The changing physical conditions in the earth’s atmosphere.
Weather is necessary to life on earth. The significant force that creates and drives weather is energy from the sun. The bulk of the world’s water is contained in its oceans and seas. This water contains salt and is not potable for humans and other animals. The sun’s heat energy warms this salty water, causing water vapor (in the form of a gas) to rise above the surface. When the water changes from a liquid to a gas, the salt is left behind. Wind blows this freshwater vapor over land, where it can condense and fall as rain.
Weather conditions are the processes and forces involved in causing this cycle of life-giving water to transfer from the sea to the sky and then to the ground. A key force in this process is the wind. Air movements and cloud formations are complex in nature, primarily due to uneven heating of land and water in different regions. Other factors are the rotation of the earth, the tilt of its axis, and its changing distance from the sun.
Both land and water start heating when the sun rises. The sun heats ocean surface water more slowly than it heats the land. This is partly due to the circulation of ocean water, whereby heat is transferred down to deeper layers, as opposed to the stability of solid land. Therefore, the ocean stores more heat energy than the land, and it gives up its heat energy more slowly than land. All land that is dark, including farmland, forests, and paved cities, readily absorbs heat energy from the sun. White regions of ice and snow mostly reflect the sun. When the sun goes down, land surfaces cool more quickly than water surfaces.
Because of gravity, the atmosphere is denser nearest the ground. Air molecules are more compacted at the surface of the globe and become less dense at higher altitudes. The density of the air at ground level, however, varies due to the uneven heating effects around the earth. This is very important, because air will flow from a high density (high-pressure) region to a low density (low-pressure) region. This pressure difference between any two areas causes an attempt to balance out the different air densities. The movement of this equalizing air is the wind, and the more the pressure differences, the stronger the wind.
The important mechanism that allows clouds to form involves the dew point. This is the temperature at which water vapor in the air will condense. The heat of the land that has received energy from the sun will warm moist air at ground level. When moist air rises, it expands due to the lesser pressure at higher altitudes. This expansion causes it to cool. Often it cools to the dew point and condenses. When a lot of water vapor condenses, clouds form. A huge number of tiny water particles float in the air, held up by air pressure and air movement. These miniature bits of water reflect light and give the cloud its appearance. These water particles can combine, forming larger water droplets. Eventually, they can become heavy enough to fall as rain (if the temperature is above freezing) or as snow or hail (if below freezing). Some clouds will form precipitation, and some will eventually dissipate as the water particles evaporate.
The two basic movements of wind are vertical and horizontal. Ground air that has warmed and expanded moves upward, traveling to areas of less pressure. This air rises vertically because the upward pushing force of denser air is greater than the downward pulling force of gravity. When this warm air also contains a large quantity of water vapor, it can condense when conditions are right. When condensing, it releases heat, which additionally warms the surrounding air, causing the upward rising process to continue.
An opposite effect occurs when liquid water particles in a cloud start evaporating. When the particles change from a liquid to a gas they absorb heat energy. This cools the surrounding air, causing it to become denser. This heavier air cannot be supported by the overall lighter pressure of thin air high up, and so it moves downward, pulled by gravity. These upward and downward vertical air movements can occur at the same time in different parts of a storm system.
Two basic types of horizontal air movement are low winds affected by friction forces and higher winds moving without friction. Near the ground, air can be swirling and turbulent as it moves around obstacles such as buildings, trees, hills, and mountains. When moving horizontally, the forward portion of a warm or cold moving air mass is called a front.
Considering weather on a global scale, there is no place that repeatedly produces more violent weather than the United States. The continental United States is situated between a subpolar region that brings cold air into the country from the north (heading southward), and a tropical region that sends hot, humid air from the Gulf of Mexico northward. When these two air masses occur at the same time and collide, violent thunderstorms and hurricanes can result. The northern climate of Alaska and the southern climate of Hawaii add to the range of weather possibilities for the United States.
Atmospheric conditions affect anything that flies, from birds to spacecraft. Prime weather conditions are clouds, precipitation, wind, lightning, heat, cold, and visibility. These are basic characteristics of weather that can combine to form events such as dust storms, thunderstorms, hurricanes, tornadoes, and blizzards.
There are many cloud types and combinations. The range of possible air temperatures, pressures, air motion, and amount of moisture can combine in complex ways. The main cloud types of interest to pilots are cumulus and stratus. Cumulus clouds, created in unstable air, are typically fluffy and often rise to great heights. While they may originally develop in blue sky and look harmless, they can transform into dark thunderstorms. They tend to produce heavy precipitation.
Stratus clouds are created in stable air and produce smooth clouds that are layered and usually flat. They tend to produce steady, long-duration light rain over a widespread area. A pilot flying through stratus clouds will experience low ceilings, meaning the area near the ground can remain cloudy, with no visibility for navigating or landing.
The basic forms of precipitation are rain, snow, hail, and sleet. While all these conditions can cause problems of visibility for the pilot, the greatest hazard is freezing rain. Sleet (partly frozen rain) can form into ice when it falls on the metal surface of an aircraft that is at or below freezing temperature. This is called icing, and it is most hazardous to flying when it forms on wings and propeller blades and interferes with fuel systems and sensing devices. The icing condition can also occur when there is no precipitation—a light plane can experience carburetor icing when flying in cold, moist weather, for example. This is a serious condition, as it can cause engine malfunction.
Ice can build up very rapidly. When it covers a wing, the lifting ability decreases, drag friction increases, and the weight of the plane increases. Ice on wings or propeller blades will cause a decrease in power. There is an increase in stall speed, the minimum speed at which a plane will automatically drop one wing and proceed into an unwanted nose-down dive. In general, a deterioration of aircraft performance results when icing occurs. The solution to icing is to immediately increase thrust, activate anti-ice and deicing equipment, and leave the area producing the icing.
Conditions vary from no wind to extremely high wind. A headwind is the situation of flying directly into the wind, and a tailwind occurs when the flight is in the same direction as the wind. A crosswind is wind coming from an angle, between a headwind and a tailwind. Flying into a large low-pressure area allows easier altitude gain due to the rising air. When flying near a large high-pressure area, it is more difficult to gain altitude due to the downward-flowing air mass.
Turbulence is a general term for air movement that is characterized by irregular or violent motion, unlike a constant-speed wind staying in one direction. Turbulence can be exhibited between ground level and 70,000 feet (the altitude of a U-2 spy plane). Turbulence is strongly associated with thunderstorms, where its most dangerous form is a condition known as wind shear. This condition occurs when wind abruptly changes direction or speed or both over a very short distance. Wind shear can occur along the boundaries of thunderstorm activity, and in the downdraft under the storm cell. The most dangerous situation for aircraft is when wind shear occurs relatively near the ground (at an altitude of 200 feet, for example) while the plane is making a landing approach or a takeoff. The direction change can be 180 degrees; the speed change can be 50 miles per hour. That is extremely hazardous for an airplane at low altitude because it can place the craft in a position where there is no time for recovery from decreased lift and misaligned attitude before impact with the ground.
Another form of turbulence is wind that changes direction near ground level. This is due to the friction between moving air and ground objects. A pilot looking at a weather chart of winds aloft may observe the prevailing wind direction. This may be a headwind to a landing pilot, for example. However, at ground level, due to friction with ground objects such as buildings, fences, and control towers, the head wind can change into a crosswind. These changes affect the attitude and speed of a landing aircraft. Dust devils are another factor in landing or departing. These are small, whirling, circular air currents, normally seen at or near ground level by the dust they kick up.
A cruising aircraft can also encounter turbulence due to larger-scale objects. Any change in terrain elevation produces wind turbulence. This is due to air being pushed up over mountains and any type of rising land from gradual inclines to abrupt cliffs. On the downwind side of mountains, air mostly descends and can be choppy and rough.
Wind shear aloft can be caused by the same system that produces the near-ground-related wind shear problems. While dangerous and potentially lethal, it is not considered to be as hazardous as at ground level due to more time for corrective action before encountering the ground.
Jet streams are fast moving, high-altitude, narrow bands of air moving globally between cold arctic regions and hot tropic regions. These jet streams often travel in a serpentine manner, generally traveling from west to east, although direction varies. They range in location from about 60 degrees latitude in the summer, down to about 20 degrees latitude in winter, with travel excursions between these locations. Their speed varies, sometimes as high as 200 miles per hour, for example, at altitudes above 30,000 feet. Strong winds exist for hundreds of miles next to the jet streams. These winds interact with slower-speed wind to cause turbulence. Thunderstorms, tornadoes, hurricanes, and warm and cold air fronts all produce turbulence at varying altitudes.
Lightning is caused by a difference in electrical charge between the ground and the sky, and even between clouds. It is most often generated when there is heavy cloud activity, as in thunderstorms. It can burn small holes in aircraft outer parts and affect sensitive instruments. However, it does not seem to strike people within a plane.
The temperature of air affects how dense it is—warmer air is less dense and cooler air is more dense. The denser the air, the better an aircraft will perform. Engines are more efficient, wings have more lift, and control surfaces provide more control. Much more runway is needed when taking off on a very hot day than is required on a very cold day.
Cold temperatures are primarily a problem for the pilot flying in freezing temperatures in areas of high moisture content. Even with the best anti-icing and deicing equipment, it is not always possible to prevent the effects of ice on an aircraft. Cold temperatures in the presence of moisture can also cause icy runways.
Besides flying in clouds, precipitation, and darkness, other atmospheric conditions can cause visibility problems. Smog is a condition created when sunlight reacts with a combination of smoke and various gaseous pollutants and then combines with fog. Smog, smoke, dust, haze, fog, and sandstorms all describe conditions that are usually low-lying and can interfere with a pilot’s visibility, especially when landing.
Thunderstorms can generate several adverse conditions for pilots of all types of craft. A thunderstorm is a cloud system containing very unstable air moving in all directions. Winds can batter and rock an aircraft, causing airsickness and aircraft attitude problems. Violent wind can also stress components of the craft, such as the wings and control surfaces. Wind can alter the course of an airplane, causing more fuel use. Strong turbulence in and around a thunderstorm may include ground-level wind shear that can make takeoffs and landings difficult or impossible. Hail can damage parts of the plane, and freezing rain can cause dangerous icing. Lightning discharges are usually frequent.
The extent of a thunderstorm depends on its type. Ordinary individual storms usually form rapidly, reach a peak of activity, and expire in about an hour. A line of thunderstorms is called a squall line. A supercell thunderstorm is one that can last for hours and travel more than three hundred miles. The supercell type can develop strong tornadoes as it travels. A tornado is a large, violent, swirling wind funnel that can easily destroy an airplane. The tops of thunderstorms range from about 33,000 feet to 80,000 feet. This means that small aircraft are not able to fly over the storm. Larger aircraft may not want to fly over the system, since rough air can extend far into the clear air above the main storm.
A weather map or chart is a drawing showing continuous lines indicating highs, lows, and fronts. The lines are called isobars, which are plot locations of similar air pressure.
A weather satellite photograph is an image printed from a picture taken by a television type of camera. The image, showing the land and tops of cloud formations, is transmitted from the satellite to the ground.
Radar in an aircraft sends out a beam that bounces off heavy moisture and rain particles, thus showing a pilot the location of the nearest intense storm cell activities.
Spherics in an aircraft is a system that detects and displays the electrical discharges of a thunderstorm. Lightning locations are shown as tiny dots of light on a screen.
Autopilot systems automatically control the aircraft’s attitude, direction, and speed. This relieves the pilot of hands-on flying and allows more time for obtaining weather reports and communicating with air traffic controllers.
Ground-based wind shear detecting devices have been developed that aid in monitoring and predicting this activity in the vicinity of airports. In the 1990’s, jet transports in the United States were required to be outfitted with wind shear detection devices. Accurate and timely predictions, while improving, are not yet an exact science in the year 2001. The storm system producing the wind shear can be more accurately monitored.
An instrument landing system (ILS) aids in organizing a landing through use of a ground radio beacon. The pilot uses cockpit instruments and a radio receiver to guide the plane to the runway.
Buck, Robert N. Weather Flying: A Practical Book on Flying in All Kinds of Weather. 4th ed. Hightstown, N.J.: McGraw-Hill, 1998. A nontechnical, fairly detailed explanation of how weather develops, what to avoid, and how to fly in and out of bad weather. Facts and insights from an experienced commercial pilot directed toward the private pilot. Gero, David. Aviation Disasters: The World’s Major Civil Airliner Crashes Since 1950. 3d ed. Sparkford, England: Patrick Stephens, 2000. Written thoughtfully and clearly in a narrative style. The nearly three hundred descriptive entries artfully bring the actual and probable circumstances to life. Many weather-related crashes. Padfield, R. Randall. Flying in Adverse Conditions. Blue Ridge Summit, Pa.: Tab Books, 1994. One of the Practical Flying Series of books, this softbound instructional guide is written in a relaxed, informal style. For the light-aircraft private or student pilot, its serious topic is spiced with personal comments and anecdotes. Weather World 2010 Project, The. (www.2010.atmos.uiuc.edu/(Gh)/home.rxml) Developed by the Department of Atmospheric Sciences at the University of Illinois, this World Wide Web site is an excellent resource of weather definitions and explanations by topic and subtopic, with color drawings, pictorials, and photographs. Williams, Jack. USA Today Weather Book: An Easy-to-Understand Guide to the USA’s Weather. 2d ed. New York: Vintage Books, 1977. Abundantly packed with eye-grabbing color illustrations, charts, diagrams, and photographs. This oversized, softbound, 227-page edition is fun and informative reading. Explains technical weather topics in layperson’s terms. Excellent for the beginning student of weather.
Pilots and copilots
Training and education
Unstable weather conditions can cause flight hazards such as icing, wind shear, and limited visibility.