocks it from escaping. This makes water vapor a greenhouse gas. Relative Humidity Relative humidity is the ratio, usually expressed as a percentage, of water vapor actually in an air parcel compared to the amount of water vapor the air parcel could hold at a particular temperature and pressure. Relative Humidity = Water Vapor Content Water Vapor Capacity Relative humidity can be confusing because it does not indicate the actual water vapor content of the air, but rather how close the air is to saturation. Page 4 of 18 My CFI Book

5 Saturation An air parcel with 100% relative humidity is saturated, while an air parcel with relative humidity less than 100% is unsaturated. Clouds and visible moisture are parcels of air, which have reached 100% relative humidity. There are three basic processes which cool air to saturation: Lifting action (the major cause of clouds); Air moving over a cooler surface; and Air cooled by radiation (the major cause of fog). Capacity An air parcel s capacity to hold water vapor (at a constant pressure) is directly related to its temperature. Warmer air can hold more water vapor than colder air. The maximum amount of water vapor that can be held by a parcel of air is called its capacity. Dew Point The dew point is that temperature to which air must be cooled to become 100% saturated by water vapor already present in the air. When the temperature is below 0 C (32 F), it is sometimes called the frost point. Temperature Dew Point Spread (Dew Point Depression) The temperature-dew point spread is important in anticipating fog. As the temperature-dew point spread decreases, relative humidity increases. Frost, fog, or low clouds will likely develop at 100% humidity. Latent Heat Water is present in the atmosphere in three states: gas, liquid, and solid. As water changes from one state to another, an exchange of heat occurs. A change of state by vaporization, condensation, sublimation, melting and freezing is called a change through the process of latent heat exchange. It is called latent heat because it is hidden in the change of state. When water is evaporated, energy is extracted from the system and added to the water molecules. When water condenses, energy is released by the water molecule and added back into the system. The Hydrologic Cycle The hydrologic cycle is the process by which water is constantly circulated in the atmosphere. The basic hydrologic process: The Sun s energy absorbs large amounts of energy from the surface of the Earth; The winds transport the moist air around the globe; When the conditions are correct, the moisture condenses out of the air and forms clouds; Some of the clouds produce precipitation; and The precipitation falls back to the surface, and the cycle begins again. Evaporation (Liquid to Water Vapor) Molecules of water are always in motion. As heat energy is added to water, the movement of molecules increases. When the water molecules bump into one another near the surface of a body of water, some of the molecules are pushed into the atmosphere. The process by which liquid is transformed into a gas is called evaporation. Water vapor that evaporates from the surface of the ocean is the source of most of the moisture in the atmosphere. Plants also provide water vapor to the atmosphere by the process of transpiration. Water enters the plant through the soil and evaporates from the plant through pores (stomates) in its leaves. Condensation (Water Vapor to Liquid) When a parcel of saturated air is cooled, the maximum amount of water vapor it can hold decreases. As a result, the excess amount of water vapor condenses into tiny droplets of liquid. When water vapor condenses on large objects, such as leaves, windshields, or airplanes, it will form dew. When it condenses on microscopic particles (condensation nuclei), such as salt, dust, or combustion by-products, it will form clouds or fog. Version: Page 5 of 18

6 Sublimation (Water Vapor to Ice) Frost forms in much the same way as dew. The difference is that the dew point of the surrounding air must be colder than freezing. Water vapor then sublimates directly as ice crystals rather than condensing as dew. Condensation Nuclei Dew and frost form on objects on the Earth s surface. For fog or clouds to form, there must be particles in the atmosphere that water vapor can condense on. These particles are called condensation nuclei. Without condensation nuclei, a relative humidity of several hundred percent would be required for condensation. Condensation nuclei can come from a variety of sources including dust, volcanoes, factory smoke, forest fires and salt from ocean spray. An abundance of condensation nuclei from combustion products makes fog prevalent in industrial areas. Condensation nuclei can be grouped into two main categories according to their reaction to water vapor. Hydrophobic particles (water-repelling) allow water vapor to condense even when the relative humidity is below 100%. Hydroscopic particles (water-seeking) resist condensation even when the relative humidity is greater than 100%. Condensation nuclei fall into one of the size categories: Aitken Nuclei: < 0.2 micrometers Large Nuclei: 0.2 to 1.0 micrometers Giant Nuclei: > 1.0 micrometers Atmospheric Stability Atmospheric stability is defined as the resistance of the atmosphere to vertical motion. A stable atmosphere suppresses the vertical motion of air parcels. Air tends to be stable is the temperature changes little or not at all with altitude. An unstable atmosphere allows an upward or downward disturbance to grow into a vertical (convective) current. A mass of air in which the temperature decreases rapidly with altitude favors instability. Characteristics of Stable and Unstable Air Unstable air has the following characteristics: Cumuliform clouds; Showery precipitation; Turbulence; and Good visibility. Stable air has the following characteristics: Stratiform clouds and fog; Continuous precipitation; Smooth air; and Fair to poor visibility in haze and smoke. Adiabatic Heating and Cooling Rising air expands and cools due to the decrease in air pressure as altitude increases. The opposite is true of descending air; as atmospheric pressure increases, the temperature of descending air increases as it is compressed. These changes are adiabatic meaning that no heat is removed from or added to the air. Determining Atmospheric Stability An air parcel can be used as a tool to evaluate atmospheric stability. A parcel is selected from a specified altitude (usually the surface) and hypothetically lifted upward to a specified test altitude. As the parcel of air is lifted upwards, it cools at the dry or moist adiabatic lapse rate, depending on atmospheric conditions. Stability can be determined by comparing the temperatures of the air parcel and the surrounding air. Stable: If the lifted parcel is colder than the surrounding air, it will be denser (heavier) and sink back to its original level. In this case, the parcel is stable because it resists upward displacement. Page 6 of 18 My CFI Book

7 Neutral: If the lifted parcel is the same temperature as the surrounding air, it will be the same density and remain at the same level. In this case, the parcel is neutrally stable. Unstable: If the lifted parcel is warmer and less dense (lighter) than the surrounding air, it will continue to rise on its own until it reaches the same temperature as its environment. In this case, the parcel is unstable. Greater temperature differences result in greater rates of vertical motion. Clouds The stability of the atmosphere determines which of two types of clouds will be formed: cumuliform or stratiform. Cumuliform: Cumuliform clouds are billowy-type clouds having considerable vertical development, which enhances the growth rate of precipitation. They are formed in unstable conditions, and they produce showery precipitation made up of large water droplets. Stratiform: Stratiform clouds are flat, more evenly based clouds formed in stable conditions. They produce steady, continuous light rain and drizzle made up of much smaller raindrops. Cloud Families Clouds are divided into four families according to their height range: Low (Stratus, Stratocumuls, Cumulus); Middle (Altocumulus, Altostratus); High (Cirrus, Cirrocumulus, Cirrostratus); and Clouds with extensive vertical development (Cumulonimbus). A further classification is the prefix nimbo or suffix nimbus, which means raincloud. Cumulus clouds are formed by vertical currents. They are billowy in appearance. Clouds formed by the cooling of a stable layer are stratus clouds. They are flat and have a sheetlike appearance. Cirrus clouds are high clouds composed mainly of ice crystals. They are least likely to contribute to structural icing (since it requires water). Cirrocumulus clouds are rows of small white fleecy clouds. They often form a pattern that resembles the scales on a mackerel s back. Estimating Cloud Bases The dew point and temperature can be estimated the above ground altitude of the cloud bases, provided that lifted unsaturated air creates the clouds. Dew point decreases about 1 F per 1,000 while unsaturated air decreases at 5.4 F (dry adiabatic lapse rate). The two lapse rates converge at approximately 4.4 F per 1,000. When the temperature and dew point converge, cloud bases form. Cloud Base Height = ((Temperature in F Dew Point in F) 4.4 F) X 1,000 Version: Page 7 of 18

8 Fog Fog is a surface-based cloud composed of either water droplets or ice crystals. A small temperature-dew point spread is essential to the formation of fog. Fog may form by cooling the air to its dew point or by adding moisture to the air near the ground. Types of Fog Radiation fog (ground fog) is formed when terrestrial radiation cools the ground (land areas only), which in turn cools the air in contact with it. When the air is cooled to its dew point, or within a few degrees, fog will form. This fog will form most readily in warm, moist air over low, flatland areas on clear, calm nights. Advection fog (sea fog) is formed when warm, moist air moves (wind is required) over colder ground or water. It is usually more extensive and much more persistent than radiation fog. It deepens as wind speed increases up to about 15 knots. Winds much stronger than 15 knots lift the fog into a layer of low stratus clouds. Upslope fog is formed when moist, stable air is cooled to its dew point as it moves upward against sloping terrain. Cooling will be at the dry adiabatic lapse rate of approximately 3 C per 1,000. Precipitation-Induced fog (frontal fog) is formed when relatively warm rain or drizzle falls through cool air. Evaporation from the precipitation saturates the cool air and forms fog. It is most commonly associated with warm fronts but can occur with slow-moving cold fronts and with stationary fronts. Steam fog is formed in winter when cold, dry air passes from land areas over comparatively warm ocean waters. Condensation takes place just above the surface of the water and appears as steam rising from the ocean. Wind and Circulation Patterns Types of Pressure Systems High (Anticyclone): Areas of high pressure generally consisting of dry, descending air. Good weather is typically associated with high-pressure systems. Low (Cyclone): Areas of low pressure generally consisting of moist, rising air. Low-pressure systems are commonly associated with cloudiness and precipitation. Trough: An elongated area of low-pressure. Ridge: An elongated area of high-pressure. Col: A neutral pressure area between highs and lows. A col forms when the high and low pattern arranges in such a way as to cause a convergence or divergence of isotherms. Wind Patterns The general circulation patterns in the Northern Hemisphere are: Air circulates in a clockwise direction around a high and counterclockwise around a low; and Air flows downward in high-pressure areas and upward in low-pressure areas. Page 8 of 18 My CFI Book

9 Forces That Affect the Wind Pressure Gradient Force (PGF): Air flows from areas of high pressure into areas of low pressure because air always seeks out lower pressure. The force that causes air to flow from high to low pressure is called the PGF. Larger pressure differences create greater PGF resulting in higher wind speeds. Coriolis Force: The Coriolis force is an apparent force that bends the track of the air over the ground to the right in the Northern Hemisphere. If the Earth did not rotate, the air would flow directly to the areas of low pressure. Since the Earth does rotate, wind is deflected by the Coriolis force. The strength of the Coriolis force is determined by wind speed and latitude. It has the least effect at the equator or when the wind speed is light. Surface Friction: Friction between the wind and the terrain surface slows the wind. The rougher the terrain or stronger the wind speed, the greater the frictional effect will be. Upper Air Winds In the atmosphere above the friction layer, only PGF and Coriolis force affect the horizontal motion of air. The instant air begins moving from high pressure towards low pressure (due to PGF), Coriolis force begins to deflect it to the right. Over a period of time, the Coriolis force will exactly balance PGF. With the forces in balance, wind will flow parallel to the isobars. This is called geostrophic wind. Surface Winds At the surface of the Earth, all three forces come into play. As frictional force slows the wind speed, Coriolis force decreases. However, friction does not affect PGF. PGF and Coriolis force are no longer in balance. The stronger PGF turns the wind at an angle across the isobars toward lower pressure until the three forces balance. The angle of surface wind to isobars is about 10 over water, increasing to as high as 45 over rugged terrain. The result is, in the Northern Hemisphere, the surface wind spirals clockwise and outward from high pressure, and counterclockwise and inward to low pressure. Global Circulation Patterns The unequal heating of the Earth s atmosphere creates a large air-cell circulation pattern because warmer air has a tendency to rise and cold air has a tendency to settle. Air at the equator rises, cools, and moves towards the poles. If the Earth did not rotate, this air moving towards the poles would stay in one circular pattern. Since the Earth does rotate, the moving air is split into three cells. While there are three cells, the net flow is from the equator to the poles at higher altitudes, and from the poles to the equator at lower altitudes. Hadely Cell: (From the Equator to around 30 North and South latitudes) Lowlatitude air movement toward the Equator that, with heating, rises vertically and moves towards the poles in the upper atmosphere. The air then cools and sinks around 30 North and South latitudes, creating semi-permanent highpressure belts. Ferrel Cell: (From around 30 to around 60 North and South latitudes) In this cell, air flows towards the poles and eastward near the surface, and towards the equator and westward at higher levels. Version: Page 9 of 18

10 Polar Cell: (From around 60 North and South latitudes to the poles) Air at around 60 North and South latitudes rises, diverges, and travels toward the poles. Once over the poles, the air sinks, forming the polar highs. At the surface, air diverges outward from the polar highs. As the air flows towards the equators, the Coriolis force deflects it to the right, creating easterly surface winds called the polar easterlies. Jet Streams A jet stream is a relatively narrow band of air with wind speeds of 50 knots or greater. Horizontal winds in a jet stream generally blow from west to east but often meander southward and northward in waves. The highest wind speeds in the jet stream are found on the polar side of the jet core. Jet streams occur in the upper levels of the troposphere and follow the boundaries between hot and cold air. As the difference in temperature increases, the strength of the wind increases. Since hot and cold air boundaries are most pronounced in winter, jet streams are the strongest for both the Northern and Southern Hemisphere winters. Jet streams form in vertical breaks in the tropopause where the global air circulation cells meet. This is where temperature changes are the greatest. A polar jet typically forms around 50 to 60 North and South latitudes. A subtropical jet forms around 30 North and South latitudes. On one side of the jet stream, air rises. On the other side, air descends. This motion on air creates a circular rotation of wind in a jet stream in addition to the high-speed horizontal winds. As air is forced upward by the jet stream, cirrus clouds will sometime form on the equatorial side. Local Weather Patterns Sea Breezes are an on-shore flow of air that occurs during the day. The unequal heating of land and water causes sea breezes. Cool air descends over the sea creating a high-pressure area. Hot air over the land rises forming a low-pressure area. Since air flows from high to low pressure, the cool air over the water flows towards the land. The air that is heated over the land rises, cools, and returns aloft to the sea. A well-developed sea breeze may reach speeds of 20 knots and altitudes of 3,000. Land breezes are an off-shore breeze occurring during the night. Land breezes are the opposite of sea breezes. Water is a much better thermal conductor than land and resists quick changes in temperature; therefore, the land cools at a faster rate than the water. Land breezes are usually weaker than sea breezes. Typical land breezes reach wind speeds of 15 knots and altitudes up to 2,000. Valley breezes are up-sloping breezes that occur in the mountains. During the day, the sun warms the mountain slopes, lowering the pressure. This causes the air in the valley to flow upward. Winds caused by mountain breezes are typically between 5 and 20 knots. Page 10 of 18 My CFI Book

11 Mountain breezes are caused by the slopes of mountains and are typically strongest at sunrise. During the night, higher terrain cools more quickly than lower terrain. Due to the pressure difference created, the air over the mountains flows downward into the valley. Winds caused by mountain breezes may reach speeds up to 25 knots. By definition, mountain breezes are katabatic winds. A Katabatic wind is any descending wind caused by inclines or mountains. These winds can consist of either cold or warm air. Air Masses When a body of air comes to rest on, or moves slowly over, an extensive area having fairly uniform properties of temperature and moisture, the air takes on those properties. This body of air is called an air mass. A source region is the area from which the air mass acquires its identifying distribution of temperature and moisture. Deserts, cold oceans, and snow-covered land make the best source regions. Middle latitudes, which typically do not let air stagnate, are poor source regions. As this air mass moves from its source region, it tends to take on the properties of the new underlying surface. The trend toward change is called air mass modification. Air masses sometimes leave their source regions and converge to spawn major weather systems. In North America, especially in the mid-west, there is often a collision of continental polar and maritime tropical air masses. Factors that can modify an air mass: Temperature and moisture; Speed of movement; Warming or cooling from below; and Addition or subtraction of water vapor. North America air mass sources: Artic (A): Very cold, very dry (winter only) Maritime Polar (Mp): Cool and dry (winter only) Continental Polar (Cp): Cool and dry (winter only) Maritime Tropical (Mt): Warm and moist Continental Tropical (Ct): Warm and dry Equatorial (E): Warm, very moist There are four latitude zones: Arctic (A): > 70 North or South Polar (P): 50 to 60 North or South Tropical (T): 20 to 35 North or South Equatorial (E): Close to the Equator There are two locales: Maritime (M): Oceans (moisture) Continental (C): Continents (dry) Fronts A front is a boundary between two air masses. All fronts lie in pressure troughs. The formation of a front is called frontogenesis. A frontolysis occurs when a front dissipates. Version: Page 11 of 18

12 Cold Front: A cold front is the leading edge of an advancing cold air mass with a steep boundary developing between the cold and warm air masses. Cold fronts are associated with cumulonimbus clouds and strong storms. Warm Front: A warm front is the leading edge of an advancing warm air mass and moves about half as fast as cold fronts. A shallow boundary will develop between the two air masses. Warm fronts are associated with stratus and nimbostratus clouds, and steady rain and a shallow boundary will develop between the two air masses. The physical characteristics of a warm or cold front can be different with each front. They vary according to the speed of the air mass and the degree of stability of the air mass being overtaken. A stable air mass forced aloft will continue to exhibit stable characteristics, while an unstable air mass forced to ascend will continue to be characterized by cumulus clouds, turbulence, showery precipitation, and good visibility. Stationary Front: A stationary front is a boundary between two air masses that are not moving. A vertical boundary between the two air masses will develop. Frontal waves and areas of low pressure will usually form on slow-moving cold fronts or stationary fronts. Occluded Front: An occluded front occurs when a cold front catches up to a warm front. When the warm front is overtaken, the cold air lifts the warm front aloft. Occluded fronts are associated with cumulonimbus and nimbostratus clouds, showers, and thunderstorms. Front Identification One of the most easily recognized discontinuities is a change in temperature. Typically, the passage of a front will be accompanied by a change in wind speed and direction. Page 12 of 18 My CFI Book

13 Typical Cold Front Weather Prior To Passage During Passage After Passage Clouds Cirrus, TCU, CU TCU, CU Cumulus Precipitation Showers Heavy Rain, Thunderstorms Light Showers Visibility Fair Poor Good Wind SSW Variable, Gusty Steady, NNW Temperature Warm Suddenly Cooler Cooler Dew Point High Dropping Rapidly Continued Drop Pressure Falling Bottoms Out, Rising Rising Typical Warm Front Weather Prior To Passage During Passage After Passage Clouds Cirrus, Stratiform, Fog Stratus Stratocumulus, Possible CU Precipitation Moderate, Snow in Winter Stops or Drizzles Little or None Visibility Very Poor Improves to Poor Fair Wind SSE Variable, Light SSW Temperature Cool to Cold Rising Steadily Remaining Warm Dew Point Rising Steadily Steady Rising then Steady Pressure Falling Becoming Steady Rise Slightly then Falls Typical Occluded Front Weather Prior To Passage During Passage After Passage Clouds Cirrus, Status Any Lifting Precipitation Light to Heavy Light to Heavy Moderate then Clear Visibility Very Poor Poor Improving Wind SE to S Variable WNW Temperature Cool to Cold Falling During Cold Occlusion, Rising During Warm Occlusion Cooler in Cold Occlusion, Milder in Warm Occlusion Dew Point Steady Slight Drop Rising then Steady Pressure Falling Becoming Steady Turbulence Types of Turbulence Dropping With Cold Occlusion, Rising With Warm Occlusion Thermal: When the surface is sufficiently warm, vertical currents of air form. These convective currents usually form cumulus clouds. Therefore, a pilot can expect turbulence below or inside cumulus clouds, especially towering cumulus clouds. Very frequent lightning, cumulonimbus clouds, and roll clouds indicate extreme turbulence in a thunderstorm. Mechanical: Strong winds, 35 knots or more, across ridges and mountain ranges can also cause severe turbulence for 100 or more miles downwind and to altitudes as high as 5,000 above the tropopause. Severe downdrafts can be expected on the lee side of the mountain. The greatest potential danger from turbulent air currents exists when flying into the wind while on the leeward side of ridges or mountain ranges. If there is enough moisture in the air, a mountain wave can be marked by an almond or lens-shaped cloud (lenticular), which appears stationary, but which may contain winds of 50 knots or more. Under the right conditions, several lenticular clouds can form one above another. One of the most dangerous features of mountain waves is a turbulent area that is created in and below rotor clouds. Version: Page 13 of 18

14 Shear: A change in wind direction or speed within a short horizontal or vertical distance creates turbulence. A common location of clear air turbulence (CAT) and strong wind shear exists with a curving jet stream. An upper or lower low-pressure trough creates the curve. The wind speed, shown by isotachs (lines of constant wind speed), decreases outward from the jet core. The greatest rate of decrease in wind speed is on the polar side as compared to the equatorial side. Strong wind shear and CAT can be expected on the low-pressure side or polar side of a jet stream where the speed at the core is greater than 110 knots. Aerodynamic: Turbulence caused by an aircraft as it moves through the air. This is also known as wake turbulence. Inadvertent Turbulence Encounter If inadvertent extreme turbulence is encountered, pilots should: Disengage the autopilot; Reduce power to a setting that maintains a recommended turbulence penetration speed; Concentrate on keeping the aircraft in a level attitude while allowing airspeed and altitude to fluctuate; Tighten seat belts and shoulder harnesses and secure any loose items; and Advise ATC. Structural Icing Structural icing refers to the accumulation of ice on the exterior of the aircraft. An in-flight condition necessary for structural icing to form is visible moisture (clouds or raindrops). Chances for structural icing increase in the vicinity of fronts. Hazards Icing and frost can increase drag, degrade control, and decrease lift by destroying the smooth flow of air over an airfoil. Airplane performance can be significantly degraded, and the airplane could stall at a lower than normal AOA. If stalled, the airplane can roll or pitch uncontrollably, leading to an in-flight upset situation. Types of Structural Icing Clear ice forms when, after the initial impact, the remaining liquid portion of the drop flows out over the aircraft surface gradually freezing as a smooth sheet of solid ice. This type forms when drops are large as in rain or in cumuliform clouds. Clear ice is hard, heavy, and tenacious. Its removal by deicing equipment is especially difficult. Rime ice forms when drops are small, such as those in stratified clouds or light drizzle. The liquid portion remaining after initial impact freezes rapidly before the drop has time to spread over the aircraft surface. The small frozen droplets trap air between them giving the ice a white appearance. Rime ice is lighter in weight than clear ice, but its weight is of little significance. However, its irregular shape and rough surface make it very effective in decreasing aerodynamic efficiency of airfoils, thus reducing lift and increasing drag. Rime ice is brittle and more easily removed than clear ice. Mixed ice forms when drops vary in size or when liquid drops are intermingled with snow or ice particles. It can form rapidly. Ice particles become embedded in clear ice, building a very rough accumulation sometimes in a mushroom shape on leading edges. Freezing rain is supercooled liquid water droplets that freeze upon impacting an object. It can result in extreme aircraft icing. The liquid water droplets often persist at temperatures much colder than 0 C and are abundant in cumulus clouds between 0 and 15 C. VFR pilots should be concerned about freezing rain because it can occur outside of clouds. Freezing rain indicates warmer temperature (above 32 F) at a higher altitude. It develops as falling snow encounters a layer of warm air. The snow completely melts and becomes rain. As the rain continues to fall, it passes through a layer of cold air and cools to a temperature below freezing. If rain falling through colder air freezes during descent, ice pellets form. The presence of ice pellets at the surface is evidence that there is freezing rain at a higher altitude. Frost is described as ice deposits formed by sublimation on a surface when the temperature of the collecting surface is at or below the dew point of the adjacent air, and the dew point is below freezing. Page 14 of 18 My CFI Book

15 Inadvertent Icing Encounter If inadvertent structural icing is encountered that does not allow aircraft to operate in icing conditions indefinitely, the pilot should: Disconnect the autopilot (autopilots can mask the effects of airframe icing); Change the flight path: Move to an altitude with significantly colder temperatures; Move to an altitude with temperatures above freezing; or Change the heading, and fly to an area clear of visible moisture. Advise ATC and declare an emergency; Turn anti-icing or deicing equipment ON including pitot heat; Give careful consideration to configuration changes that might produce unanticipated aircraft flight dynamics; and Consider a no-flap landing (flaps can aggravate the loss of lift on the tail). Thunderstorms Three conditions necessary to the formation of a thunderstorm are: Sufficient water vapor; An unstable lapse rate; and An initial upward boost (lifting). The initial upward boost can be caused by heating from below, frontal lifting, or by mechanical lifting (wind blowing air upslope on a mountain). Stages of Thunderstorms Thunderstorms follow a three-stage process: cumulus, mature, and dissipating. Cumulus Stage The cumulus stage consists of continuous updrafts that can reach 3,000 FPM. The updrafts cause clouds and low-pressure areas to develop. As raindrops and ice pellets in the clouds grow, their weight begins to overpower the lifting force. As the drops fall through the cloud, they cool the air making it denser than the surrounding updrafts. This process causes downdrafts to form within the cloud. Mature Stage When the downdraft s become strong enough to allow the first precipitation to reach the Earth s surface, the mature stage has begun. Thunderstorms reach their greatest intensity during the mature stage. Updrafts in the storm can reach 9,000 FPM. Dissipating Stage Rain keeps the downdrafts cool which accelerates their downward velocity. Eventually, the downdrafts cut off the updrafts, and the storm loses the source of warm air that is its driving force. The dissipating stage of a thunderstorm is characterized mainly by downdrafts. Thunderstorm Cells There are three principal thunderstorm cell types: single cell, multicell (cluster and line), and supercell. A single cell thunderstorm consists of only one cell. Single cell thunderstorms are rare; almost all storms are multicell. A multicell line thunderstorm can extend laterally for hundreds of miles. New cells continually re-form at the leading edge of the system. A multicell cluster thunderstorm consists of a cluster of cells at various stages of their life cycle. A supercell thunderstorm consists primarily of a single, rotating updraft that persists for an extended period of time. Nearly all supercells produce severe weather, and about 25% produce a tornado. Version: Page 15 of 18

16 Types of Thunderstorms Air mass thunderstorms result from local surface heating. On a clear, sunny day, local hot spots form that are capable of making the air over them unstable enough to generate a thunderstorm. When an air mass thunderstorm reaches its mature stage, rain falls through or immediately beside the updraft. Falling precipitation induces frictional drag that retards the updrafts, and reverses it to a downdraft. Such self-destructive cells usually have a life cycle of 20 minutes to 1.5 hours. Steady-state thunderstorms are associated with weather system line fronts, converging wind, and troughs aloft. The precipitation falls outside the downdraft, allowing the cell to last for several hours. Steady-state thunderstorms form in squall lines that develop ahead of cold fronts. Squall line thunderstorms are the most violent type of steady-state thunderstorms. They are the most likely storms to produce cumulonimbus mamma clouds, funnel clouds, and tornadoes. Thunderstorm Motion The movement of individual cells which comprise a storm may deviate substantially from the motion of the storm. Overall, the storm motion equals the combined effects of both advection and propagation. Advection: The component of storm motion due to individual cells moving with the average wind throughout the cloud. The wind at FL180 usually provides a good approximation of direction and speed. Propagation: The component of storm motion due to old cell dissipation and the new cell development. Thunderstorm Avoidance Airborne weather radar precisely measures rainfall density, which can be related to turbulence associated with the radar echoes. The most intense echoes are severe thunderstorms. A severe thunderstorm is defined as a storm that produces surface winds of 50 knots or more or hail 3/4 inches or more in diameter. Pilots should avoid severe thunderstorms by: Over-flying by at least 1,000 for every 10 knots of wind speed; Remaining at least 20 miles away; or Not flying between cells unless they are separated by at least 40 miles. Sometimes thunderstorms can penetrate overlying bands of stratiform clouds. These are known as embedded thunderstorms. Since these thunderstorms are obscured by other clouds, it may be impossible for a pilot to detour around them visually. Inadvertent Thunderstorm Encounter If an inadvertent thunderstorm encounter occurs, the pilot should: Disconnect the autopilot; Reduce power to a setting that maintains a recommended turbulence penetration speed; Concentrate on keeping the aircraft in a level attitude while allowing airspeed and altitude to fluctuate; Tighten seat belts and shoulder harnesses and secure any loose items; Maintain an instrument scan; Advise ATC and declare an emergency; Turn anti-icing or deicing equipment ON including pitot heat; Turn up cockpit lights to the highest intensity to resist the urge to look outside; and Avoid turning maneuvers that increase structural stress on the aircraft. Lightning When lightning occurs, the cloud is classified as a thunderstorm. Lightning bolts reach temperatures of 30,000 to 40,000 F. Lightning bolts travel from the ground up to the cloud, from the cloud to the ground, and between clouds. Thunder occurs because lightning bolts heat and expand air instantaneously. After lightning occurs, air rushes back in to fill the void. The rushing air sends out shock waves perceived to us as sound. Page 16 of 18 My CFI Book

17 Hail Hail is formed inside cumulonimbus clouds by the constant freezing, melting, and refreezing of water as it is carried around by updrafts and downdrafts. Hailstones may be thrown outward from a storm cloud for several miles. Tornadoes Before thunderstorms develop, a change in wind speed and direction creates an invisible, horizontal spinning effect. Rising air within the storms tilts the rotating air from horizontal to vertical, forming a tornado. Tornadoes are violently rotating columns of air extending from a thunderstorm to the ground. Tornadoes are visible because of debris picked up by the spiraling winds. Tornadoes that form over water are called waterspouts. Tornadoes can: Produce winds of over 250 miles per hour; Be a mile wide; and Stay on the ground for more than 50 miles. Tornadoes can occur at any time, but are normally: Developed between 3 and 9 pm; Most common in spring; Least common in winter; Most violent in March and April; and Most numerous in May. Wind Shear Fujita Scale of Tornado Intensity Rating Wind Strength Relative Damage F0 Weak Light F1 Weak Moderate F2 Strong Considerable F3 Strong Severe F4 Violent Devastating F5 Violent Incredible Wind shear is defined as a change in wind direction or speed in a very short distance in the atmosphere. It can occur at any level of the atmosphere and can exist in both horizontal and vertical directions. Note: The horizontal wind shear critical for turbulence per 150 miles is greater than 18 knots. Common Sources of Wind Shear Low-level wind shear can be expected during strong temperature inversions, on all sides and directly under a thunderstorm. Low-level wind shear can also be found near frontal activity because wind changes. In warm front conditions, the most critical period is before the front passes. Warm front shear may exist below 5,000 for about 6 hours before surface passage of the front. The wind shear associated with a warm front is usually more extreme than those in cold fronts. The shear associated with cold fronts is usually found behind the front. Potentially hazardous wind shear may be encountered during periods of a strong temperature inversion with calm or light surface winds, and strong winds above the inversion. Eddies in the shear zone will cause airspeed fluctuation as an aircraft climbs or descends through the inversions. Wind Shear Detection The pilot can detect wind shear by a sudden change in indicated airspeed. During an approach, the most easily recognized means of identifying possible wind shear conditions includes monitoring the rate of descent (vertical velocity) and power required. Version: Page 17 of 18

18 Some airports can alert pilots to the possibility of wind shear on or near the airport with Low-Level Wind Shear Alert Systems. When a tower reports a boundary wind that is significantly different from the airport wind, there is a possibility of hazardous wind shear. Types of Wind Shear Encounters Loss of Tailwind: A tailwind may shear to either a calm or headwind component. Initially, the airspeed will increase by an amount equal to the change in wind velocity. The aircraft will pitch up, and altitude will increase; lower than normal power will be required initially, followed by a further decrease as the shear is encountered. Loss of Headwind: A headwind may shear to a calm or tailwind component. The decrease in headwind will cause a loss in airspeed equal to the decrease in wind velocity. Initially, the airspeed will decrease, the aircraft will pitch down, and altitude will decrease. Microbursts Microbursts are intense, small-scale downdrafts. They can occur anywhere that convective weather conditions exist. Five percent of all thunderstorms produce microbursts. Microbursts are highly localized but very hazardous. An aircraft that encounters a microburst can be subjected to wind shear and downdrafts as strong as 6,000 FPM. Recovering from a microburst encounter at low altitude may be impossible. Types of Microbursts There are two types of microbursts: wet microbursts and dry microbursts. Wet microbursts form by the downward acceleration air created by the drag of precipitation. Microbursts that occur with virga, rain that evaporates before it reaches the ground, are called dry microbursts. Evaporation causes the air to cool, which produces the downward acceleration. Life Cycle of a Microburst 1. Formation: Evaporation and precipitation drag creates a downdraft; 2. Impact: The accelerating downdraft strikes the ground; then 3. Dissipation: The air moves outward from the point of impact. Microburst Encounters during Landing A microburst is particularly dangerous during landing. The first indication of a microburst could be a change in aircraft performance while the power remains constant. This emphasizes the importance of maintaining a stabilized approach. Pont 1: An increasing headwind causes the airplane to balloon above the glideslope. The pilot reacts by reducing power and pitching down. This leaves the aircraft in a nose-low, power-low configuration. Pont 2: A sudden downdraft causes the airplane to sink. Point 3: The downdraft and sink rate decrease, but the increasing tailwind causes the airplane to lose airspeed. It can cause the aircraft to stall or land short of the runway. Page 18 of 18 My CFI Book