There is, in fact, no true upper boundary to the atmosphere but there is a transitional region where it merges into interplanetary space. This region is greatly affected by changes that occur during the solar cycle.
Atmosphereic pressure is an important factor in governing the behaviour of the weather and also in its prediction. This pressure is the weight of the column of air above any particular point and is measured by one of the various types of barometer. Pressure was originally described in terms of the height of the column of mercury it would support, but it is now defined in scientific units as the force exerted on a given area. Atmospheric pressure is normally measured in millibars, or thousands of a bar. One bar is equivalent to the pressure exerted by 750.06mm of mercury at 0*C. Although pressure is by no means constant throughout the atmosphere, the average at sea level is about 1013mb.
In the atmosphere as a whole, pressure declines upward. at first rapidly and later much more gradually. When you fly in an aircraft at a height of about 9km the outside pressure is around 350mb.This is a measure of the amount of air above you - roughly 65 per cent of the atmosphere lies below.
Temperature also varies throughout the atmosphere and the way in which it changes with height is used to define the various layers. The lowest layer, the troposhere, contains most of the mass of the atmosphere and here, too, the changes that form the weather patterns of the world largely take place.
The tropopause, the boundary between the trosposhere and the stratosphere, is usually fairly distinct, although its height may vary considerably. It is normally found at altitudes of about 18 to 30 km over equatorial regions 8 km over the poles. Variations occur with the seasons - especially at higher latitudes - but the decline in height from Equator to poles is never uniform. There are always major 'steps' between the levels over the tropics and those over the middle latitudes, as well as lesser ones nearer the poles and many other irregularities. The locations of these breaks in the tropopause have a great effect upon the development of weather systems.
The Earth is unique among the planets in that its atmosphere largely consists of the heavy gases. nitrogen (78 per cent) and oxygen (21 per cent), with mere traces of other gases. The relative proportions are fairly constant up to about 80 km, but the heavy gases become rare at greater heights and only hydrogen and helium are found in the outermost regions. In the stratosphere there is a high concentration from of ozone at 25km. This acts as a shield against damaging ultraviolet radiation from the Sun, only a small fraction of which actually reaches the Earth's surface.
Another important constituent of the atmosphere, water
vapour, varies in concentration from place to place and from time to time.
Because the tropopause is a strong inversion (a condition in which a layer
of warm air holds cooler air near the Earth's surface); water vapour is
largely trapped below it. For this reason, nearly all the clouds you see
are in the troposphere. Some cloud formations may stretch up to the tropopause,
but in the stratosphere the air is dry and clouds are rare.
Setting the aneriod barometer-
If you want to compare your barometer readings with those at other places, check the instrument's setting from time to time and adjust it if neccessary. To do this, choose a day when the weather is fairly quiet (when the pressure is not changing rapidly), contact the nearest meteorological station and ask for the current pressure reading. Set the pointer on your barometer to this pressure, using the adjusting screw on the back of it. If the station is far away there may be a small pressure difference, but this will be insignificant. The pressure you are given will also be corrected to mean sea-level, so you will not have to worry about the drop in pressure with the increasing altitude.