Thursday, March 8, 2012

Pressure Systems: From High to Low


Note: all pictures from the public domain

The atmosphere has weight. Put another way, the air exerts a pressure on the surface of the earth. How much? Per square inch, the air weighs 14.7lbs at sea level. Or, the weight of the air causes a column of mercury (mercury barometer) to rise 29.92” at sea level. Or, for you metric folks, the air exerts a force of 1013.2 kilopascals or mb.

Imagine a column of air. The higher you rise in that column, the less air there is above you pushing down on you, or the less pressure exerted. The average decrease in pressure is 1”Hg per thousand foot ascension.

Now this of course is in an ideal world. In reality, the air pressure is continuously changing at various rates at various locations – and this information can tell us a lot about current and future weather conditions.

For this reason, pressure readings are taken at weather reporting stations, and transmitted to forecasting stations where they are plotted on maps. Areas of like pressure are joined by lines called isobars. When these isobars are drawn, they form patterns on the map. They never cross, but form concentric circles depicting areas of high and low pressure. Like lines on a contour map, high pressure areas correspond to hills and low pressure areas to valleys. It is important to note that high and low pressure systems are only high or low relative to the air surrounding them – thus a low could be 1012mb or 992mb, depending on the air surrounding it.


But what exactly is a low pressure area? A low pressure area is defined as a cyclone, depression or simply as a low. It is a region of relatively low pressure, with the lowest pressure at the centre. Lows are areas of ascending air, with the air churning inwards towards the centre of the low in a counter-clockwise fashion. As we learned from Weather 101, rising air expands, cooling as it does so, and often condensing. As a result, lows are often associated with low cloud, poor visibility and precipitation. Just remember this: lows = bad weather.


A high pressure area conversely is defined as an anti-cyclone or a high. It is a region of relatively high pressure, with the highest pressure at the centre, decreasing towards the outside. The winds around a high travel clockwise, spinning into the centre. The air in a high is descending, increasing in pressure and consequently warming. This means that the saturation point increases (the air can hold more moisture) and there is very little, if any, precipitation. High pressures are associated with weather that is fair and clear, with light to moderate breezes. Highs tend to move slower and remain more stationary than lows. Just remember this: highs = good weather.


So the next time you’re flipping channels, stop for a just a moment longer at The Weather Network. See if you can make a bit more sense of the weather man’s forecast. And then wait and see if he’s right!

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