700 mb is considered by many to be the top of the lower atmosphere. An air pressure of 700 millibars is commonly equivalent to 10,000 feet (3,100 meters) in elevation, but the height typically ranges from near 7,700 to 10,500 feet (2,350 to 3,150 meters).
As with all other constant pressure charts, height values have the last digit, a zero, truncated. So, a height value of 282 represents the 700 MB elevation of 2,820 meters.
700 mb is low enough in the atmosphere that the elevation of some of the mountains in the Rockies will be higher. Because 700 mb in the mountains would be ground level, there are no values for wind, temperature, and relative humidity in these locations. However, the heights at the 700 mb level in these regions are still estimated as if there were no mountains.
Meteorologists look at both moisture and vertical motion of the air to find and forecast precipitation. While the 700 mb chart is not the sole source of this information, it can be used to quickly identify precipitation potential by looking at relative humidity values to determine the amount of moisture available.
Like the 500 mb level, the humidity contours are 50% relative humidity (line), 70% relative humidity (light green shading), and 90% relative humidity (dark green shading). However, remember this is only at the 700 mb level. There may be more or less moisture at other levels.
Omega (Ω) depicts vertical motion relative to the Earth's surface. Motion away from the surface (rising air) is shown as with minus (-) sign (red color). Motion toward the surface (sinking air) is not given a sign but is understood to be positive (+) (blue color).
The units for Omega are in microbars per second (µb/s). Microbars per second nearly equates to centimeters per second, so the values can be read as such.
