While typically plotted on surface charts, the ‘thickness’ line 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. (red on my maps)
This is the distance between the elevation of the 1,000 mb and 500 mb levels.
The “540 thickness” contour closely follows the surface freezing temperature (32°F/0°C).
Typical thickness layers used in weather analysis include the following:
a. 1000-500 mb thickness: rain-snow discrimination (5400 m isoline) and thermal gradient on the cold side of fronts. (red on my maps)
b. 1000-850 mb thickness: rain-snow discrimination (1300 m isoline) and rain/sleet/snow determination. (magenta on my maps)
c. 850-700 mb thickness: rain-snow discrimination (1540 m isoline) and rain/sleet/snow determination. (purple on my maps)