Tuesday, October 11, 2005

Weather lesson and more?

Good friend Chris Arbizzani is on an epic journey out west, car camping and exploring. He sends really great e-mails that go into detail describing the people and places he's seen. And, because he's something of a geology buff and weather nut, he tends to go into greater detail about those subjects. Here is an interesting tidbit I gleaned from his latest post, culled, of course, because it has something remotely to do with mountains:

Meteorology Lesson: How can a snowstorm happen in Colorado in October? I am going to try to explain the phenomenon the best I can, it can get kind of technical, but is interesting.
A large snowstorm is happening all across the high country of Colorado. It may seem odd, because there are no other major winter storms in the country, and in fact, temperatures all around here are much warmer. There are a number of specific weather phenomenon that have to occur together to form a storm like this one. The escence of all weather phenomenon is Air Pressure. High Pressure generaly means fair weather, and Low Pressure often means storms. The contrast between pressure systems produces wind, and often precipitation. In North America, most Low Pressure System come down from the north, with the notable exception of hurricanes. In this case, a Low pressure system has stalled over Colorado, called by Meteorologists a "Cut off Low", as there is higher pressure all around it, and no significant colder or warmer air mass behind it. The Low is stalled over the mountains, creating the specific conditions needed for large amounts of precipitation. Storms in the Northern Hemisphere rotate counter-clockwise (you may remember this from the satelite photos of hurricanes), and suck air into them because of the pressure difference. Moist air is being drawn into the storm from the southeast, and east. Cold air is being drawn in primarily from higher in the atmosphere, a normal, and common occurence with storms. In the flatlands, the colder air is kept 5 or 10 thouseand feet or greater, up in the air, making the storm a rain event. Due to elevation, the mountains interact with parts of the storm that would otherwise never touch the ground, thus producing snow. Another factor related to elevation is the difference between warm and cool air. The air out on the great plains of Kansas and South Dakota is comparatively warmer than the air on the mountains. The rotation of the storm forces the warmer air from the plains, via a strong northeast wind, to be forced up the front range mountains. As the air rises, it cools, and condences water out of it, forming clouds. The already cold upper air causes the clouds to almost immediately create precipitation. So we esentialy have large range of conditions present in this storm, it is really something for the textbooks.