The common explanation is that as the land warms up during the day, the warm air above the land rises. Over the remainder of the day this wind not only slowly gets stronger, but also slowly turns right (due to the Coriolis effect, left in the southern hemisphere).
On a classic sea breeze day, we go from no wind in the morning to suddenly seeing a light wind developing from an onshore direction. The other important coastal effect that models struggle to get right is sea breezes. After the sun goes down, the surface starts to cool down again, and the mixing process becomes weaker. By the late afternoon the mixing reaches up to 2000m and brings winds down from these heights.
The last bit is of course what sailors are interested in!Īs the day progresses, the heating effect gets stronger (above), and the mixing slowly reaches higher altitudes, bringing stronger winds from higher levels down to the surface, which gives an increase in the wind over the day. The sinking bubbles not only bring colder air from higher levels, but also stronger winds. We end up with a mixing process with constantly rising and sinking bubbles of air. The air that rises is replaced with colder air that is sinking down from higher levels. This creates a layer of warm air underneath the still colder air above it which, like a hot air balloon, will rise until it finds air of the same temperature (above). When the sun comes up in the morning, it heats the surface of the earth quicker than the air above. To understand why this difference in windspeed changes we need to understand ‘mixing’. The first fact is that due to less friction, winds at higher altitudes are stronger than near the earth’s surface. This difference in windspeed with height is significantly greater in the early mornings than in the afternoon – certainly much more than can be explained by friction alone. So, what is going on? Why was the forecast so far off in the morning? There are two principles behind this. But by starting time the wind has picked up to 12 knots and it’s a rather gustyday. Some mornings, the sea can look like a perfect mirror even though the forecast is for a north-westerly 10 knots, gusting 18.
LAND BREEZE FULL
With big summer races such as the Round the Island approaching we speak to pro navigator, meteorologist and sailor Wouter Verbraak about his top tips on understanding the behaviour of winds for coastal racing, paying particular attention to mixing and sea breezes.įor more on this coastal weather forecasting, see the full piece from Wouter in Yachts and Yachting May 2014.
0 kommentar(er)
