Although light and elastic, air does have weight. Because of its weight, the atmosphere has a
pressure of approximately 14.7 pounds per square inch at sea level. Put another way, imagine a square 4
inches on one side and 5 inches on the longer side. This is a space 20 inches square, and at sea level, the
atmospheric pressure on the square is 294 pounds (14.7 x 20). Standard air is 15C., and normal
barometric pressure at sea level is 29.92 inches of mercury. The three figures (14.7 pounds, 15C., an
29.92 inches) make up "standard conditions," a term often used instead of the three figures. Assuming a
constant temperature, the density of a volume of air varies directly with the pressure. If the pressure is
doubled, the density is doubled.
The letter C in 15 stands for Centigrade, a scale in the metric system for measuring temperature.
The temperature of 15 C is equal to 59 Fahrenheit. The conversion formula to change Centigrade to
Fahrenheit is: 9/5 x (15) + 32 = 59F. To convert from Fahrenheit to Centigrade, the formula is: C =
59 - 32 x 5/9).
Figure 1.2 shows the atmosphere divided into layers circling the earth. The layers, starting with the
one next to the earth, are the troposphere: most of our weather occurs in this layer. Next are the
stratosphere, ionosphere, and exosphere. The tropopause is a narrow area between the troposphere and
the stratosphere. Notice in figure 1.2 that the various cloud formations appear to straddle the line
identifying the tropopause location. The jet stream is near the upper portion of the troposphere. The
troposphere varies in height from 60,000 feet above sea level over the Equator to 25,000 feet over the
poles. Its height varies with the seasons; it is higher in summer than in winter. In the temperate zones, it
is about 35,000 feet above sea level.
1.4 TEMPERATURE VARIATION WITH ALTITUDE
The temperature in the troposphere decreases as an aircraft gains altitude, because the air closest to
the earth is warmer and receives the largest amount of the sun's energy. The variation in temperature
with altitude is called the temperature lapse rate and is usually expressed in degrees per thousand feet. If
observations taken day after day at thousands of locations around the world were averaged, the average
temperature lapse rate would be about 2C or 3 1/2F per thousand feet.