The average pressure at the surface is 1013 millibars. There is no "top" of the atmosphere by strict definition. The atmosphere merges into outer space. There are 5 slices of the troposphere that meteorologists monitor most frequently. They are the surface, 850 mb, 700 mb, 500 mb, and 300 mb (or 200 mb). Why are these slices monitored and not others more frequently? Why not have a 600 mb and a 400 mb chart? Each of the primary 5 levels have a reason they are studied over other slices of the troposphere (sort of).

The surface is obviously important because it gives information on the weather that we are feeling and experiencing right where we live.

The 850 mb level represents the top of the planetary boundary layer (for low elevation regions). This is near the boundary between where the troposphere is ageostrophic due to friction and the free atmosphere (where friction is small). For low elevation regions the 850 mb level is the best level to assess pure thermal advection.

The 500 mb level is important because it is very near the level of non-divergence. This allows for an efficient analysis of vorticity. Actually the level of non-divergence averages closer to the 550 mb level, but 500 mb is a more "round" number as compared to 550 mb so it was used. The 500 millibar level also represents the level where about one half of the atmosphere's mass is below it and half is above it.

A level is needed to depict the jet stream. The polar jet stream has a vertical thickness of at least 200 millibars with the core of the jet averaging at about 250 millibars. Either the 200 or 300 mb chart can be used to assess the jet stream / jet streaks. In winter, the 300 mb chart works best and in the summer the 200 mb chart works best for analyzing the core of the jet. The jet stream is at a higher pressure level (closer to the surface) in the winter because colder air is more dense and hugs closer to the earth's surface.

It is important to have an understanding of the average height of each of these important levels. 1000 mb is near the surface (sea level), 850 mb is near 1,500 meters (5,000 ft), 700 mb is near 3,000 meters (10,000 ft), 500 mb is near 5,500 meters (18,000 ft), 300 mb is near 9,300 meters (30,000 ft). All of these values are in geopotential meters; Zero geopotential meters is near sea level. The height of these pressure levels on any given day depends on the average temperature of the air and whether the air is rising or sinking (caused by convergence / divergence). If a cold air mass is present, heights will be lower since cold air is denser than warm air. Denser air takes up a smaller volume, thus heights lower toward the surface. Rising air also decreases heights. This is because rising air cools. Rising air could be the result of upper level divergence. Upper level divergence lowers pressures and heights because some mass is removed in the upper troposphere from that region. This causes the air to rise from the lower troposphere and results in a cooling of the air. If the average temperature of a vertical column of air lowers, the heights will lower (trough).