Weather Tutorials: Synoptic Charts | COLA Charts | Radar Images

How to interpret COLA Mean Sea Level Pressure, Thickness, Precipitation Charts

Read to the bottom of this page to for an understanding of how snow is predicted*

The thick black contours indicate sea level pressure in millibars.

Low sea level pressure indicates cyclones or storms near the surface of the earth. High sea level pressure indicates calm weather.

The thin grey contours indicate the vertical distance, or thickness, between the 1000 millibar surface and the 500 millibar surface, measured in tens of meters. Since air behaves nearly as an ideal gas, and vertical distance is proportional to volume over a specified surface area, the thickness between two pressure levels is proportional to the mean temperature of the air between those levels. Thus, low values of thickness mean relatively cold air.

The 540 vertical distance line is highlighted, since this line is often used as a rule of thumb to indicate the division between rain and snow for low terrain. When there is precipitation where the thickness is below 540dam, it is generally snow. If the thickness is above 540dam, it is usually rain (or sleet if the air next to the surface is below freezing).


This information was sourced from; Center for Ocean-Land-Atmosphere Studies, Maryland, USA.