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.
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
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.
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
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
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.
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
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.