Get expert advice on weather forecasting from Chris Tibbs and David Houghton
There’s no such thing as a ‘typical’ Cowes Week, says meteorologist Chris Tibbs. Still he offers his best forecasting advice for the Week
Almost completely surrounded by land, the Solent is a tricky place to forecast for. Early August sees some of the warmest days of the year, adding a thermally driven component to the local wind and the shape of the land adds a geographical modification.
But no two Cowes Weeks are ever the same. The days when the thermal effect is small are when we have a large synoptic scale system affecting the area – this is when a depression arrives from across the Atlantic. The strong wind and minimal land heating reduces the thermal effect, but the geographical effect can be significant.
With the wind from the south-west or west there is a funnelling effect up the western Solent. This gives us a stronger wind than may be expected.
When the wind is closer to south, the funnelling effect is lost, as the Isle of Wight shelters the Solent, but large shifts and gusts are to be expected. Once the fronts are through, and the wind veers to the north-west, squally conditions predominate.
While we may well get a depression passing through, most of Cowes Week is likely to be sailed in slack pressure as the Azores High sits to the south and the depressions are closer to Iceland than Ireland. This gives us the classic conditions of light breezes in the early morning and a good sea breeze in the afternoon.
The tricky part is predicting just where and when the sea breeze will pick up. The easy one to predict is if we already have a light south-westerly in the morning. This will not generate a true sea breeze but the wind will slowly increase through the day to reach a peak by mid-afternoon. How strong it becomes is a combination of the gradient wind and the thermal effect.
Our classic case of a true sea breeze from the south-west is when the early morning breeze is from north or north-west. However, the wind will go calm for a while before the sea breeze starts and there will be some big shifts around until the heating of the mainland overrides the island’s influence.
Watching the clouds build over the land shows the beginning of the sea breeze; the earlier the clouds appear, the earlier and stronger the sea breeze is likely to be.
Forecasting a sea breeze when the early morning wind is from east of north is more difficult. This is when there is high pressure over the land and lower pressure over the sea. Although we may think of the sea breeze as the land heating up, the air rising, to be replaced by cool air from the ocean, this is an over-simplification. It is driven by a pressure gradient, with lower pressure over the land than the sea. If we start with higher pressure over the land we need to reverse this gradient before a sea breeze is possible.
So when we have a light easterly breeze at breakfast time, we are likely to have a poor sea breeze – probably from the south-east to start with and maybe swinging to the south-west later. The last two years have seen a greater than average number of days like this, giving the peculiar Solent sight of boats sailing from the west and from the east under spinnaker.
If the mainland remains free from cloud, no matter how hot the land gets, the breeze will stay very light all day and we are likely to need the kedge.
No two days are ever the same. Keep listening to Cowes Radio for the latest weather updates and the racecourse commentary often has tips about wind strengths in different parts of the Solent. Other yachts make great wind indicators and, although the forecasts will give a general idea of what to expect, keep watching, as there will be many local variations.