The atmosphere is the envelope of air that surrounds the earth. Over one-half of the air, by weight, is within the lower 18,000 feet of the atmosphere; the remainder is spread out over a vertical distance in excess of 1,000 miles. No definite outer atmospheric boundary exists: the air particles become fewer with increasing altitude until they gradually overcome the earth’s gravitational force and escape into space.
The atmosphere rotates with the earth as a gaseous outer cover. Within the atmosphere, other air movement occurs. Differences in the temperature of the earth’s surface affect the density of the atmosphere and cause a continuous internal air movement called circulation. The force of this circulation varies between breezes and winds. The latter can have forces that range from mild to gales, hurricanes, and typhoons. As shown in figure 1.1. a given volume of dry air contains approximately 78 percent nitrogen; 21percent oxygen; and 1 percent argon, carbon dioxide, and minute amounts of other gases. Air contains, in addition to the gases just mentioned a variable amount of water vapor, most of which is concentrated below 30,000 feet. The maximum amount of water vapor the air can hold depends primarily upon the temperature of the air; the higher the temperature, the more water vapor the air can hold. Water vapor in the atmosphere varies from insignificant amounts to 4 percent by volume (100 percent humidity). Air also contains variable amounts of impurities such as dust, salt particles, and products of combustion. These impurities are important because of their effects on visibility and especially because they are necessary to cause the water vapor to form into clouds and fog and to condense into rain and snow. If the air were absolutely pure there would be little condensation.
Fig. 1.1. Composition of the Atmosphere
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 15°C., and normal barometric pressure at sea level is 29.92 inches of mercury. The three figures (14.7 pounds, 15°C., 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° = 59°F. To convert from Fahrenheit to Centigrade, the formula is: C = 59° – 32° x 5/9). 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. 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. 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 2°C or 3 ½°F per thousand feet.
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