The atmosphere is the ocean of air that surrounds the earth. Air is a fluid, which flows, and the atmosphere flows with currents of air.
Air in our atmosphere is a mixture of gases, mostly nitrogen (78%), an invisible, odorless gas. Oxygen is also common, comprising 21% of the air. The remaining 1% is carbon dioxide and other gases.
The atmosphere has several layers...
Troposphere: the portion of the atmosphere closest to the earth. It contains almost all of earth's weather. Near the poles, the troposphere extends upward to around 20,000 feet. Near the equator, it extends higher, to around 48,000 feet.
Tropopause: the upward limit of the troposphere, dividing the troposphere from the stratosphere.
Stratosphere: extends upward from the tropopause to an altitude of around 160,000 feet. Inside the stratosphere, the air is very stable and almost no weather exists.
Stratopause: divides the stratosphere from the mesophere.
Mesosphere: extends upward from the stratopause to around 280,000 feet. Inside the mesosphere, temperature decreases, reaching temperatures as low at -90 degrees Celsius.
Mesopause: separates the mesosphere from the thermosphere.
Thermosphere: the atmosphere above the mesopause. It has no defined upper limit, as the atmosphere gradually fades away into space.
Atmospheric Pressure
The weight of the air above exerts pressure on the air below. This is the reason the air pressure becomes lower and lower with increasing altitude. Atmospheric pressure refers to the pressure exerted on the ground or sea by the atmosphere above.
A barometer is the device used to measure atmospheric pressure. It gives a pressure reading in inches of mercury, which refers to the height liquid mercury is raised by atmospheric pressure inside the barometer. The higher the pressure, the higher the mercury will rise inside the tube of the barometer.
Atmospheric pressure varies with changing weather. It is read at stations all over the world, as these pressure readings are widely used in weather forecasting and aviation.
Every physical process of weather is accompanied by, or is a result of, unequal heating of the Earth's surface. As a result, unequal heating of the Earth's surface is also responsible for these variations is atmospheric pressure between reporting stations.
International Standard Atmosphere (ISA)
The international standard atmosphere provides a common, widely used reference. The standard atmosphere has a temperature of 15 degrees Celsius, which is 59 degrees Fahrenheit, and a pressure of 29.92 inches of mercury. The standard rate of temperature drop with increasing altitude, known as the standard temperature lapse rate, is 2 degrees Celsius per thousand feet of altitude gain. The standard rate of pressure drop with increasing altitude, known as the standard pressure lapse rate, is 1 inch of mercury per thousand feet.
The temperature and pressure lapse rates may be used to calculate the standard temperature and pressure at altitude. For example, at 5,000 feet, the standard temperature is 5 degrees Celsius, while the standard pressure is 24.92 inches. These standards are often used as a reference and for rough estimations.
The Standard Atmosphere Temperature: 15C / 59F Pressure: 29.92 inches
Temperature Lapse Rate: 2C per 1,000 feet
Pressure Lapse Rate: 1 inch per 1,000 feet
Standard Temperatures and Pressures Aloft
Sea Level: 15C and 29.92 inches
1,000: 13C and 28.92 inches
2,000: 11C and 27.92 inches
3,000: 9C and 26.92 inches
4,000: 7C and 25.92 inches
5,000: 5C and 24.92 inches
Etc....
Example: If the temperature at 5,000 were actually 8C, it could be referred to as ISA+3.
Sea Level Pressure (SLP)
Since pressure drops with altitude, any reporting station having an elevation higher that sea level will show a barometer reading that is affected by the station's elevation. If the actual pressure readings were reported, they would not be useful. Instead, pressure readings are converted to and reported in sea level pressure, which is what the pressure reading would be if the reporting station were at a sea level elevation. For example, the pressure at a station with an elevation of 5,000 feet might be 24.95 inches. By adding 1 inch per thousand feet of altitude drop, the station reports a SLP of 29.95". This makes the station easily comparable to other stations, allowing areas of higher and lower pressure to be determined.