|
||
|
||
 |
 |
|
 |
||||||
| Ever wondered how some surfers seem to insticively know
when big surf is about to arrive? Actually it's not all that hard, once
you've learnt how to read weather maps. Waves are caused by wind blowing over the surface of the ocean. The size and type of the waves generated depends on three factors: the wind speed; how long the wind blows in a constant direction; the 'fetch' (the distance that the wind blows over the water's surface in a constant direction). If the wind is strong and blows for a long time, across a long distance in the direction of your beloved beach, then you should be in for plenty of swell! Thanks to Michael Fish and his mates at the BBC, you can see detailed weather maps on the TV every day. Most quality newspapers also print useful maps, and you can even have a forecast chart faxed to you by the Met office by dialling 0336 400 445 (about £1.50 per fax). Swell is generated by low pressures systems (storms) that constantly travel around the globe, moving from west to east. These 'lows' are shown on weather maps by isobars (air pressure contours) which indicate the wind direction and wind speed. In the Northern Hemisphere, winds always rotate anti-clockwise around lows. The closer the isobars, the stronger the wind. High-pressure systems (which usually bring settled, sunny weather) have winds which rotate around them in the opposite (clockwise) direction. Looking at the weather chart shown above, we see an intense low-pressure system situated out in the mid-Atlantic, with its centre measuring 969 millibars. As the isobars are packed tightly together, gale force winds will be howling around it, and blowing in a westerly direction around the underside of it; this will produce a big swell moving in a westerly direction. Open ocean swells travel at 20-25 mph, and since the low is about 1000 miles away from Britain, the swell being generated will start to arrive in approximately two days' time. Swells like this tend to fan out away from the generating wind direction, by perhaps 30 or 40 degrees. So this swell will provide waves for most of the west-facing coasts of Europe, from the Hebrides right down to the Canaries. Our chart also shows an area of high pressure centred over Denmark. If this was to stay there for the next few days it would provide light south-easterly winds (remember, the winds follow the isobars and rotate clockwise around highs), which would blow offshore along the north coasts of Devon and Cornwall, giving classic surfing conditions. Swells that are generated a long way out in the ocean, then travel for days before hitting land, will be well lined-up and 'clean' when they arrive (provided there's little or no wind when they do). These are called groundswells, and provide the best conditions for surfing. Strong local onshore winds will also generate swell, but it will be of the messy and choppy variety, with the waves close together. This kind of swell is called a storm-swell, and the waves will be erratic and hard to ride. |
|||||||
|
 |
|
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
