The 540 line is in reference to a 5,400 geopotential meter thickness between 1000 and 500 millibars. Thickness is a primary function of the temperature of the air and a secondary function of the moisture content of the air. Temperature and moisture are combined together to produce the virtual temperature. The average virtual temperature from 1000 to 500 millibars determines the thickness displayed on analysis and model progs. Warming the temperature or adding moisture to the air will increase the virtual temperature and will therefore increase the 1000 to 500 mb thickness. When the thickness becomes low enough, snow can reach the surface. Through researching the correlation between thickness and precipitation type, the 540 thickness is used "generally and loosely" as the non-snow / snow line. Thicknesses of 540 or lower indicate snow is most likely (50% of time a 540 thickness will produce snow at elevations below 1000 feet) and thickness values of greater than 540 most likely indicate non-snow precipitation. There are many circumstances in which a lower than 540 thickness can produce rain and a higher than 540 thickness can produce wintry precipitation. NEVER should a forecaster determine the precipitation type solely by the thickness value. Here are a few items the forecaster must also consider:
(2) Warm biasing of 540 thickness
(3) Cold biasing of 540 thickness
(4) Temperature in the PBL
(5) Evaporative cooling potential and
(6) Forecast model error
In high elevation regions, snow is common even when the 1000 to 500 millibar thickness is 546 or 552. This is because high elevation regions are located at lower pressure levels closer to mid-level colder air (precipitation does not have to fall as far to reach the surface as compared to low elevation regions).
A 540 thickness can have a warm or cold bias. Generally, temperatures decrease with height in the troposphere. However, differential advection can cause layers in the troposphere to be warmer or colder than they otherwise would be. For example, WAA can occur between 800 and 650 millibars, which causes the temperature in that layer to increase and thus results in the thickness increasing above 540. If the temperature between the surface and 650 millibars continues to stay below freezing, the precipitation will fall as snow. Whether the 700 mb temperature is -20 C or -5 C, the precipitation will fall as snow if the temperatures are below freezing at all levels aloft in the troposphere. WAA in the mid-levels of the troposphere can cause the temperature to remain fairly constant with height, from the surface to 700 millibars instead of the "normal" decrease of temperature with height. This is an example of a warm thickness bias.
As a second example of biasing, in this case cold thickness biasing, a shallow layer of warm air in the PBL can melt snow before it reaches the surface. The thickness may be 540 because of plenty of cold air aloft, but the precipitation still reaches the ground in the form of rain because of the shallow warm air in the PBL. Click here for an example of cold thickness biasing.
Temperatures in the PBL are critical to the precipitation type. A shallow layer of polar air at the surface can cause freezing rain or sleet when the thickness is well above 540. This is because deep layers of the troposphere will have more "weight" at determining the thickness than shallow layers.
Evaporative cooling causes the thickness value to decrease. If precipitation falls into a deep layer of dry air, expect thicknesses to lower. Lastly, there is forecast model error. It is wise to check several forecast models for the positioning of the 540 line. Use the model guidance plus your meteorology knowledge of the biases of the 540 line and the mechanisms that cause thicknesses to lower or rise when making a precipitation type forecast.
On the forecast models, thickness (thik) is plotted with surface standardized pressure and precipitation. The 540 line is not dashed such as the other lines of thickness are (except 510 and 570 lines are also solid).