what are pressure systems, troughs, highs & lows when the weather report comes on?
In: Physics
Air in the atmosphere rises up and falls back down. Where are is falling, air pressure – the weight of air pushing down on everything – is a little higher, where air is rising, the pressure down here is a little lower. It isn’t much higher or lower – normally the pressure only rises or falls by a few percent.
This is what the highs and lows are – areas where air is rising upwards, and areas where air is falling down. Air up high is dry, so high pressure areas are going to be dry; when the hot damp air down here rises up it cools and the water falls out, causing cloud and rain. Wind also goes from those high pressure areas to low ones, which also causes weather.
Often an area of pressure – high or low – will form in a line. A line of lower pressure is a trough, a line of high pressure is a ridge.
Lots of things cause these areas of high and low pressure. At the root of it is temperature differences – heated air expands and so weighs less and rises causing a low pressure zone, cooled air shrinks and so weights more and falls causing high pressure. This happens locally – the ground heats up in the day causing a low pressure, for instance – and it happens globally. The cold air at the poles falls and the hot air at the equator rises, and this drives a number of circulations causing bands of high and low pressure around the globe. Because of the effect of the earth’s rotation on airfows, bands of low or high pressure break up into cells.
There is one more thing on a weather map that is important – warm and cold fronts. This is where winds pull cold air from polar areas down into warmer ones, or warm air from temperate or tropical areas down into cooler areas. The boundaries between these pools of cold air and the warmer surroundings (or warm air and cooler surroundings) drives severe weather and are an important feature of weather.
High pressure systems usually means it not going to rain. Low pressure means it’s more likely that there will be rainfall.
High pressure systems are caused by cooler air falling and compacting air in the atmosphere (increasing pressure). Low pressure is where warm air rises (think evaporation) releasing pressure.
A trough is like a burst of low pressure pushing out from a low pressure system, it usually carries strong winds and sometimes downpours.
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