We started our discussion at 300 mb for the upper atmosphere and now we are making our way down towards the surface.

The next level that meteorologists often plot is the 500 mb constant pressure map. An upper level weather chart is plotted in constant pressure because it is easier to explain. This one tells a lot to a meteorologist who’s about to make a forecast.

500 mb is considered the middle of the atmosphere and ranges in elevation. You’ll notice that height is plotted on the map and height values range from about 5,000 meters to 6,000 meters. If it’s plotted you’ll notice that the last zero is dropped.

In certain parts of the country, the 5400 meter contour is important because it often divides the rain/snow line in the winter months. Heights are also lower in colder regions and higher in warmer.

You’ll also find that vorticity is plotted. The definition of vorticity is basically the amount of spin in the atmosphere or energy. The higher the vorticity, the higher the amount of spin in the atmosphere.

If you’re downstream of high levels of vorticity it is often anticipated that you can expect rain or cloudy skies. If it’s the opposite, you’re upstream from the vorticity then you can expect sunnier skies and nicer weather.

You can also find troughs and ridges on a 500 mb map. You can first determine a trough or a ridge when the height contours are plotted. A ridge is where the height contours are moving north on a map and are often associated with high pressure systems. A trough is the opposite with a southward trend or a dip in the atmosphere.

A ridge is also often associated with areas of high pressure… which usually means light winds, sunnier skies, and warmer temperatures. If it’s a trough then it’s a low pressure with precipitation, cloudy skies, or cooler temperatures.

Within theses troughs and ridges are also shortwaves. They’re surges of energy in the atmosphere and are embedded within the trough or ridge. And can disrupt the weather pattern.

There may be days where the 500 mb chart has no troughs and ridges. Those days are considered to be zonal. Zonal flow means winds flows from west to east. It if were meridional, then troughs and ridges would exist with a north and south flow. Also leading to a fairly active weather pattern.

The weather pattern is complex and always changing. We will discuss the 700 mb pressure map next.