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Highs and Lows - Wild Weather

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Each week Steve Symonds takes a local's look at our region's weather.

Ask Steve a weather question here.

Highs and Lows

I was asked the other day whether high and low pressure systems were determined by the central pressure. For instance if the pressure were 1010hPa would that be high or low? It could, in fact be either, I would need to know more about the system.

High and low pressure refers to the pressure caused by the mass of air above a point on the surface. If there is less air, the pressure is low, if there is more, the pressure is high. How do we get more or less air above a point?

Here we have to consider a concept that doesn't really exist but helps to explain what is happening. Imagine two columns of air, one at the equator and one further away. The column at the equator is going to be heated by the sun and it will expand. At the surface, the pressure remains the same but higher up, at any level, there will be more air in the expanded column than in the unaffected one. More air means higher pressure so air will flow, at some height above the ground, from the expanded column towards the other. What happens at the surface? Air has moved away from the equatorial column so there is less air above the surface so the pressure is lower. In the other column, more air has moved into the upper parts so the pressure at the surface is higher. At the surface, the air will flow from the higher pressure towards the lower pressure. This completes the circuit. Air rises at the equator, moves towards the poles, sinks back to earth and moves back towards the equator. The characteristics of low pressure systems, therefore are air flowing in at the surface, rising at the centre and flowing out at the top. High pressure systems are reversed. Air flows in at the top, descends and flows out at the bottom.

You may have noticed that I have been carefully avoiding any mention of the Coriolis effect. The earth rotates on its axis, one revolution per day. Because of this, anything moving over the surface of the earth is deflected to the left in the southern hemisphere and to the right in the northern hemisphere. The effect is weak and strongest at the poles. I will leave the ramifications of Coriolis and water going down plugholes to another time but one of the effects of Coriolis is to position the high pressure systems such that the air rising at the equator sinks back to earth between 30°S and 40°S depending on the time of year. This belt of high pressure around the globe in both hemispheres is called the Sub-Tropical Ridge.

Because the air in high pressure systems is descending, it is very stable. Cloud will not form and without cloud there is no rain. All the world's major deserts, apart from the polar deserts, lie under the sub-tropical ridge in both hemispheres. Much of Australia lies under the sub-tropical ridge.

The other effect of Coriolis is to deflect the air blowing into the lows and out from the highs. In the southern hemisphere, the air will spiral clockwise into a low and anti-clockwise out of a high. The reverse is true in the northern hemisphere.

In other parts of the world, the terms "cyclone" and "anticyclone" are used instead of "low" and "high". This is quite correct. Unfortunately we cannot do this in Australia. There is a special kind of low pressure system called a Tropical Cyclone. This term is used in the South Pacific and Indian Oceans. In other parts of the world they are called Typhoons and Hurricanes. Years ago, the news media decided that "tropical cyclone" was too complicated so they dropped the "tropical". This means that in Australia the term "cyclone" is only used in reference to tropical cyclones. It also means we can't use the term "anticyclone" at all.

Now we know the difference between a high and a low, can we put figures on them? Can we say if 1010hPa is high or low? You will not find a high pressure system with a central pressure below 1000hPa nor will you find a low pressure system with a central pressure above 1020hPa but in between you can find both. A low pressure system of 1010hPa, however, would be surrounded by highs of 1030-1040hPa while a high of 1010hPa would be surrounded by lows of less than 1000hPa. You won't find the two together.

Finally, the highest sea level pressure recorded anywhere in the world was 1084hPa in Agata, Siberia in December 1968 while the lowest sea level pressure was 870hPa in Typhoon "Tip" in October 1979. Tropical cyclone "Tracy" had a central pressure of 950hPa.

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Last Updated: 27/08/2004 11:52:00 AM AEST

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