Originally posted by Anna
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I've noticed that so-called "secondary weather systems" here in the UK usually follow pretty closely on from main ones, such as fronts. This would appear to be because what is known in the "trade" as "returning maritime polar air" moves faster around the low pressure centre - the reason being, as you can see on today's map, the parent low has tighter isobars than are ahead of the main fronts, hence the winds there are stronger than those ahead (i.e. to the south and east) of the weather system.
One reason which has been posited for these secondary features is that the cold air is not constituted as a simple airmass of more-or-less similar temperatures at ground level, but is cold air moving into air which is still to some extent warm at ground level, even after the passage of the main front(s) - or at any rate warmer than the replacement air. As this colder replacement air meets the lingering warmer air ahead, it lowers the ground temperature in a sequence of steps, each one of which is marked by a line of showers, advancing in parallel with the retreating cold front, and marked as a line without warm or cold front symbols, or namely, a squall line. As each line moves through, the temperature near ground level falls; and as it successively falls, convective (shower-generating) activity (which is dependent on a sharp temperature gradient from ground to cloud formation level) is gradually reduced, and the air stabilised.
Sometimes on a weather chart you will spot several of these squall lines pursuing the main fronts of a low pressure system. In some instances concentrations of heavy showers along the squall line can produce sufficiently severe weather, with correspondingly stronger inflows at certain points along the line than at others, as to cause significant lowering of barometric pressure at those points, which in turn can be sufficient to lead to the formation of a secondary low, or "arctic low", at that point.
A couple of years ago, following a series of TV programmes in which British storm chasers went off to America to observe thunderstorm and tornado activity over there, I was briefly in touch with an American storm chaser forum, and had the opportunity to observe in some detail differences between the conditions which lead to frontal genesis in the States and here. The factors leading to tornadic thunderstorms in America are very different from those which we have here. Air feeding into their storm-producing cold fronts to produce gigantic tornadoes originates from three sources, which we just don't have in the UK - cold, very dry air from the region of the Rockies; hot, very dry air from the California and Arizona desert regions; and hot, very moist air from the Gulf of Mexico. One thing I noticed - and this may go part way to ansering your question, though I don't have a full explanation - was that the resultant storm systems cover a much wider geographical area at any given time over there than is the case here. This may have something to do with the massive areas heated up to Tropical temperatures over the huge, almost flat American landmass, and the slack associated wind systems creating conditions in which weather systems can develop on a much bigger scale than we have here in the UK, where the influence of the Atlantic Ocean in keeping the jet stream on the move makes things happen in more concentrated forms, and generally over shorter periods of time, though, tbh, this is conjecture on my part.
but a cousin I grew up with is coming over from Oz next week and will be around for three short weeks only, some here and some time in other parts ofthe country. He is the nearest I have to a younger brother and it will be great to see him. He went out on the £10 scheme 40 years ago, last visited us in 2005.
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