What is "Critical Thickness?"

Pressure decreases with height upward through the atmosphere. If you take any two pressure levels in the atmosphere, say for example the 1000mb level and the 500mb level, and find the vertical distance between the two, then you have calculated a variable called atmospheric thickness.

Thickness appears on many weather maps and forecast models. As to why, first remember that when an object is heated, it will expand. Likewise, when an object is cooled, it will contract. The atmosphere adheres to these rules too; and so, if you hold the 1,000mb layer steady near the surface, the 500mb layer will be higher in a warmer air mass and lower in a colder air mass.

Thickness then is so widely used because it provides insight into what the temperatures are like over a deep layer of the atmosphere. This becomes important in winter storm forecasts, as thickness can help meteorologists determine what type of winter precipitation will fall, be it rain, sleet or snow.

The critical thickness then is a thickness value that can be used as a de facto rain/snow line. For many spots across the country, with the 1000-500mb thickness, the 5400dm thickness line is used, dubbed the "540 line". The 540 line however is not always appropriate to use, and will not work in some locations, especially in higher elevations.