The 850 mb chart

The 850 mb chart, representing weather conditions in the lower troposphere, is at a level that is above approximately 15 percent of the atmosphere in terms of mass. At an altitude of approximately 1500 meters (5000 feet), this level is above most of the influences of surface friction in the many sections of the country. Unfortunately, the 850 mb intersects and goes below the terrain in the Rocky Mountains. For example, the "Mile High City" of Denver, CO usually has a surface pressure - a measured value not corrected to sea level - of approximately 830 mb, which places it at a higher altitude than the 850 mb surface. Meteorologists often look at the analyzed temperature field of this level, because over the non-mountainous regions, the diurnal temperature cycle is much less than at the surface. They can frequently tell correctly that precipitation falling in regions with an 850 mb temperature of 0 degrees Celsius will probably fall as snow, while rain would more than likely fall at warmer temperatures.

The 500 mb chart

The 500 mb chart represents weather conditions in the mid- troposphere, at a level where approximately half the mass of the atmosphere lies below this level. This level is at an altitude of approximately 5,500 meters (18.000 ft). This level is often used to represent upper level flow conditions because the level is well above the effects of topography and friction and the level is below the region in the upper troposphere where the air flow may experience strong accelerations and decelerations when in the vicinity of the upper jet streams. Since many weather systems tend to follow the wind flow at this level, this level is often considered to symbolize the steering level of these systems.

The 300 mb chart

The 300 mb chart is in the vicinity of the tropopause, at the top of the troposphere. Only 30 percent of the mass of the atmosphere lies above this level. The altitude of the 300 mb surface is near 9000 meters (30,000 ft) - at a level where many long-distance commercial jet aircraft fly. This level also corresponds the level of the upper tropospheric jet stream, a region of very fast winds that move across the country. Inspection of the isotach patterns at these levels not only reveal the location of the jet streams, but aid the meteorologist in locating the regions of largest acceleration, deceleration and wind shear (rapid changes in wind speed and/or direction); these regions contribute to the upper level horizontal divergence and convergence patterns that influence surface weather systems.