weather - page 2

8.2 Weather Systems – Lows and Highs

Synoptic charts are drawn using data sent in by weather stations worldwide. Information on barometric pressure, temperature, humidity and cloud types is all taken into account to produce a chart that shows centres of high and low pressure and any associated disturbances caused by air masses of different temperatures meeting (photo 8.2.1). Very noticeable is the circular pattern of isobars – the lines joining points of equal barometric pressure that enable us to calculate the wind speed and direction.


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Photo 8.2.1

Low Pressure

An area of low pressure is formed when warm air and cool air meet and interact with each other. The warm air rises above the cool air, leaving slack pressure at the surface, and the cool air flows in to replace it (photo 8.2.2). The Earth’s rotational effect (Coreolis force) kicks in and causes the rising warm air to go into an anti-clockwise and upward spiral in the northern hemisphere. South of the Equator it reverses to blow in a clockwise direction around a low.

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Photo 8.2.2

Air flows from the area of high pressure to the area of low pressure in an attempt to equalise the pressure, just as it does when a hole is made in a bicycle tyre – the air leaks out. However, the air does not pass directly from one to the other; instead, it more or less follows the line of the isobars except that it pushes out from the high and ‘toes into’ the low. Follow the flow on the weather chart and you will see that the flow is similar to a figure of eight. Notice that the wind blowing in from the west between the high and the low converges and the isobars are closer. This means that the wind will be stronger in this area – the closer the isobars are the stronger the wind will be.

The Sequence from A to F

We shall assume that we are at position A on the right-hand side of photo 8.2.3, with a clear blue sky and very little wind. Someone on the boat is keeping a good log and is recording everything that happens as the depression passes over.  



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Photo 8.2.3

At position A

1. We see high cirrus cloud that looks hooked by the wind. This means that there is moisture in the upper atmosphere and a possible frontal system may be approaching (photo 8.2.4).

2. We are aware that aircraft vapour trails are remaining in the sky and not disappearing just behind the aircraft. There is also a halo around the sun (photo 8.2.5). This is caused by a very thin layer of cirrostratus cloud veiling the sun. (The prefix cirro = high cloud, and stratus = layer.).

3.The wind is light, but has changed direction from southwest to south. This means that it has ‘backed’ – ie, it has swung anticlockwise.

4.The barometer has started to fall.

5.The visibility is reasonably good – more than 5 miles.



Photo 8.2.4

Photo 8.2.5

At position B

1. The sky is now completely covered with high cloud and a thickening lower layer – altostratus. (The prefix alto = medium-height layer cloud.) The sky in photo 8.2.6 looks as if some nasty weather is on its way.

2. The wind is rising now and the pressure is continuing to fall.

3. It has begun to rain and the visibility has fallen to about 3 miles.



Photo 8.2.6

At position C

1. We are now completely soaked and the cloud has become thicker and lower. It is nimbostratus cloud (photo 8.2.7). (The prefix nimbo = rain-bearing.).

2. We are getting quite close to the warm front and the wind is howling. The pressure has now fallen 5 millibars in the last 3 hours. 3.The visibility has become quite poor and it is quite misty in the rain, but we are feeling a little warmer as we get close to the warm front.



Photo 8.2.7

At position D

1. Life is beginning to look up a bit – the heavy rain has stopped and it’s now just drizzle or light rain.

2. The cloud has thinned and looks a little lumpier than it did as it is now stratocumulus cloud (photo 8.2.8). (The prefix strato = layer, and cumulus = accumulating.) Stratocumulus could be described as layer cloud that has a small amount of vertical development.

3. We noticed that the pressure had stopped falling when we did the last log entry – it has steadied now. 4It is still blowing hard, but the wind has veered round to become southwesterly.

5. The visibility is quite poor – about 1 mile.

6. It is definitely a lot warmer.




Photo 8.2.8

At position E

1. Just when we thought the weather was improving, it has started to pour with rain again (photo 8.2.9).

2. The wind is even stronger than it was and is getting quite gusty too.

3. It doesn’t feel so warm now – quite chilly in fact.

4. Despite the heavy rain, a line of blue sky appears to be creeping towards us from the northwest.




Photo 8.2.9

At position F

1. The rain has stopped and the sky is blue, except for quite a few towering cumulus and cumulonimbus clouds, which have been giving us some heavy squally showers (photo 8.2.10). (The prefix cumulo = accumulating or growing, and nimbus = rain-bearing.)

2. The wind has veered yet again and is now blowing from the northwest. We think that it has moderated a little.

3. The barometer has risen a lot in the last couple of hours, but is slowing now.

4. The visibility is now very good – we can see coastline about 15 miles away, but in the showers we can hardly see anything. We had hailstones during the last squall.

5. There are not so many clouds to the northwest – the showers will die out and we will have blue skies again.






Photo 8.2.10

High Pressure (an Anticyclone)

A cyclone is always associated with extreme weather, and an anticyclone is just the opposite, with fair or fine weather and light winds at its centre. Highs that build in pressure slowly but steadily often remain in one place for a time and give long periods of stable weather. They also form ridges between two depressions (photo 8.2.11). This should provide a day of sunny weather before the next depression arrives. During the summer, a high generally has blue skies and calm nights associated with it. In the winter months, though, it can be cold, overcast and gloomy if the air is coming from the north and has passed over a large expanse of sea.






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Photo 8.2.11