Correlation does not prove causation

Editor:

One of the notable, and not so pleasant, features of our local climate is the presence of the near-daily spring winds, which at times can reach howling proportions and are a real nuisance. Everyone who has been here for some time comments on it and over the years I have repeatedly heard the local folklore that the winds will stop as soon as the snow in the nearby mountains has melted. Beware: one of the cardinal principles of science, if not common sense, is that correlation does not prove causation.
As we go from winter to spring, the Earth continues its orbit around the sun, the tilt of the Earth's axis changes, and we get more direct radiation from the sun. This begins to melt the snow in the valley (not in the mountains, at least, at first) and there is less reflectivity (albedo) from the snow and ice. Both conditions warm the ground – and warming starts from the bottom up. As the ground heats up, heating the air above it, the warmer air rises because it's less dense. 

This sets up a type of heat transfer called convection and in this case a type of air circulation. Since air is now rising, air from the jet stream (which is starting to weaken in the spring but still moderately strong) will start to replace it and air starts flowing down. This causes our spring winds!!! The snow melt out in the Gunnison valley floor typically occurs by late March, and the spring winds don't usually start until late April or May, so the snow is not really a factor here. The sun’s angle is the critical factor. Furthermore, note that we don't usually get our winds until mid-late morning, and then increasing in the afternoon, because the ground must be heated up to a certain degree before the convection occurs. As we move into June and July, we get more and more direct solar radiation and now the jet stream is no longer overhead, as it is in the winter, due to the continuing tilting of the earth's axis. We don't have the stratification with cold air on the bottom and strong winds aloft. With less temperature difference comes more stability. So basically, our spring winds are a result of a complex interplay of the angle of the sun’s rays (due to the earth’s wobbling orbit), heating of the earth on the valley floor causing convection, which then destabilizes the overlying jet stream and brings strong spring winds down to the surface. In the evening, we cool a bit; convection stops, and the overturning of the air stops with it. The jet stream is currently slamming us from both the Pacific northwest and the desert southwest. However, we still have our typical spring convection system funneling wind down our valleys. Dry, low humidity air mixes easily with the entire column and makes it easier to move, and put together, it creates exactly what we’ve got: weeks of persistent, often daily wind.

Weather is complicated!

Bruce Bartleson

Gunnison