In the spring, when many are yearning for milder temperatures to take hold, a back door front is looked upon as set-back, with temperatures falling from unseasonably balmy levels back to chilly or even cold levels. Such a scenario is called a “back door” cold front and usually occurs during the spring and summer months. The result is a cold front that advances toward the west and south. Winds out ahead of a cold front tend to blow from the south and southwest, and then shift after the frontal passage (called a “fropa” by meteorologists) into the northwest.īut on occasion, a cool air mass might build up over eastern Canada and sink southward through northern New York and New England. toward the east - faster in the winter than summer - and are usually oriented along a northeast to southwest line. Then we might speak of the frontal boundary not so much as a “cold” front as opposed to representing a “dry” front.Ĭold fronts generally advance at average speeds of 20 to 25 mph. In some cases, there doesn’t seem to be much of a difference temperature-wise between the air ahead of a cold front and the air moving in behind it so it’s not so much the difference in temperature as is the difference in terms of a moist and humid air mass being displaced by a significantly drier and less humid air flow. With the cold front, warm air is rapidly forced upward (like the shavings) in advance of the actual front (the “cutter”), creating towering cumulus clouds, some hard showers and quite possibly a few gusty thunderstorms followed by a push of cooler and drier air in its wake. When the cutter or sharpened metal plate is pushed forward over a wood surface it slices shavings of wood that curl upward in advance of the cutter. Another good metaphor for a cold front is that it’s like a hand plane. But Bjerknes realized that there were masses of colder and warmer air that swept across the globe and bumped into each other and in the process produced zones of unsettled weather near and along their respective boundaries.Ĭold fronts are very much like atmospheric plows, pushing away warm, moist air and replacing it with a cooler and drier air mass. Or if today turned out to be warmer, then it was assumed that it was yesterday’s air with some heat added in. Prior to that, it was thought that if today happened to be colder, it was simply yesterday’s air with some heat that was lost to space. In 1919, Jacob Bjerknes, son of a noted Norwegian meteorologist, Vilhelm Bjerknes, announced his discovery of air masses and fronts. If you were to check out the maps that were published prior to Aug, 1, 1941, you might notice that something was missing. Virtually every weather government map that was published during that 132 year time span - more than 48,000 of them - are available here. The website provides access to historical daily weather maps from 1871 thru 2003. Daily Weather Maps Project is an interesting and potentially valuable weather resource for researchers.
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