by the National Weather Service

“ 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 from the 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". ”

“ 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 depends on characteristics of the substance. Each substance has its unique temperature change for the specific change in heat. ”

“ It behooves every pilot to learn to appreciate good weather, to recognize and respect marginal or hazardous weather, and to avoid violent weather when the atmosphere is on its most cantankerous behavior. ”

“ The decrease in air density with increasing height has a physiological effect which we cannot ignore. The rate at which the lungs absorb oxygen depends on the partial pressure exerted by oxygen in the air. The atmosphere is about one-fifth oxygen, so the oxygen pressure is about one-fifth the total pressure at any given altitude. Normally, our lungs are accustomed to an oxygen pressure of about 3 pounds per square inch. But, since air pressure decreases as altitude increases, the oxygen pressure also decreases. ”

“ Pressure Variation Pressure varies with altitude and temperature of the air as well as with other minor influences which we neglect here.
Altitude
As we move upward through the atmosphere, weight of the air above becomes less and less. If we carry a barometer with us, we can measure a decrease in pressure as weight of the air above decreases. Within the lower few thousand feet of the troposphere, pressure decreases roughly one inch for each 1,000 feet increase in altitude. The higher we go, the slower is the rate of decrease with height. ”

“ An increase in temperature with altitude is defined as an inversion, i.e., lapse rate is inverted.
An inversion often develops near the ground on clear, cool nights when wind is light. The ground radiates and cools much faster than the overlying air. Air in contact with the ground becomes cold while the temperature a few hundred feet above changes very little. Thus, temperature increases with height. Inversions may also occur at any altitude when conditions are favorable. For example, a current of warm air aloft overrunning cold air near the surface produces an inversion aloft. ”

“ Introduction Weather is perpetual in the state of the atmosphere. All flying takes place in the atmosphere, so flying and weather are inseparable. Therefore, we cannot treat aviation weather purely as an academic subject. ”

“ Variations with Topography Not related to movement or shape of the earth are temperature variations induced by water and terrain. As stated earlier, water absorbs and radiates energy with less temperature change than does land. Large, deep water bodies tend to minimize temperature changes, while continents favor large changes. Wet soil such as in swamps and marshes is almost as effective as water in suppressing temperature changes. Thick vegetation tends to control temperature changes since it contains some water and also insulates against heat transfer between the ground and the atmosphere. ”

“ When temperature is colder than standard, you are at an altitude lower than your altimeter indicates. When temperature is warmer than standard, you are higher than your altimeter indicates. When flying cross country, keep your altimeter setting current. This procedure assures more positive altitude separation from other aircraft. When flying over high terrain in cold weather, compute your true altitude to ensure terrain clearance. When your aircraft is heavily loaded, the temperature is abnormally warm, and/or the pressure is abnormally low, compute density altitude. ”

“ Altitude If it were possible for a pilot always to determine mean temperature of the column of air between the aircraft and the surface, flight computers would be designed to use this mean temperature in computing true altitude. However, the only guide a pilot has to temperature below him is free air temperature at his altitude. Therefore, the flight computer uses outside air temperature to correct indicated altitude to approximate true altitude. ”

“ At the top of the troposphere is the tropopause, a very thin layer marking the boundary between the troposphere and the layer above. The height of the tropopause and certain weather phenomena are related. Chapter 13 discusses in detail the significance of the tropopause to flight. Above the tropopause is the stratosphere. ”

“ It is evident that the average heat gained from incoming solar radiation must equal heat lost through terrestrial radiation in order to keep 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.
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. ”

“ Low pressure systems quite often are regions of poor flying weather, and high pressure areas predominantly are regions of favorable flying weather. A word of caution, however—use care in applying the low pressure-bad weather, high pressure-good weather rule of thumb; it all too frequently fails. ”

“ Since the Earth is essentially spherical, the sun is more nearly overhead in equatorial regions than at higher latitudes. Equatorial regions, therefore, receive the most radiant energy and are warmest. Slanting rays of the sun at higher latitudes deliver less energy over a given area with the least being received at the poles. Thus, temperature varies with latitude from the warm Equator to the cold poles. ”

“ A companion manual, Aviation Weather Services, Advisory Circular 00-45, supplements Aviation Weather. This supplement (AC 00-45) periodically is updated to reflect changes brought about by latest techniques, capabilities, and service demands. It explains current weather services and the formats and uses of weather charts and printed weather messages. The two manuals are sold separately; so at a nominal cost, a pilot can purchase a copy of the supplement (AC 00-45) periodically and keep current in aviation weather services. ”

“ Beatrice Emery deserves special recognition for her relentless effort in typing, retyping, proofing, correcting, and assembling page after page of manuscript. Many other offices and individuals have contributed to the preparation, editing, and publication of the two volumes. ”

“ The authors have devoted much of the book to marginal, hazardous, and violent weather which becomes a vital concern. Do not let this disproportionate time devoted to hazardous weather discourage you from flying. By and large, weather is generally good and places little restriction on flying. ”

“ 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 usually occurs after sunrise, sometimes as much as one hour after. The continued cooling after sunrise is one reason that fog sometimes forms shortly after the sun is above the horizon. ”

“ The millibar is rapidly becoming a universal pressure unit.
Station Pressure Obviously, we can measure pressure only at the point of measurement. The pressure measured at a station or airport is "station pressure" or the actual pressure at field elevation. We know that pressure at high altitude is less than at sea level or low altitude. For instance, station pressure at Denver is less than at New Orleans. Let's look more closely at some factors influencing pressure. ”

“ Temperatures also vary with differences in topographical surface and with altitude. These temperature variations create forces that drive the atmosphere in its endless motions.
Diurnal Variation 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 by terrestrial 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. ”