Wednesday, December 24, 2008

The cruel irony of a December warm front

It's been cold lately. Bitter cold. Almost Minnesota cold. It's understandable that people are gratified to hear the forecast of an approaching warm front and welcome the arrival of more temperate weather. What they disregard, though, is the effect of the passing of the front. Many people visualize the passing of a front as simply turning up the thermostat: presto, it's warm now. They visualize the front a a vertical wall separating the cold air from the warm air. That's not the case. In actuality, the front approaches as a wedge, with the warm air up high climbing over the cold air. You can see it in this (modified) diagram:

Much like a cold front in the summer, there is often precipitation at the leading edge of a warm front. In the summer, that spells thunderstorms. In the December warm front, it spells freezing rain. The warm, moist air of the front is above freezing, so the precipitation comes in the form of rain. Until the cold air closer to the ground is moved out of the way by the full passage of the front, the rain falling from the higher temperatures above freezes as it passes through the frigid air hugging the ground. What you end up with is a city-wide ice rink. This invariably occurs right around the evening rush hour, of course.

This is all by way of saying that we had a warm front come through yesterday, and had it not been for the lure of Irish Egg Rolls (essentially a deep fried Reuben sandwich- so yummy!) at the Dublin Village Tavern, I would never have left the house. Today we're sitting under a steady rain and enjoying, such as can be accomplished on a rainy December day) moderate temperatures in the 50s.

Not flying weather at all.

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