Tutorial
2(b). Atmosphere
The differences in the amount of solar energy received by the various regions of the Earth throughout a day or
year cause temperature variations that power our dynamic atmosphere. There are 5 temperature variations that need
to be considered by pilots whenever planning a flight:
Diurnal variation is the change in air temperature that occurs from day to night brought about by the
Earth's rotation. A unique exchange takes place on the Earth's surface in regards to solar and terrestrial (from
or on Earth) radiation. The sun gives off energy to the Earth in the form of solar radiation. Fifty-five percent
of the solar radiation received by the earth and its atmosphere is reflected while the remaining 45% is absorbed
and converted into heat. The Earth itself gives off radiation that is referred to as terrestrial radiation. The
exchange is worldwide and maintains a delicate balance in the Earth's atmospheric temperature. That is, the average
amount of heat gained through solar radiation is roughly equal to the amount of heat lost through terrestrial radiation.
This unique exchange keeps the Earth's atmosphere from becoming progressively hotter or colder. During the day,
terrestrial radiation is exceeded by the solar radiation and the Earth's surface becomes warmer. At night, the
part of the Earth facing away from the sun receives no solar radiation. 
Despite the continuance
of terrestrial radiation, the Earth's surface cools. The cooling of the Earth's surface continues until about 1
hour after sunrise. Shortly after sunrise, the solar radiation of the next day exceeds that of the terrestrial
radiation, and the temperature increases. The continued cooling after sunrise can cause the formation of fog shortly
after the sun is above the horizon. Pilots need to keep this in mind when planning flights with an early morning
departure. Such morning ground fog can delay the departure time due to limited visibility.
Seasonal variation occurs due to the Earth's approximate 23.50 degree tilt
and its position relative to the sun during its revolution. The hemisphere that
receives more direct rays of sunlight will have warmer temperatures than the
opposite hemisphere at that same time of year. Knowing the average daily temperature
of a region from which or to which you are flying will assist in your flight
planning calculations.
Along
with the seasonal variations are the latitudinal variations. The shape of the Earth directly affects the amount
of solar radiation received during certain segments of its revolution around the sun. Equatorial regions receive
more direct rays of the sun, thus more solar radiation. Moving farther south or north of the equator will change
the angle at which the rays strike the Earth, thus decreasing the amount of solar radiation received at that latitude.
Remembering that water absorbs and radiates energy with temperature changes that are less than land, a pilot must
look closely at the terrain over which the flight will progress, especially if flying at lower altitudes. The topographical
variations are influenced also by diurnal and seasonal variations and can cause a change in wind direction or wind
strength. In general, the partial list below lists some of the general topographical variations:
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