Tip of the Month
April, 2010 - Whether the weather is hot, but usually not…
This subject is truly extensive. There are a huge number of books that have been written about weather and many, many people far more knowledgeable than I who can wax lyrical about world weather systems.
Needless to say, it’s a good idea to keep an eye on the weather. Nobody wants to be caught out in bad weather and frankly, with so much good quality forecasting these days, it’s almost inexcusable. Weather forecasts can be found in newspapers, on the radio and on the television, on regular VHF broadcasts, on the internet, but even easier than all these options, try using ‘Mk 1 eyeball’; stick your head out of the window!
Essentially though, the weather we need to know about falls into three categories.
1. Depressions
2. Sea breezes
3. Fog
Depressions occur when warm, wet wind crossing the Atlantic, picking up moisture as it goes, reaches the Polar Front to the north of our latitudes. The cold Polar winds try to overtake these warm, wet winds from the southwest and set up an anti-clockwise, upwards spiral of wind. As the warm, wet winds rise, they cool. Cold air isn’t capable of carrying as much moisture as warm air and so the moisture is released as precipitation - that’s rain to you and me. This cycling air produces steep pressure gradients and as such, the resulting wind can be very strong.
The classic timeline of a depression moving through is as follows:
• Falling barometer
• Lowering cloud base
• Complete cloud cover
• Rain
• Veering wind direction as the warm front arrives
• Steadying barometer
• Easing of the rain to a continuous lighter rain or drizzle, in the warm sector
• Rising barometer, as the cold front arrives
• Thunder clouds, often with the thunder
• Gusty winds
• Showery rain
• Veering wind direction
• Crystal clear skies, with fluffy white clouds
Keep your eye on the barometer. Make a regular note of the readings when you fill in your deck log and you will instantly notice a change. A fall of 6mb in a two-hour period means head for port; there’s some bad weather due soon.
Sea breezes occur on sunny, summer days, when rapidly rising air, which has been warmed by the land heating up, sucks in air from the sea, producing the onshore breeze. The rising air eventually cools, falling back down over the sea and so the process continues until the evening when the sun goes in. Katabatic wind is the wind that falls down hillsides and cliffs, and thence out to sea, as the land, which during the day was hot, now cools, so cooling the air above it. This wind doesn’t last particularly long and personally, I’ve only experienced this in the Med.
Fog is caused when warm, wet air meets cold air. Such as around the UK in the spring. The waters are at their coldest at this time, and as the air coming across the Atlantic starts to warm up the resultant coming together causes fog. It normally burns off by mid afternoon. Likewise, cold air falling down hillsides into waterways in autumn produces fog when it hits the warm air hanging over the now warm water in our rivers and estuaries.
richard.corbett@sealine.com
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