Skipper’s Tips #15 – Whether the weather is hot…

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This subject is truly extensive.  There are a huge number of books that have been written about weather and there are many, many people far more knowledgeable than I, who can wax lyrical about world weather systems.  That said, I do know some of the pertinent bits and that’s what I hope to pass on to you today. 

It goes without saying that 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, as boaters, 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, comes into contact with the Polar Front, which lies generally to the north of our latitudes.  The cold Polar winds will get underneath these rising, 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 eventually, the moisture is released as precipitation – that’s rain to you and me.  The difference in air pressure from the outer edge of the depression to the inner centre varies hugely over relatively short distances – we can see this when we look at synoptic weather charts showing the clustering of isobars around the centre of the depression. These tightly packed isobars indicate the large pressures gradients involved, which to you and me essentially means strong winds.  In fact, the closer together the isobars, the stronger the winds.  If you can get hold of a print out of a synoptic chart, there should be a scale on it, which will allow you to measure the precise wind speeds.

Here’s something to consider the next time you get a chance to look at a synoptic chart. In the Northern Hemisphere, the wind cycles anti-clockwise around a low and clockwise around a high.  So, if you get a High and a Low next to each other the wind is likely to be great where they meet as you will have two wind systems effectively going in the same direction.  However, when you get two Low’s next to each other, they cancel each other out at the point at which they meet.  The resulting wind, at this point, will most likely be light and variable in direction.

If we know what to look for we should be able to spot a weather system coming and this will allow us to make an informed decision on whether to go to sea or not.  So, what does an approaching depression look like and what are the clues?

The classic timeline of a depression moving through is as follows:

  • Falling barometer
  • Lowering cloud base
  • Rain
  • Reducing visibility
  • Complete cloud cover
  • Veering (clockwise) change in 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
  • Excellent visibility

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.

Clearly, given the basic level of our weather forecasting skills (up to Yachtmaster level), I would still recommend that you also compare what the traditional weather sources are telling you with your new found skill of being able to spot a depression – the weather we experience in and around the UK is very varied and subject to quick changes. Weather systems continually speed up and slow down, often arriving early or not at all.  

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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 blows out to sea from the land. As the land, which during the day was hot, cools down, it cools the air above it. This, now heavy, cool air tumbles down the hillsides and coastal cliffs, rushing out to sea.  This wind effect doesn’t last particularly long and personally, I’ve only experienced this in the Med. 

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Fog is caused when warm, wet air gets chilled revealing the moisture content as fog. The classic example of this is in coastal UK waters in Spring, we call this Advection Fog or more affectionately, Sea Fog . The waters around the UK are at their coldest at this time. As the warm, moist air coming across the Atlantic meets the cold waters around the UK, the chilling effect reveals the moisture as fog.  It can often burn off by mid-afternoon but occasionally it can hang around for a few days until the wind direction changes and the temperature differential changes.  I’ve seen waves of fog plague islands in the early Summer months and even in August on Jersey one time when I was delivering a new boat to a customer and got fog-bound myself. 

Radiation Fog or Land Fog, as some call it, happens mostly in the Autumn months. After a warm, sunny September day, the land, which has been warming up during the day, will chill down quickly under clear skies.  This chilling cools the air lying over the ground, which in a Katabatic Wind style, tumbles down into valleys and estuaries where it meets warm, moist air lying over streams and rivers.  The chilling effect of this cold air produces the fog – which gives us those romantic looking photographs that people like me love to capture.

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Please don’t base your boating plans on a forecast you had five days ago – 12 to 24 hour forecasts are going to be the most accurate and this is what you should be looking at before you decide to head down to your boat. Then, before setting off on your journey, get the latest forecast as published by the marina.  Remember too, that the weather you’re feeling inside the marina will often bear no relation to what’s actually going on at sea.

This has been another Captain Corbett’s Adventure.  If I’m not on Jersey teaching a private tuition Day Skipper theory or Yachtmaster theory course, then I’m either spending time with someone on their boat, giving them the confidence to take their boat out with their family and friends on board or I’m off somewhere exotic delivering a boat. Either way, I’ll write it up and put it on the Blog for you all to see, so keep popping back to see my most recent adventures.