540 thickness line

Typically plotted on surface weather maps, the ‘thickness’ contours are derived from constant pressure charts. Thickness is the measurement of the distance (in meters) between any two constant pressure surfaces.

The thickness of the layer is proportional to the mean temperature of the layer.

One of the most common thickness charts used in meteorology is the 1000-500 mb thickness. This is the distance between the elevation of the 1,000 mb and 500 mb levels.

Of particular interest is the 540 thickness line.

The 1000-500 mb thickness value of 540 (5,400 meters) is the traditional line to determine rain verses snow. The 540-contour closely follows the surface freezing temperature (32°F/0°C).

The 540-thickness line (5,400 meters deep) is approximately the point where precipitation changes from liquid to frozen or vice versa.

If precipitation is expected for locations poleward of the 540-thickness line, the rule of thumb is to predict snow.

More at https://www.weather.gov/jetstream/thickness