Reading a 500 mb Weather Chart: Heights, Vorticity and Humidity in the Middle Atmosphere

Of all the charts of the upper atmosphere, the 500 millibar (mb) constant pressure chart is the forecaster's favourite: if meteorologists could look at only one, this would be it. The 500 mb level sits between about 16,000 feet (4,980 meters) and nearly 20,000 feet (6,000 meters) up, and counts as the "middle" of the atmosphere: halfway, by pressure, between the 1,000 mb of sea level and the zero millibars of the top of the atmosphere, more than 30 miles (50 kilometers) up.

A 500 mb constant pressure chart. Image from NOAA JetStream.

Heights

A constant pressure chart maps not pressure but height: how high above sea level the 500 mb level lies. On the chart the last digit, a zero, is dropped. Add it back to read the height in meters: "564" stands for a height of about 18,503 feet.

Height contours across North America. Image from NOAA JetStream.

Wind barbs at 500 mb. Image from NOAA JetStream.

Vorticity: the air's spin

The chart also measures vorticity, the rotation of air. It can turn in any direction, but forecasters care about spin around an axis pointing straight up from the ground. The 500 mb chart shows absolute vorticity, the combination of spin from wind shear and from curvature that gives the total at any place. Its units, 10⁻⁵ per second, are dropped on the chart, so a value of 16 × 10⁻⁵ per second appears simply as "16". The higher the number, the faster the spin, and the vorticity maximums help locate short-wave troughs riding within the long waves of the atmosphere.

Absolute vorticity at 500 mb. Image from NOAA JetStream.

What matters most is not the value but how fast it is changing:

• Downstream of a vorticity maximum, where vorticity is increasing fastest, air converges low down and rises, and precipitation may follow.
• Upstream, where it is decreasing fastest, air sinks and spreads out near the ground, bringing fair or improving weather.

Humidity

Moisture decides where precipitation will or will not fall, and the 500 mb chart shows it as relative humidity: in the usual example, a line at 50 percent, light green shading at 70 percent and dark green at 90 percent. It describes this level only; there may be more or less moisture above and below.

Relative humidity at 500 mb. Image from NOAA JetStream.

Surface weather compared with the pattern at 500 mb. Image from NOAA JetStream.

Sources

• NOAA National Weather Service, "Constant Pressure Charts: 500 mb," JetStream, an online school for weather; rewritten in hubnx's own words. Charts from NOAA.