FOR PILOTS AND FLIGHT OPERATIONS PERSONNEL
DOC. AC 00-6 FEDERAL AVIATION ADMINISTRATION (Washington, D.C.)
Chapter 2 – TEMPERATURE
Why do we stress temperature in aviation weather? Look at your flight computer; temperature enters into the computation of most parameters on the computer. In fact, temperature can be critical to some flight operations. As a foundationfor the study of temperature effects on aviation and weather, this chapter describes commonly used temperature scales, relates heat and temperature, and surveys temperature variations both at the surface and aloft.
Two commonly used temperature scales are Celsius (Centigrade) and Fahrenheit. The Celsius scale is used exclusively for upper level air temperatures and sometimes for ground leveltemperatures.
Traditionally, two common temperature references are the melting point of pure ice and the boiling point of pure water at sea level. The melting point of ice is 0ºC or 32ºF; the boiling point of water is 100ºC or 212ºF. Thus, the difference between the melting and boiling is 100º Celsius or 180º Fahrenheit; the ratio between degrees Celsius and Fahrenheit is 100/180 or 5/9. Since0ºF is 32 Fahrenheit degrees colder than 0ºC, you must apply this difference when comparing temperatures on the two scales. You can convert from one scale to the other using one of the following formulae: C = 5/9 (F-32) ; F = 9/5C +32 Many flight computers provide for direct conversion of temperature from one scale to the other. Temperature is measured with a thermometer. What makes athermometer work? Simply, the addition or removal of heat.
HEAT AND TEMPERATURE
Heat is a form of energy. When a substance contains heat, it exhibits the property we measure as temperature – the degree of “hotness” or “coldness” A specific amount of heat absorbed by or removed from a substance raises or lowers its temperature a definite amount. However, the amount of temperature change dependson characteristics of the substance. Each substance has its unique temperature change for the specific change in heat. For example, if a land surface and a water surface have the same temperature and an equal amount of heat is added, the land surface becomes hotter than the water surface. Conversely, with equal heat loss, the land becomes colder than the water.
The Earth receives energy fromthe sun in the form of solar radiation. The Earth and its atmosphere reflect about 55 percent of the radiation and absorb the remaining 45 percent converting it to heat. The Earth in turn, radiates energy, and this outgoing radiation is “terrestrial radiation.” It is evident that the average heat gained from incoming solar radiation must equal heat lost through terrestrial radiation in order tokeep the earth from getting progressively hotter or colder. However, this balance is world-wide; we must consider regional and local imbalances which create temperature variations.
The amount of solar energy received by any region varies with time of day, with seasons and with latitude. These differences in solar energy create temperature variations. Temperaturesalso vary with differences in topographical surface and with altitude. These temperature variations create forces that drive the atmosphere in its endless motions.
Diurnal variation is the change in temperature from day to night brought about by the daily rotation of the Earth. The Earth receives heat during the day by solar radiation but continually loses heat byterrestrial radiation. Warming and cooling depend on an imbalance of solar and terrestrial radiation. During the day, solar radiation exceeds terrestrial radiation and the surface becomes warmer. At night, solar radiation ceases, but terrestrial radiation continues and cools the surface. Cooling continues after sunrise until solar radiation again exceeds terrestrial radiation. Minimum temperature...