We’ve all borne witness to the occasional failed snowfall forecast.Of course, there are numerous reasons why a snowfall forecast can go bad. I won’t even bother listing them here, because it would take up most of my article space. The element of snowfall forecasting that I’ll focus on today is called the critical thickness.
First, we have to toss out our way of looking at a map. When forecast model data relating to thickness is plotted on a map, it looks like wavy lines with the occasional circle. But we have to use our imagination, and view it as a 3D image! Those wavy lines are plotting contours in which the thickness is the same. So, it’s beneficial to look at a thickness map as a topographical map of the atmosphere! The thickness troughs, which look “U”-shaped on the 2D map, are actually atmospheric valleys in the 3D world.
Now, let’s talk about what “thickness” means in this situation.The thickness is the height, in dekameters, of an airmass from the point at which the pressure is 1000mb up to the 500mb level. Normally, the thickness values will be 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.
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 Connecticut, 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, such as the one above. Thickness values will be lower to the north of this line, and higher to the south of it. Here in Connecticut, 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 to see snowflakes falling all the way down to the ground level.
Of course, there are plenty of other factors that help determine whether snow will fall in your area. But the placement of that critical thickness line plays a crucial role in our forecasting process.