Weather Theory

Air pressure is how often and how hard air molecules strike a surface. The Aviation Weather Handbook: "The atoms and molecules that make up the various layers in the atmosphere are always moving in random directions. Despite their tiny size, when they strike a surface, they exert pressure," and "As the density of the air increases, the number of strikes per unit of time and area also increases" 1. More molecules in the space means more strikes, and more strikes means more pressure.

Warm air is less dense

Warm air is less dense than cool air. In the open atmosphere, heated molecules move faster and spread farther apart. The handbook: "Density is inversely related to temperature. Assuming constant mass and pressure, an air parcel with a higher temperature is less dense than an air parcel with a lower temperature (see Figure 8-10). This is because the warmer air occupies a large volume" 2.

The three factors

Pressure, temperature, and humidity set air density. The handbook: "In general, the density of an air parcel can be changed by changing its mass, pressure, or temperature" 2. Humid air is less dense than dry air, because a water molecule weighs less than the nitrogen or oxygen molecule it displaces. The handbook: "air with a greater amount of water vapor is less dense than air with a lesser amount of water vapor. This is because dry air molecules have a larger mass (weight) than water vapor molecules, and density is directly related to mass (see Figure 8-11)" 2.

Humidity is the weakest of the three factors. North Aero's figure: fully saturated warm air is about 1 percent lighter than dry air, and about 2 percent on the hottest days. The effect is real and small.

Any one of high, hot, or humid raises density altitude. The three do not need to occur together. A cool, dry day at a high airport has a high density altitude from pressure alone. A hot, humid day at sea level has one from temperature and humidity.

The atmosphere is about 78 percent nitrogen, 21 percent oxygen, and 1 percent other gases including water vapor. The handbook's table gives "Nitrogen N2 78.081% Oxygen O2 20.945%" 3. Nearly all weather happens in the troposphere, the lowest layer. The handbook: "The troposphere begins at the Earth's surface and extends up to about 11 kilometers (km) (36,000 ft) high," and "Almost all weather occurs in this region" 4.

The standard lapse rate

Temperature normally falls with altitude at an average of about 2 degrees Celsius per 1,000 feet. The handbook: "the temperature decreases 6.5 °C/km (3.57 °F/1,000 ft) in the standard atmosphere. But since this is an average, the exact value seldom exists. In fact, temperature in the troposphere sometimes remains constant or even increases with height" 5. The standard rate is an average, not a measurement of today's air.

Adiabatic cooling

Rising air cools by expansion with no heat exchange. The handbook: "As a bubble or parcel of air ascends (rises), it moves into an area of lower pressure (pressure decreases with height). As this occurs, the parcel expands. This requires energy (or work), which takes heat away from the parcel, so the air cools as it rises (see Figure 12-1). This is called an adiabatic process. The term 'adiabatic' means that no heat transfer occurs into, or out of, the parcel" 6. This process builds the clouds that form by lifting.

Sinking air warms by compression, and the warming lowers its relative humidity. The handbook: "A descending (subsiding) air parcel compresses as it moves into an area of higher pressure. The atmosphere surrounding the parcel does work on the parcel, and energy is added to the compressed parcel, which warms it," and "The parcel's temperature-dewpoint spread increases, while its relative humidity decreases" 6.

The two adiabatic rates

The dry adiabatic lapse rate is about 3 degrees Celsius per 1,000 feet. The handbook: "The lapse rate of a rising unsaturated parcel (air with relative humidity less than 100 percent) is approximately 3°C per 1,000 ft (9.8°C per km). This is called the dry adiabatic lapse rate" 6. Saturated rising air cools more slowly, because condensation releases latent heat. The handbook: "the parcel now cools at the moist adiabatic lapse rate, which varies between approximately 1.2°C per 1,000 ft (4°C per km) for very warm saturated parcels to 3°C per 1,000 ft (9.8°C per km) for very cold saturated parcels" 7.

Environmental and adiabatic

The environmental lapse rate is the measured temperature profile of the actual air. It is a different quantity from the adiabatic rates a parcel follows. A sounding gives the actual profile: "An atmospheric sounding, or simply sounding, is a plot of the vertical profile of one or more atmospheric parameters, such as temperature, dewpoint, or wind above a fixed location" 5. The environmental rate is the balloon's measurement. The adiabatic rates describe a lifted parcel. Stability is the comparison of the two.

Dewpoint is the temperature air must cool to for saturation. The handbook: "Dewpoint is the temperature an air parcel must be cooled at constant pressure and constant water vapor pressure to allow the water vapor in the parcel to condense into water (dew)" 8. Dewpoint measures the actual water vapor content. A pilot uses the temperature-dewpoint spread to predict fog, cloud bases, and structural ice before they appear. The handbook: "The temperature greatly affects the air parcel's ability to hold water vapor, while the dewpoint indicates the actual quantity of water vapor in the parcel. As the spread decreases, relative humidity increases. When the spread decreases to zero, relative humidity is 100 percent, and the air parcel is saturated" 8.

The cloud-base estimate

In a rising convective parcel the spread closes about 2.5 degrees Celsius per 1,000 feet. The temperature falls 3 degrees and the dewpoint falls half a degree: "for each 1,000-ft increase in elevation, the parcel's temperature decreases by 3°C. Concurrently, the dewpoint decreases approximately 0.5°C per 1,000 ft (1.8°C per km)" 6. Cumulus bases are near the spread divided by 2.5, times 1,000 feet. The handbook's example: "at the surface the air parcel has a temperature of 18°C and a dewpoint of 13°C," and the parcel reaches "saturation at its LCL of 2,000 ft" 7. A 5-degree spread gives 2,000 feet.

The estimator works for convective cumulus only. It fails for stratus and frontal decks, because parcels rising from the surface do not make those clouds. Frontal lift and moisture set the base of a stratus deck under a warm front. The surface spread does not predict that base.

Relative humidity

Relative humidity changes with temperature even when moisture does not. The handbook: "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," and "It is possible to change an air parcel's relative humidity without changing its water vapor content" 9. Dewpoint, not relative humidity, is the measure to plan with. Warmer air can contain more water vapor: "An air parcel's capacity to hold water vapor (at a constant pressure) is directly related to its temperature" 9.

Cloud droplets form when vapor condenses onto condensation nuclei in cooling air. The handbook: "This process is triggered by the presence of microscopic cloud condensation (and ice) nuclei, such as dust, clay, soot, sulfate, and sea salt particles" 7. Condensation releases latent heat. The handbook: "Evaporation is the phase transition by which a liquid is changed to a vapor. During evaporation, water absorbs 2,501 J/g due to the latent heat of vaporization. Condensation, the reverse process, releases 2,501 J/g back to the environment" 10. That release is the energy source of thunderstorms 10.

Freezing rain means warm air above

Freezing rain at your altitude means a warmer-than-freezing layer is above you. AC 91-74B: "Freezing rain forms when rain becomes supercooled by falling through a subfreezing layer of air. Ordinarily, air temperatures decrease with increasing altitude, but freezing rain requires a temperature inversion, which can occur when a warmer air mass overlies a colder air mass," and "When flying in freezing rain, normally there is warm air (above 0 °C (32 °F)) above" 11. The rain was liquid in that layer. North Aero's exit is up into the warm layer above, or back along the route already flown.

Frost

Frost forms by deposition. The handbook: "Deposition is the phase transition by which vapor (a gas) is changed into a solid without passing through the liquid phase" 12. On clear, cold nights the surface cools below both freezing and the dewpoint. The handbook on those nights: "At night, insolation is absent but outgoing terrestrial radiation continues, and the Earth's surface continues to cool," and cooling is greatest "over land, at low latitudes, with a clear sky, dry air, and light wind" 13.

Frost on a wing is rough enough to reduce lift. The handbook: "frost, snow, and ice accumulations on the leading edge or upper surface of the wing that are no thicker or rougher than a piece of coarse sandpaper can reduce lift by 30 percent and increase drag up to 40 percent" 14. The pilot removes all frost, ice, and snow from the lift surfaces before flight. AC 91-74B: "When performing an aircraft preflight inspection, remove all frost, snow, and ice from the aircraft surfaces because even very small amounts may adversely affect the aerodynamic properties of a wing" 15.

Stability is the atmosphere's resistance to vertical motion. The handbook: "Atmospheric stability is the property of the ambient air that either enhances or suppresses vertical motion of air parcels and determines which type of clouds and precipitation a pilot will encounter" 16.

The parcel test

The parcel test compares a lifted parcel to its surroundings. The handbook: "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," and "If the lifted parcel is warmer and, therefore, less dense (lighter) than the surrounding air, it will continue to rise on its own until it reaches the same temperature as its environment. This final case is an example of an unstable parcel" 16. A parcel that stays warmer keeps rising. One that turns cooler sinks back.

The numeric comparison

A pilot judges stability by comparing the environmental lapse rate to the adiabatic rates. The handbook's definitions: "Absolute stability (see Figure 13-1) is the state of a column of air in the atmosphere when its lapse rate of temperature is less than the moist adiabatic lapse rate," "Absolute instability (see Figure 13-3) is the state of a column of air in the atmosphere when it has a superadiabatic lapse rate of temperature (i.e., greater than the dry adiabatic lapse rate)," and "Conditional instability (see Figure 13-4) is the state of a column of unsaturated air in the atmosphere when its lapse rate of temperature is less than the dry adiabatic lapse rate but greater than the moist adiabatic lapse rate" 17.

An examiner who asks how a pilot determines stability wants this comparison. An environmental lapse rate greater than 3 degrees per 1,000 feet is absolutely unstable. A rate less than the moist rate is absolutely stable. A rate between the two is conditionally unstable.

Inversions

An inversion marks strongly stable air. The handbook: "A temperature inversion, or simply inversion, is a layer in which the temperature increases with altitude," and "The principal characteristic of an inversion layer is its marked stability, so that very little turbulence can occur within it" 18. Inversions trap haze, smoke, and pollutants. Beneath one the air is smooth and the visibility is poor. The handbook on haze: "Haze occurs in stable air and is usually only a few thousand feet thick," with "a definite ceiling above which in-flight (air-to-air) visibility is unrestricted" 19.

The two signatures

Stable air shows four signs:

• smooth air
• stratiform clouds
• steady precipitation
• poor visibility.

Unstable air shows the opposite four:

• turbulence
• cumuliform clouds
• showery precipitation
• good visibility.

The handbook: a warm moist air mass over a cold surface "produces stable air associated with stratiform clouds, fog, and drizzle" 20, and "Billowy cumuliform clouds, usually seen over land during sunny afternoons, are signposts in the sky indicating convective turbulence" 21. The cloud form shows the stability from the ground.

Moisture destabilizes. Because saturated air cools more slowly as it rises, moist air turns unstable more easily. A lifted parcel that reaches saturation stops cooling at 3 degrees per 1,000 feet and starts cooling at the moist rate. It therefore stays warmer than a dry parcel would, and warmer than more of its surroundings 7.

Conditional instability

Conditional instability is air that is stable while dry and unstable once lifted to saturation. The handbook: "An air parcel lifted upward would be initially stable, but at some point, above its LCL, it would become unstable. The term 'conditional' means the parcel must be lifted to a particular level (altitude) before it becomes unstable and rises because of its own buoyancy" 17. The level of free convection is the height above which a lifted parcel rises on its own. The handbook: "The LFC is the level at which a parcel of saturated air becomes warmer than the surrounding air and begins to rise freely. This occurs most readily in a conditionally unstable atmosphere" 22. The LFC explains elevated and embedded storms. The handbook on elevated convection: "Elevated convection often occurs when air near the ground is relatively cool and stable," and "stability indices based on near-surface measurements (such as the LI) will typically underestimate the amount of instability present" 22.

The stagnant high

A stagnant high-pressure system can mean poor VFR conditions. Air sinks in a high and warms, an inversion forms under it, and the inversion traps haze for days. The handbook on visibility under the inversion: "At or below this level, the slant range (air-to-ground) visibility is poor," and "Fog evaporates, but haze and smoke must be dispersed by the movement of air" 19. High pressure on the chart does not guarantee good visibility.

Uneven heating of the Earth's surface is the cause of all weather. The handbook: "the heating is somewhat uneven because certain areas of the Earth's surface absorb more heat from the Sun than others," and by latitude, "the Earth absorbs more solar radiation at lower latitudes than higher latitudes, which creates heat imbalances and temperature gradients between the Equator and the poles" 23. Land heats and cools faster than water. The handbook: "land heats up faster than water," and "land cools faster than water" 24.

Sea breeze and land breeze

A sea breeze is daytime flow from the cool water toward the heated land. The handbook: "A sea breeze (see Figure 10-13) is a coastal local wind that blows from sea to land and is caused by temperature differences when the sea surface is colder than the adjacent land" 24. A land breeze is nighttime flow from the cooled land toward the warmer water: "A land breeze (see Figure 10-16) is a coastal breeze blowing from land to sea caused by the temperature difference when the sea surface is warmer than the adjacent land" 24.

Clear, calm nights

On clear, calm nights the surface radiates its heat away. The cooling produces surface inversions and radiation fog. The handbook: "A surface-based inversion typically develops over land on clear nights when wind is light. The ground radiates and cools much faster than the overlying air" 18, and "Factors favoring the formation of radiation fog are: 1) a shallow surface layer of relatively moist air beneath a dry layer, 2) clear skies, and 3) light surface winds" 25. The same night makes both.

Wind is air moving from high pressure toward low pressure. The handbook: "Wind is driven by pressure differences, which create a force called the PGF. Whenever a pressure difference develops over an area, the PGF makes the wind blow in an attempt to equalize pressure differences" 26. Coriolis force turns moving air to the right in the Northern Hemisphere: "The force deflects air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere" 27.

Aloft and at the surface

Above the friction layer, wind blows roughly parallel to the isobars. The handbook: "However, the instant air begins moving, Coriolis force deflects it to the right. Soon the wind is deflected a full 90° and is parallel to the height contours" 28. Surface friction slows the wind and angles it across the isobars toward low pressure: "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" 28. The angle is "about 10° over water, increasing to as high as 45° over rugged terrain" 28.

Wind veers and strengthens on the climb out of the friction layer. At the surface, friction slows the wind and turns it left of the isobars. A few thousand feet up it blows at full speed along them. The climb from one to the other is a clockwise shift and an increase in speed. That shift is why the surface wind and the winds aloft forecast differ in the same direction on most days.

Highs and lows

Northern Hemisphere circulation is clockwise and outward around highs, counterclockwise and inward around lows. The handbook: "in the Northern Hemisphere, the surface wind spirals clockwise and outward from high pressure and counterclockwise and inward into low pressure (see Figure 10-11)" 28. Air sinks in highs and becomes drier. Air rises in lows and forms cloud. Sinking air warms by compression and its spread widens. Rising air cools and its spread closes 6.

Closely spaced isobars mean a steep gradient and strong wind: "Closely spaced contours/isobars indicate strong winds, while widely spaced contours/isobars mean lighter wind" 26.

Mountain waves form when strong wind crosses a ridge into stable air. The handbook: "if the wind is sufficiently strong and the surrounding atmosphere is stable, a wave will develop" 29. The rotor zone under the wave crest is the most dangerous part: "it is quite common for a rotor zone to develop near or below ridge level on the downwind side of the mountain, under a wave crest and associated lenticular cloud (if sufficient moisture is present). This is an area of potentially severe-to-extreme wind shear and turbulence" 29. Lenticular and rotor clouds mark a wave. A dry wave has no visible warning: "extremely severe wind events can occur with little or no visual warning of their presence," and the clouds "may be absent if the air is too dry" 29.

Wind shear

Low-level wind shear is a defined term. The handbook: "Non-convective LLWS is defined as a wind shear of 10 kt or more per 100 ft in a layer more than 200 ft thick that occurs within 2,000 ft of the surface" 30. Wind shear can occur at any altitude and in any direction. It is not only a thunderstorm product. The handbook: "It is important to remember that wind shear can affect any flight at any altitude" 30. Its causes are frontal passage, thunderstorms, inversions, and terrain: "These wind variations can result from a large variety of meteorological conditions such as topographical conditions, temperature inversions, sea breezes, frontal systems, and strong surface winds" 30.

A shear that removes headwind on final costs airspeed and lift at the worst time. The airplane is slow, low, and configured for drag. The headwind ends. The airspeed drops by the amount of the shear. The airplane, already low, sinks toward the runway. The handbook calls the low-level kind "especially hazardous due to the proximity of an aircraft to the ground" 30.

The flight of N4257C

On October 11, 2008, at 5:02 in the afternoon, a Maule MXT-7-180A took off from Doña Ana County Airport at Santa Teresa, New Mexico. An isolated thunderstorm was off the departure end of the runway. The NTSB: "The private pilot and private pilot rated passenger were fatally injured" 31. The pilot was 46 and not instrument rated, with 1,755 hours, 1,293 of them in this airplane 31. The two flew in from Glendale, Arizona, that day "to visit the 'Land of Enchantment RV Fly-in' being held there" 31.

"Witnesses observed the airplane taxi and perform an intersection takeoff directly towards an isolated thunderstorm located off the departure end of the runway. The airplane appeared to climb to 150 to 200 feet above the ground before disappearing from view as it went into the storm. One witness stated that the airplane's wings were rocking back and forth and that it appeared the pilot was having difficulty controlling the airplane, just before it went out of view. Five to ten minutes later, after the storm moved past, smoke was seen rising approximately one mile west of the airport" 31.

Weather radar measured the storm. "Weather radar data showed the thunderstorm was producing an approximate 60 knot wind shear at the time of the accident" 31. A special observation seven minutes later gave "winds 300 degrees at 14 knots, gusting to 31 knots, 10 miles visibility, broken clouds at 6,000 feet, overcast at 25,000 feet, thunderstorm within 10 miles northwest moving northeast at 35 miles per hour" 31. The radar showed "a divergent wind pattern over the accident site with a shear of greater than 60 knots between the inbound and outbound wind components" 31. That pattern matches the handbook's description of a downburst spreading outward at the surface. The airplane climbed into it at 200 feet.

The warning existed. "Convective Significant Meteorological Information (SIGMET) 42C was current and applied to the accident site at the time of the accident. It warned of an area of thunderstorms moving from 220 degrees at 10 knots with maximum tops to 43,000 feet. The advisory implied severe and greater turbulence, low-level wind shear, and local instrument meteorological conditions. No evidence was found indicating either the pilot or passenger had obtained weather information or a weather briefing prior to departure" 31. The probable cause: "The pilot's decision to takeoff and fly into a thunderstorm" 31.

Where the pilot could have prevented the accident

Two pilots took off toward a storm they could see. Each decision below is marked with the PAVE category that applied to it.

An air mass takes on the temperature and moisture of the region it forms over. The handbook: "The longer the air mass stays over its source region, the more likely it will acquire the properties of the surface below," and "Air masses are classified according to the temperature and moisture properties of their source regions (see Figure 11-1)" 20.

A front is a zone

A front is the boundary zone between two air masses. It has depth. The handbook: "A front is a boundary or transition zone between two air masses," and "Fronts do not exist only at the surface of the Earth; they have a vertical structure in which the front slopes over the colder (denser) air mass" 32. Three things show frontal passage. The handbook: "significant temperature gradients, or differences, exist along fronts (especially on the cold air side); winds usually converge, or come together, at fronts; and pressure typically decreases as a front approaches and increases after it passes" 32. The three signs are:

• a wind shift
• a temperature change
• a pressure fall, then a rise.

Cold fronts

A cold front is steep and fast. The handbook: "A cold front occurs when a mass of cold, dense, and stable air advances and replaces a body of warmer air. It is so dense, it stays close to the ground and acts like a snowplow, sliding under the warmer air and forcing the less dense air aloft. Cold fronts have a steep slope, and the warm air is forced upward abruptly (see Figure 11-6). This often leads to a narrow band of showers and thunderstorms along, or just ahead of, the front if the warm rising air is unstable" 33. After the front, skies clear: "Behind a fast-moving cold front, the skies usually clear rapidly, and the front leaves behind gusty, turbulent winds and colder temperatures" 33.

Squall lines form ahead of fast cold fronts. The handbook: "A continuous line of thunderstorms, or squall line, may form along or ahead of the front. Squall lines present a serious hazard to pilots as squall-type thunderstorms are intense and move quickly" 33.

Warm fronts

A warm front is shallow and slow. The handbook: "Warm fronts move slowly, typically 10 to 25 mph. The slope of the advancing front slides over the top of the cooler air and gradually pushes it out of the area. Warm fronts typically have a gentle slope, so the warm air rising along the frontal surface is gradual (see Figure 11-5). This favors the development of widespread layered or stratiform cloudiness and precipitation along, and ahead of, the front if the warm rising air is stable" 34. The weather arrives far ahead of the surface front, and it is layered and steady.

Ahead of a warm front the clouds lower and thicken in sequence. The handbook: "Generally, prior to the passage of a warm front, cirriform or stratiform clouds, along with fog, can be expected to form along the frontal boundary" 34. The Pilot's Handbook names the families: "They are classified according to the height of their bases as low, middle, or high clouds, as well as clouds with vertical development" 35. Cirrus comes first, then the middle layers, then the low stratus and the rain. The sequence is the front's slope seen from underneath.

Winter warm fronts

Winter warm fronts produce freezing rain. Rain falls from the warm layer aloft into freezing air below. AC 91-74B: freezing rain "requires a temperature inversion, which can occur when a warmer air mass overlies a colder air mass. This situation can occur along a warm front, where a warm air mass overruns a cold air mass" 11. The warm front's shallow slope is the warm layer over the cold one.

Stationary and occluded

A stationary front moves little and its weather is prolonged. The handbook: "When the forces of two air masses are relatively equal, the boundary or front that separates them remains stationary and influences the local weather for days" 36. An occluded front is a faster cold front overtaking a warm front and lifting it. The handbook: "Cold fronts typically move faster than warm fronts, so in time they catch up to warm fronts. As the two fronts merge, an occluded front forms," and "As the occluded front approaches, warm front weather prevails but is immediately followed by cold front weather" 36.

Warm-front occlusions produce embedded thunderstorms. The handbook: "A warm front occlusion occurs when the air ahead of the warm front is colder than the air of the cold front. When this is the case, the cold front rides up and over the warm front. If the air forced aloft by the warm front occlusion is unstable, the weather is more severe than the weather found in a cold front occlusion. Embedded thunderstorms, rain, and fog are likely to occur" 36. The storms are inside the stratiform deck and not visible from outside it.

Clouds are classified by the height of their bases, and the two handbooks count the groups differently. The Aviation Weather Handbook: "the part of the atmosphere in which clouds are usually present has been divided into three levels: high, middle, and low (see Table 12-3)" 37. The Pilot's Handbook keeps a fourth group: "Clouds with extensive vertical development are cumulus clouds that build vertically into towering cumulus or cumulonimbus clouds" 35. One book has three levels, the other four families. The examiner can use either.

Reading the names

Nimbo in a cloud name means the cloud produces precipitation. The handbook: "Nimbus comes from the Latin word meaning 'rain.' These clouds typically form between 7,000 and 15,000 ft (2,100 to 4,600 m) and bring steady precipitation" 37. Cumulus forms indicate instability and stratus forms indicate stability. The handbook: cumuliform clouds "show the vertical motion or thermal uplift of air taking place in the atmosphere" 37. Towering cumulus indicates strong convective turbulence and possible storm development. The Pilot's Handbook: "Towering cumulus clouds indicate areas of instability in the atmosphere, and the air around and inside them is turbulent" 35.

The causes and the four words

Turbulence has three causes in the handbook, and frontal turbulence is a wind-shear kind. The handbook: "Turbulence is caused by convective currents (called convective turbulence), obstructions in the wind flow (called mechanical turbulence), and wind shear," and inversions "commonly occur within the lowest few thousand feet above ground due to nighttime radiational cooling, along frontal zones, and when cold air is trapped in a valley" 21. The four names a pilot hears are convective, mechanical, frontal, and wind shear. The intensities are defined terms. The handbook: "Light − Causes slight, erratic changes in altitude and/or attitude (pitch, roll, or yaw)," "Moderate − Similar to Light but of greater intensity. Changes in altitude and/or attitude occur, but the aircraft remains in positive control at all times," "Severe − Causes large, abrupt changes in altitude and/or attitude. It usually causes large variations in indicated airspeed. Aircraft may be momentarily out of control," and "Extreme − The aircraft is violently tossed about and is practically impossible to control. It may cause structural damage" 38. Severe means momentarily out of control. A pilot reports severe only when the airplane was momentarily out of control.

Clear air turbulence

Clear air turbulence occurs near the jet stream, with no cloud to mark it. The handbook: "CAT is defined as sudden severe turbulence occurring in cloudless regions that causes violent buffeting of aircraft. CAT is a higher altitude turbulence (normally above 15,000 ft) particularly between the core of a jet stream and the surrounding air," and it "is often encountered unexpectedly and frequently without visual clues to warn pilots of the hazard" 39.

A thunderstorm needs three ingredients. The handbook: "Thunderstorm cell formation needs three ingredients: sufficient water vapor, unstable air, and a lifting mechanism (see Figure 22-1)" 40. The three are moisture, instability, and lift. Without any one of the three, no storm forms.

The life cycle

The life cycle has three stages. The handbook: "It undergoes three distinct stages during its life cycle (see Figure 22-2): towering cumulus, mature, and dissipating. The total life cycle is typically about 30 minutes" 40. The cumulus stage is updrafts: "The distinguishing feature of the towering cumulus stage is a strong convective updraft" 40. The mature stage begins when precipitation reaches the surface, and updrafts and downdrafts coexist: "The cell transitions to the mature stage when precipitation reaches the surface. Precipitation descends through the cloud and drags the adjacent air downward, creating a strong downdraft alongside the updraft," and "Weather hazards reach peak intensity toward the end of the mature stage" 40. The dissipating stage is downdrafts: "The dissipating stage is marked by a strong downdraft embedded within the area of precipitation" 40.

The mature stage is the most violent, and surface precipitation marks its start. A single cell lasts about 30 minutes, and its hazards peak at the end of the mature stage. The downdraft that spreads out ahead of the cell is the gust front: "The arc-shaped leading edge of downdraft air resembles a miniature cold front and is called a gust front" 40.

The distances

A pilot avoids severe storms by at least 20 miles. The handbook: "Avoid by at least 20 mi any thunderstorm identified as severe or giving an intense, heavy, or extreme radar echo. This is especially true under the anvil of a large cumulonimbus. Such echoes should be separated by at least 40 mi before flying between echoes" 41. Never fly under a thunderstorm or under its anvil: "Do not attempt to fly under a thunderstorm, even if you can see through to the other side. Turbulence and wind shear under the storm could be hazardous," and "Do not attempt to fly under the anvil of a thunderstorm. There is a potential for severe and extreme CAT" 41.

Hail can fall in clear air beneath and beside the anvil. The handbook: "Eventually, the hailstones fall and may be encountered in clear air several miles from the thunderstorm," and "Pilots should anticipate possible hail with any thunderstorm, especially beneath the anvil of a large cumulonimbus" 42. Storm tops at or above 35,000 feet are extremely hazardous: "Regard as extremely hazardous any thunderstorm with tops 35,000 ft or higher, whether the top is visually sighted or determined by radar" 41. When thunderstorms cover more than half the area, go around it: "Circumnavigate the entire area if more than half the area is covered by thunderstorms" 41. Embedded storms need radar: "Do not fly without airborne radar into a cloud mass containing scattered embedded thunderstorms" 41. A Warrior has no airborne radar, so its pilot must not enter a cloud mass with embedded storms.

The microburst

A microburst is a small, intense downdraft. The handbook: "A microburst (see Figure 22-8) is a small-scale, intense downdraft that, when reaching the surface, spreads outward symmetrically (see Figure 22-9) or asymmetrically (see Figure 22-10), in all directions from the downdraft center. It is the most severe type of wind shear" 43. The handbook's numbers: "The lifespan of a microburst is about 5–15 minutes, during which time it can produce downdrafts of up to 6,000 fpm; increasing headwind and headwind losses of 30–90 kt, seriously degrading performance" 43. A flight through one takes the airplane from headwind to tailwind, and the airspeed loss is the sum of the two. The clues are visible: "Microburst activity may be indicated by an intense rain shaft at the surface, but virga (i.e., streaks of precipitation falling from a thunderstorm cloud but not reaching the ground) at the cloud base and/or a ring of blowing dust is sometimes the only visible clue (see Figure 22-11)" 43.

Structural icing needs two things: visible moisture and an airframe surface at or below freezing. AC 91-74B: "Nearly all aircraft icing occurs in supercooled clouds. Liquid drops are present at outside air temperatures (OAT) below 0 °C (32 °F) in these clouds," and "Ice forms on aircraft structures and surfaces when supercooled droplets adhere to them and freeze" 44. The Aviation Weather Handbook: "Supercooled water will readily freeze if sufficiently agitated. This explains why airplanes collect ice when they pass through a liquid cloud or precipitation composed of supercooled droplets" 45.

Clear and rime

Clear ice is large supercooled drops spreading before they freeze. The handbook: "Clear ice (or glaze ice) is a glossy, clear, or translucent ice formed by the relatively slow freezing of large, supercooled water droplets," and "Since it is clear and difficult to see, the pilot may not be able to quickly recognize that it is occurring" 46. AC 91-74B: "This type of ice is denser, harder, and sometimes more transparent than rime ice. With larger accretions, clear ice may form 'horns'" 47. Rime ice is small drops freezing on impact. The handbook: "Rime ice is rough, milky, and opaque ice formed by the instantaneous freezing of small, supercooled water droplets after they strike the aircraft," and "Rime ice grows into the air stream from the forward edges of wings and other exposed parts of the airframe" 46.

Freezing rain is the most dangerous accumulation. The handbook: "A favored location for severe clear icing is freezing rain and/or freezing drizzle below a front" 46. Its drops are the largest, and they reach the farthest aft. The handbook: "These are water droplets in a subfreezing environment with diameters larger than 50 microns, such as freezing drizzle (50–200 microns) and freezing rain (>200 microns). These larger droplets can flow along the airfoil for some distance prior to freezing," and "SLD ice tends to form aft, beyond the reach of deicing equipment" 46.

The temperatures

Most icing occurs between 0 and minus 20 degrees Celsius. The handbook: "almost all icing tends to occur in the temperature interval between 0°C and -20°C, with about half of all reports occurring between -8°C and -12°C," and "The only physical cold limit to icing is at -40°C because liquid droplets freeze without nuclei present" 45.

Severe, and the tail

Icing intensities are defined terms, and severe icing requires immediate exit. AC 91-74B: "Severe Icing. The rate of ice accumulation is such that ice protection systems fail to remove the accumulation of ice and accumulation occurs in areas not normally prone to icing, such as aft of protected surfaces," and "Immediate exit is required by many Airworthiness Directives (AD), flight manuals, and operations under part 91" 48. Tailplane icing has a separate hazard. AC 91-74B: "With ice on the tailplane, it may stall after deployment of flaps," and "it is important that the pilot be alert to the possibility of tailplane stall, particularly after full flap deflection, on airplanes not evaluated for susceptibility. A no-flap landing should be considered to avoid a tailplane stall, consistent with AFM procedures" 49. The pilot knows the airplane's flap guidance before extending flaps in ice.

Carburetor ice is a warm-day hazard. The Pilot's Handbook: "Carburetor ice is most likely to occur when temperatures are below 70 degrees Fahrenheit (°F) or 21 degrees Celsius (°C) and the relative humidity is above 80 percent. Due to the sudden cooling that takes place in the carburetor, icing can occur even in outside air temperatures as high as 100 °F (38 °C) and humidity as low as 50 percent" 50. It is not a freezing-weather problem. The venturi and the evaporating fuel do the cooling.

Fog

Fog is a cloud at the surface. The handbook: "Fog is a visible aggregate of minute water droplets that are based at the Earth's surface, and it reduces horizontal visibility to less than 5/8 SM (1 km)," and "Fog differs from a cloud only in that its base must be at the Earth's surface, while clouds are above the surface" 51. It rarely forms until the spread closes: "Fog seldom forms when the temperature-dewpoint spread is greater than 2°C (4°F)" 51.

Radiation fog

Radiation fog needs a clear night, light wind, and moist air. The handbook: "Radiation fog (see Figure 18-1 and Figure 18-2) is a common type of fog, produced over a land area when radiational cooling reduces the air temperature to or below its dewpoint. Thus, radiation fog is generally a nighttime occurrence and often does not dissipate until after sunrise" 25. It forms in low spots and clears after sunrise: "Ground fog usually burns off rather rapidly after sunrise. Other radiation fog generally clears before noon unless clouds move in over the fog" 25.

Advection fog

Advection fog is moist air flowing over a cooler surface. It needs wind, and it can last for days. The handbook: "Advection fog (see Figure 18-5 and Figure 18-6) forms when moist air moves over a colder surface and the subsequent cooling of that air to below its dewpoint," and "It can remain over the water for weeks, advancing over the land during night and retreating back over the water the next morning" 52. Wind above about 15 knots lifts it instead of clearing it: "Advection fog deepens as wind speed increases up to about 15 kt. Wind that is generally stronger than 15 kt lifts the fog into a layer of low stratus or stratocumulus clouds" 52.

Upslope fog is moist air cooled adiabatically as terrain forces it up. The handbook: "Upslope fog forms as a result of moist, stable air being adiabatically cooled to or below its dewpoint as it moves up sloping terrain (see Figure 18-7)" 53. Steam fog is cold air over warm water: "When very cold air moves across relatively warm water, enough moisture may evaporate from the water surface to produce saturation," and "Steam fog is associated with a shallow layer of unstable air; thus, pilots can expect convective turbulence flying through it" 53. It carries low-level turbulence.

Frontal fog forms when rain saturates the cold air beneath a front. The handbook: "If the cold air below is near its dewpoint, evaporation (or sublimation) from the precipitation may saturate the cold air and form fog (see Figure 18-8). A fog formed in this manner is called frontal (or precipitation-induced) fog" 53.

Mist and haze

Mist is the same droplets as fog, with visibility of five-eighths of a mile or more. The handbook: "Mist is a visible aggregate of minute water droplets or ice crystals suspended in the atmosphere that reduces visibility to less than 7 SM (11 km), but greater than, or equal to, 5/8 SM (1 km)" 19. The METAR code is BR.

Haze makes slant-range visibility poor. Looking straight down, the pilot sees the ground. Looking ahead, the pilot cannot see the horizon or traffic. The handbook: "A haze layer has a definite ceiling above which in-flight (air-to-air) visibility is unrestricted. At or below this level, the slant range (air-to-ground) visibility is poor" 19.

Smoke, dust, and blowing obscurations reduce visibility fastest in stable air, which holds them near the surface. The handbook: "Haze occurs in stable air," and "Fog evaporates, but haze and smoke must be dispersed by the movement of air" 19. Under an inversion nothing disperses them.

Volcanic ash

Never fly through volcanic ash. The handbook: "Flying into a volcanic ash cloud can be hazardous. Volcanic ash is composed of silica (glass)," "Piston-powered aircraft are less likely to lose power, but severe engine damage is likely after an encounter with a volcanic ash cloud that is only a few hours old," and "Particles impacting the windshield can sandblast the surface into a frosted finish that obscures the pilot's view" 54. The pilot avoids it by wide margins, because the ash is not always visible.

A sounding plots temperature and dewpoint as two lines against height. The handbook: a sounding "is a plot of the vertical profile of one or more atmospheric parameters, such as temperature, dewpoint, or wind above a fixed location" 5. Where the temperature line and the dewpoint line converge, the air is saturated and cloud is present. The slope of the temperature line is the environmental lapse rate, and its slope against the dry and moist adiabatic lines shows the stability 17.

The freezing level and the inversion

On a sounding, the temperature line crossing 0 degrees Celsius marks the freezing level. Where the temperature line warms with height, the layer is an inversion. The handbook warns against estimating the freezing level from the standard rate: "Caution should be taken when using the standard lapse rate to estimate the freezing level. Quite often the boundary layer is dry adiabatic, and the estimate of freezing level could be in error" 5. The inversion is "a layer in which the temperature increases with altitude" 18. The pilot reads both from the line rather than computing them.