Aviation Meteorology
From Pilots Almanac
Contents |
1. Introduction
Weather is a fascinating phenomenon that greatly influences our lives and yet, is something we have absolutely no control over. Most of the weather that occurs on our planet happens below 15,000 feet. That's great if you are flying a commercial jetliner at a high altitude of perhaps 30,000 feet where there is hardly any significant weather. For the majority of us we are "under the weather" most of the time. Weather is the utmost consideration of all pilots when planning a flight. Whether the pilot is flying a small airplane or flying a commercial jetliner, the weather at the departure airport, all along the route and at the destination airport must be constantly monitored. We will consider here the major weather conditions that affect pilots whether flying under conditions of VFR (visual flight rules) or IFR (instrument flight rules).
2. Atmosphere
Though we live on the surface of the earth, we actually live at the bottom of an ocean of air. Dynamic layers of air interact with the earth's surface and the sun's energy to produce the phenomenon of weather. The atmosphere is classified into layers based upon the characteristics each layer exhibits. Most weather occurs in the troposphere and most flying occurs in the troposphere and stratosphere.
The atmosphere is comprised of a mixture of gases referred to as "air". It is about 80 percent nitrogen and 20 percent oxygen. Water vapor and many other gases constitute the remainder of the gas mixture. Note that the molecular weight of water vapor is less than that of both oxygen and nitrogen. For this reason, moist air is less dense than relatively drier air. Airfoils and air-breathing engines exhibit reduced performance in moist air because of this reduction in density.
Air is made up of matter and has weight. Since air is gaseous, it is compressible. This means that the air pressure nearer the surface of the earth is greater than the air pressure in the stratosphere.
Air exerts pressure on everyone and everything. At the earth's surface the pressure is 14.7 pounds per square inch. That does not sound like much, but it means the air pressure per square foot is 2116.8 pounds. Increase the air pressure and the air's density is increased. Because of air's compressibility flight conditions will vary depending upon the altitude. This is due to the air density. More molecules in the air will generate greater lift with less thrust. Fewer molecules in the air will require greater thrust to generate adequate lift.
The decrease in air density with increase in altitude also affects people physiologically. Decrease the air pressure and the oxygen pressure is also decreased. The rate at which our lungs absorb oxygen depends on the partial pressure exerted by the amount of oxygen in the air. Since our atmosphere is about 1/5 oxygen, the oxygen pressure at any given altitude will be about 1/5. Under normal conditions our lungs function under 3 pounds per square inch of oxygen pressure. As an airplane climbs higher into the troposphere, it will encounter less oxygen. Without supplemental oxygen the people on board such a flight will suffer from hypoxia, a deficiency in oxygen. The symptoms of hypoxia are a feeling of acute exhaustion with an immediate impairment in vision and judgment resulting in unconsciousness and death if the proper amount of oxygen is not soon administered. Prolonged flights at or above 10,000 feet and even short flights above 12,000 should use auxiliary oxygen.
The ocean of air we live in can be calm, delightfully warm and pleasant or it can be turbulent and rainy like a thunderstorm, hurricane or tornado. The air temperature varies from below -100 Celsius to above 1500 Celsius (-150 Fahrenheit to 2700 Fahrenheit). These variations are caused by the uneven heating of the earth's surface by the sun's energy as well as how the earth reacts to this energy. The characteristics of a substance (for example water or land) will affect the amount of heat absorbed or released by that substance. Let's say we have a land surface and water surface of equal temperature and we apply an equal amount of heat to each. The land surface will become hotter at a much faster rate than the water surface. The opposite is true when both substances release the same amount of heat. Under equal heat loss the land will become colder at a faster rate than the water.
3. Wind
Variation in the temperature comes from uneven heating of the earth's surface by the sun. The warmer air expands becoming less dense than the cooler air around it. The cooler air (which has greater density) moves toward the ground. The rising air spreads out above, becomes cooler and eventually descends while the cooler air below warms and rises. This process of convection plays itself out worldwide from hemispheric circulations to local airflows. Horizontal movement of the air is known as wind.
Airflow around Pressure Systems
This convection process causes changes in the air density, and those variations cause winds. Winds flow out of higher air density areas into lower air density areas. Because of the Coriolis effect in the Northern Hemisphere, the wind flows clockwise around higher air density areas, called a "high," and counterclockwise around lower air density areas or a "low." For general reference, highs bring clear weather and lows bring stormy weather. Because this is not always true, pilots perform a thorough check of all available weather data when planning a flight.
Jet Stream
Running through these high and low pressure areas high in the troposphere (near the tropopause) is the jet stream. On the average, winds tend to increase speed with height in the troposphere culminating in maximum speeds near the tropopause. These high-speed winds also become concentrated, narrow bands that wind their way through the atmosphere. The jet stream varies from 100 to 400 miles wide and is usually found above 30,000 feet. Its general motion is from west to east with a speed range of 150 to 300 miles per hour. It has seasonal migrations within the United States. Closer to the earth's surface the jet stream causes local disruption in the airflow and some additional turbulence. However, during high altitude flights riding with a jet stream increases overall ground speed.
Circulation Patterns
There exist general circulation patterns and seasonal surface pressure systems worldwide, but of equal importance to the airplane pilot are those influences closer to the surface such as friction, local wind patterns and wind shear which all affect flight. Friction occurs between the wind and the terrain surface acting in opposition to the wind's direction. Friction slows the windspeed. The rougher the terrain the greater the friction. The greater the windspeed, the greater the friction. Knowing the terrain over which the flight is to take place will help the pilot account for patches of rough air along the way.
Terrain features such as mountains, valleys and shorelines also generate local wind patterns of which low flying pilots need to be aware. Land and sea breezes are important when flying above coastal regions. During the day, the land is warmer than the sea; therefore the wind blows from the cooler water toward the warmer land. This is called a sea breeze. At night, the wind reverses as the land cools more quickly than the water generating a land breeze.
Day versus Night
The mountain and valley breezes are also diurnal. The radiated ground heats air next to a mountain slope in the daytime. Colder, denser air farther away from the mountain slope located at the same altitude as the warmer air settles down upon the warmer air forcing it to move up the mountain slope. This is referred to as a valley wind because it flows up the mountain slope out of the valley. At night, the opposite movement occurs. The air on the mountain slope is cooled, becomes heavier than the surrounding air and follows the mountain slope down into the valley. Mountain winds are usually stronger than valley winds.
Air Currents and Local Air Circulation
Rising and descending air currents affect local air circulation. Surfaces such as planted fields, meadows and water tend to retain heat and cause descending air currents. Meanwhile rocky or sandy terrain, plowed fields and barren land reflect heat and cause ascending air currents. These will cause a landing aircraft to overshoot or undershoot the runway if not accounted for.
Wind Shear
Wind shear is encountered in an area where two winds moving in opposite directions "rub" or mix together. This shear zone creates small eddies and whirling masses of air that move in various directions. This generates tremendous turbulence. Some wind shears are predictable, but others may occur unexpectedly. Getting caught in wind shear can be devastating to an aircraft, especially if the wind shear occurs close to the ground. Currently airports are being outfitted with wind shear alarms that warn controllers and pilots of the potential windshear existence within runway takeoff and landing corridors.
4. Moisture
Flying would be much easier if moisture were not such an influential component found in the atmosphere. Moisture in the air creates more hazards during flight than any other weather phenomenon. Water in the atmosphere is measured by relative humidity and dew point accompanied by a temperature-dew point spread. Knowing the conditions during which water changes state also helps pilots to avoid moisture-related problems during flight.
Temperature and moisture
Relative humidity relates the actual amount of moisture in the air (in the form of a percent) to the total amount of moisture that could be held in the air (a ratio). Relative humidity expresses the degree of saturation. As a rule, cold air holds fewer water molecules than does warmer air. That is, in two equal-sized volumes, the colder volume of air will contain less water vapor than the warmer volume. If air is completely saturated with water molecules the humidity is 100%.
In relationship to the humidity is dew point. Dew point is the temperature (in degrees) to which air must be cooled in order to be saturated with water vapor already in the air. Weather reports for pilots usually include the dew point as well as the temperature. When the two are compared, the difference reveals to the pilot how close the air is to being 100% saturated. This difference is called the temperature-dew point spread.
Lake effect weather
On a clear night when the dew point colder than 32° F and the temperature-dew point spread is 50° F or less and decreasing, then frost will form. Fog is most likely when the temperature-dew point spread is 50° F or less and decreasing. The fog would be lifting when the temperature-dew point spread begins increasing. Fog usually forms when the dew point and the temperature are within a few degrees of each other. The air temperature being lowered to the dew point, or the dew point being increased to the air temperature causes fog formation. Air temperature can be lowered as the air crosses over a colder surface like cold lake waters or a snow-covered area. Increasing the atmospheric moisture occurs when air flows from a water source (large lake, ocean) then moves over land.
Pilots need to be mindful of the conditions which cause radiation fog and advection fog. Of the two types of fog, radiation fog does not hang around as long, is less hazardous and more localized. This means that when flying at low altitudes, a pilot will encounter patches of it and be able to fly through it quickly.
Advection fog (also known as ground fog) occurs most often during clear, cool autumn nights while the earth's surface is rapidly cooling. It may hang in the air through the morning, but dissipates a few hours after sunrise. Advection fog however, forms when air laden with moisture from a maritime area moves from the water area over higher terrain while gradually cooling. As the air temperature is reduced to the dew point advection fog forms. This happens most often during the winter months over the eastern half of the United States as moist air flows northward from the Gulf of Mexico across the land increasing in elevation and cooling as it moves. This same phenomena occurs along the coastal region of California as warm winds blow across the chilled California Current resulting in advection fog that can stretch from San Francisco to San Diego.
Precipitation refers to all different kinds of moisture that falls from the sky: drizzle, rain, snow, ice pellets, hail and ice crystals (whether they contact the ground or not).
As air becomes saturated from cooling temperatures or increasing dew point, water vapor starts to condense on the microscopic particles that are suspended in the air. Once the water droplet or ice crystal forms it will continue to grow by continued condensation or sublimation. When enough of these water or ice particles merge they form a visible collection called a cloud. Depending upon air currents and temperature, clouds can take on a sheet-like shape or become towering masses. Precipitation that forms and remains liquid will fall as drizzle or rain. If water vapor condenses at temperatures below freezing, snowflakes can form. Precipitation can change its state (vapor, liquid or solid) depending upon the temperature of its immediate environment. As ice pellets fall through a warmer layer of air they will turn to raindrops. In order to generate significant precipitation, clouds are commonly 4,000 feet or more in thickness. The heavier the precipitation, the thicker the cloud cover is likely to be. When the destination airport is reporting precipitation that is light to moderate, expect the cloud cover to be greater than 4,000 feet thick.
Icing
One of the major weather hazards to aviation is icing. Icing is the formation of ice on parts of a vehicle. Pilots and controllers need to be aware of the icing process, under what conditions ice will form on an aircraft, the different forms it takes on an aircraft and its effects on the aircraft's flight characteristics. Icing occurs when an aircraft flies through visible water and the temperature at the point where the moisture strikes the aircraft is 32° F (0° C) or colder. Even though the air temperature around the airplane may be a few degrees warmer than freezing, aerodynamic cooling can occur (due to the rapid movement of the airplane through the air creating a wind chill effect) and lower the temperature of the airplane's surface thus inducing icing. Supercooled water increases the rate of icing. As a supercooled water droplet hits the airplane's surface a part of it freezes instantaneously. The manner in which the remaining portion of the water droplet freezes determines whether the ice formation is clear ice, rime ice or mixed ice.
- Clear Ice - After the initial impact of supercooled droplets from large raindrops strike the surface, the remaining liquefied portion flows out over the surface and freezes gradually. This freezes as a smooth sheet of sold ice. It is hard and heavy and is difficult to remove.
- Rime Ice - formed from small supercooled droplets when the remaining liquefied portion after initial impact freezes rapidly before the drop has time to spread over the surface. This traps air between the droplets, and gives the ice a white appearance. It is lighter in weight than clear ice. Its formation is irregular and its surface is rough. It is brittle and more easily removed than clear ice.
- Mixed Ice - formed when supercooled water droplets are of various sizes or are intermingled with snow or ice particles. After initial impact, the remaining portion freezes rapidly and forms a mushroom shape on the leading edges of a wing. Ice particles are imbedded in clear ice and form a hard and rough-edged mass.
Icing is considered a cumulative hazard as it takes time for the ice to build up on the aircraft and increasingly changes the aircraft's flight characteristics.
Basically all clouds with temperatures that are sub-freezing have the potential for icing conditions. See the cloud sub-section for specific cloud types. Other conditions that are conducive to icing include mountainous regions and certain fronts. Icing is most hazardous and most probable over mountainous regions than any other type of terrain. Rapid movements of upward air coupled with cooling support large water droplets. Each mountain region has its own areas that are prone to icing conditions. Pilots need to be familiar with these zones. The most dangerous icing takes place over the windward side of ridges and above the crests. This icing zone can extend 5,000 feet or higher above the top of the mountain.
The effects of icing can be devastating to a flight. A pilot can help to avoid conditions in which icing occurs or upon encountering icing by reacting quickly and ascending or descending to an altitude where the temperature is warm enough to melt the ice. Avoiding quick maneuvers when icing occurs is recommended, as the aircraft is not operating at peak efficiency and is more likely to stall. If choosing to ascend to a warmer flight level, increasing airspeed to a faster than normal speed when climbing will help to avoid a stall.
5. Clouds
After air begins to cool and then becomes saturated, sublimation or condensation starts the cloud-forming process. The air within the newly formed cloud layer is either stable or unstable. This stability or instability will determine the type of cloud structure. This is because stable air resists convection while unstable air prefers convection.
Cloud formation in stable air develops horizontally in uniform, sheet-like layers called "strata". When an unstable layer of air is forced upward by convection, a cloud forms vertically. How high into the atmosphere the cloud shapes itself depends upon the depth of the unstable layer. These clouds tend to pile up in a heap or "cumulus." They are characterized by their lumpy, billowy shapes. Vertical heights of cumulus clouds vary from the shallow fair weather cumulus to the giant thunderstorm cumulonimbus clouds. The convection process occurring in unstable air will give a bumpy ride to an airplane passing through it. In stable air flying is usually smooth.
All in all, clouds give pilots and others who monitor the weather an indication of air motion, stability and moisture. For basic identification purposes, clouds are divided into four families:
- high clouds,
- middle clouds,
- low clouds and
- vertically advanced clouds.
Cloud Name Prefixes
- Cirro is the prefix given to high clouds, those with bases above 20,000 feet, composed of ice crystals. High-level clouds are typically thin and white in appearance, but can appear in a magnificent array of colors when the sun is low on the horizon.
- Alto is the prefix given to mid-level clouds, those between 6,000 and 20,000 feet. They are composed primarily of water droplets, however, they can also be composed of ice crystals when temperatures are low enough.
- Strato is the prefix given for low clouds, those with bases below 6,000 feet. These low clouds are of mostly composed of water droplets.
- Nimbo added to the beginning, or nimbus added to the end of a cloud name means the cloud is producing precipitation.
