Snowfall forecasting might be the toughest part of a meteorologist's job. Luckily, we have one little tool that can be a big help when predicting snow. It's called the Critical Thickness.
First, let's talk about what "thickness" means in this situation.The thickness is the height, in dekameters (10s of meters) from the point at which the pressure is 1000mb, up to the 500mb level. This is a vertical value, rising upward from the Earth's surface. Normally, thickness values are between 500 dekameters (dam) and 600dam, though lower thickness values are certainly possible in the United States and are actually common during a Canadian winter.
To understand thickness values, we have to look at maps in a completely different way-- sort of like a 3D image. When thickness lines are plotted on a map, they look like wavy or squiggly lines (sometimes you'll see a few circles, too). Each of those lines represents a contour in which the thickness is the same. So you can imagine that the thickness lines are sort of like a topographical map of the atmosphere!
OK, so... why is the thickness ever "critical"? The critical thickness corresponds to an airmass that is equally able to support snow and rain through its vertical column. For this reason, the critical thickness line is sometimes called the "50/50 Line", because you are just as likely to have either rain or snow occurring. (This is a slight simplification, because temperature and humidity also play a role). In the DC Metro area, the critical thickness is approximately 540dam. This level, often called the 540 line, is such a common critical thickness value throughout the United States that it is highlighted on most forecast model maps. In the Beltway region, once you are north of the critical thickness line, the chances for snow increase astronomically. South of the critical thickness line, it's very difficult for snowflakes to fall all the way down to the ground without melting.
Take a look at the maps in this article. Map #1 has several thickness lines plotted on it (this is called a composite). Notice how the lines are to the south of the DC Metro area. A forecaster can determine that this pattern will easily support snow. Map #2 has lots of lines on it! The yellow ones represent thickness contours... not just the critical thickness. The contours are at 60dam intervals. The solid yellow line is the 540dam line, or critical thickness line. The shaded areas indicate precipitation potential. If you look closely, you can see the shaded area north of the solid yellow line-- a clear indication of snow.
Of course, there are plenty of other factors that help determine whether snow will fall in your backyard. But the placement of that critical thickness line plays a crucial role in our forecasting process. Next time snow is in the forecast, you can check out the model data yourself, find the critical thickness line, and make your own prediction!