Aviation

Anyone who flies an airplane, a glider, or a balloon should take the weather into account before leaving the ground. Commercial and military pilots have ready access to the necessary information. Airplanes are designed to withstand the normal stresses imposed by violet weather events of the kind discussed in this chapter. Unfortunately there still are those rare circumstances where inclement weather, combined with pilot errors, leads to fatal accidents. The more a flier knows, through study and experience, the smaller the likelihood of such an episode. The major meteorological factors that affect an airplane in flight are wind, turbulence, lightning, hail, and aircraft icing (that is, the accumulation of ice on the leading edges of an airplane flying through a super cooled cloud). But weather considerations must begin at the runway. The length of runway needed for takeoff depends on the type and weight of the airplane, the elevation of the airport, the wind velocity, the air temperature, and the humidity. The lift imparted to an airplane's wings at any speed depends on the as density that is, the mass of air in unit volume of space which decreases with height. Departing airplanes must move faster in Denver (elevation about 5,300 feet) than in Chicago (elevation about 670 feet) to take off. To reach the higher speed, airplanes in Denver need longer runways. In addition, as the air temperature and (to a lesser extent) the humidity rise, the air density decreases; so longer takeoff runs are needed in summer than in winter. The altitude of an aircraft is usually measured by a pressure altimeter, which is calibrated according to the average pressure height relationship of the atmosphere.  Although labeled in units of altitude, the instrument actually responds to air pressure, which is a (unction of height. Since vertical distributions of temperature and density vary with time and place, so do the pressure at the ground and the rate at which pressure decreases with height. To take the deviations from average into account, a pilot adjusts the altimeter before takeoff and landing. Knowing the atmospheric pressure at the airport, the pilot sets the altimeter so that it reads the field elevation  when the airplane is on the ground. Up-to-date "altimeter settings" are normally supplied, via radio, from airport control towers. When air temperatures at flight altitudes are lower than the averages used to calibrate the altimeter, the actual altitude of the airplane is less than the one given by the altimeter; air temperatures above the averages have the opposite effect. Knowing the temperature, a pilot can easily compute a corrected altitude. Two important concerns in air travel are ceiling, which is defined as the height of the base of the lowest layer of clouds or other obscuring phenomenon that hides more than half the sky, and visibility, the greatest distance at which one can see and identify prominent objects. Low ceilings and visibility most often result from fogs or stratus clouds, but heavy snow or rain, blowing sand, or dense smoke and haze can also be causes. These conditions arc most significant during landings and takeoffs. In very dense fogs, ceilings and visibility can be nearly zero, bringing traffic to a halt. Fortunately, these "zero zero" conditions are not common. More often, ceilings are reduced to a few hundred feet and visibility to less than a mile, in which cases air traffic continues, though at a slower rate, in and out of airports having suitable radio or radar systems that can guide an airplane to a safe touchdown. Some types of fogs and low cloud layers form or become more dense on clear nights as a result of cooling by thermal radiation to the night sky. For this reason, ceilings and visibility may drop during the night and early morning. Pilots of small airplanes, who are often required to fly with the ground in sight, should schedule flights out of fog-prone airports for periods after the sun has burned off the fog and before fog begins to form in the evening or night.