Constant Pressure Charts: 700 mb

Constant Pressure Charts: 700 mb

700 mb considered by many to be the top of the lower atmosphere. Height values have the last digit, a zero, truncated. So, a height value of 282 represents the 700 MB elevation of 2,820 meters.

An air pressure of 700 millibars is commonly said to occur near 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).

700 mb is now low enough in the atmosphere where the elevation of some of the mountains in the Rockies will be higher than this level. The values for the heights of 700 mb in this region are estimated as if there were no mountains. But, there is no value for wind, temperature and relative humidity.

This is also getting into the portion of the atmosphere where we are looking for both moisture and vertical motion to find and forecast precipitation. For moisture, the 700 mb chart will indicate the relative humidity.

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) represents vertical motion. The sign on Omega is relative to the earth's surface. Motion away from the surface (rising air) is given minus the (-) sign (red color). Motion toward the surface (sinking air) is not given a sign and is therefore positive (+) (blue color).

The units for Omega are in microbars per second (µb/s). Fortunately, microbars per second nearly equates to centimeters per second so the values can be read as such.

Map of the North America
Wind barbs
Surface weather in relation to the constant pressure chart.
Relative Humidity
Height contours