10 WIND AND WAVES Thunderstorms


These potentially dangerous monsters are also driven by heat convection, and can be recognised by the towering (cumulonimbus) clouds that can reach up to 12km high into the sky. Thunderclouds often have anvil shaped heads, caused by the jet stream tugging the cloud top to one side. There are various types of thunderstorm, but they all work by a similar mechanism: hot air is carried upwards by convection, then descends with great force after it cools at high altitude. Depending on the temperatures, you’re very likely to get heavy rain or hail.


Fig 10:8


Since they’re fuelled by heat, thunderstorms in Britain are a fairly rare feature of the summer months. In hotter climates they’re to be found year-round and are a lot more destructive. The thunder itself is caused by lightning, with the time lag between the flash and the thunderclap being due to the


difference between the speed of light and the much slower speed of sound – about six seconds per mile. Actually, this gives you a useful tool. By timing the delay in seconds and dividing it by six, you can estimate how far away the thunderstorm is. If the delays gets shorter, you know it’s getting closer.


FOG


There are few meteorological phenomena mariners dread more than fog. With the arrival of radar and GPS, the threats may have diminished but they certainly haven’t disappeared. The skipper of a small boat may hear the throb of a large ship or see a strong echo on his radar screen, but has the ship seen him? Almost anyone will feel vulnerable. We shall be looking at safety tactics in the next chapter but for now let’s investigate how fogs form.


Advection fog Fogs are nothing more than very low clouds and are made up of minute water droplets suspended in the air. For mariners, the most common type is ‘advection fog’ (Fig 10:9). This is formed when the wind blows warm, moist air over a colder surface where it condenses. Coastal fog and sea


Fig 10:9


fog are both advection fogs and can only occur when the wind speed is below about 15 knots. Higher wind speeds will lift the fog to form low cloud.


Valley Fog


Although more commonly an inland feature, valley fog can cause problems within hill flanked estuaries. At night the air cools and becomes denser, sliding down the hillsides into the valley. If the temperature of the valley floor – water in an estuary – is cold enough, condensation occurs and fog is formed. Since estuaries are often sheltered from the wind, the fog may linger even when it’s perfectly clear outside.


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RYA Seamanship for Sea Anglers


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