Thursday, July 8, 2010

Got Rain??

The fact that we usually get a fair amount of rain goes against most general concepts that define a humid climate. The prevailing westerlies cross land, not water, to get to us. They also pass over the Coastal Range, the Sierra Nevada's, the Rockies, the Appalachians and any other bump in the road that should wring out any bit of moisture that may have been left. Ideally, we should be a desert! The fact of the matter is, we are fortunate enough to be at the collision center of two different air masses, that is of course as long as the jet stream decides to meander a little as opposed to going straight from point A to point B like it is now. The irony of this is, its the cool dry air that sweeps down from Canada that causes our rain. Our textbook mid-latitude cyclone has two arms that spin in a counterclockwise manner. The left arm is the cold front bringing the Canadian air to the south. The right arm is the warm front that brings that tropical, humid air up from the Gulf. The key here is that the cold dry air travels faster than the warm air and overtakes,..actually undertakes it. What I mean by this is that the key to our rain is when the continental polar air mass catches and goes under the maritime tropical air mass it lifts the air, cools it, condenses it, and if all goes well, brings us rain. No collision of air masses means no rain and a jet stream moving in a fairly straight line high above our degree latitude. That is what is happening now. Therefore, our normal pattern of hot humid days, followed by rain and a few clear days has turned into desert dry hot air from the southwest that has traveled across the dry land, without collisions, to our region.
Well the next question is why does the jet stream decide to go straight instead of meandering diagonally across the country?
Well, it all has something to do with a South Pacific Ocean current that sits right on the equator and the strength of the trade winds. If the trade winds weaken, the current doesn't move, warm water stays on top of the pacific, and magically the jet stream meanders like mad and brings us weather like we had this winter,...El Nino. If the trade winds begin to blow again, faster than normal, more upwelling occurs off the coast of Peru, stretching cold water across the Pacific and magically, the jet stream starts moving in a straighter line,...La Nina 
The moral of the story is, the second you see a blue anomaly stretching from South America almost the whole way to Australia, start conserving your water. If you see a red anomaly stretching across, break out your mud boots and snow shovels. If you see a nice mix, blue next to Peru (rhyme?), red near the Indo-Australian area, it should make for a nice normal year.

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