How to predict combination afternoon breezes
by John Jamieson on 10 Oct 2012
How can you predict the effects of a sea breeze when combined with a coastal wind? Will this mean sail reefing this afternoon or a wind light enough to fly a spinnaker? Learn how to forecast coastal wind effect with these sailing secrets from John Jamieson (Captain John):
Coastal sailing - how will I know what the wind will do this afternoon? .. .
Winds caused by pressure differences create prevailing winds. After blowing for several hours, a prevailing wind aligns itself in a direction, parallel the coastline.
If local sea breezes are strong enough, they can modify the direction and speed of the prevailing coastal wind.
These effects might not be included in a marine weather forecast. Use these easy steps to make your own sailing weather forecast:
1. Listen to the forecast prevailing wind
Tune in to the closest weather channel on your marine radio. Copy down the forecast coastal winds direction and speed.
For example, let's say that a northerly wind has been blowing for several hours along the coast. The marine forecast calls for northerly winds at 15 knots this afternoon.
2. Estimate the afternoon sea breeze
Make a determination as to whether to modify the forecast based on cloud cover. Cloudless days mean stronger afternoon sea breezes. Partly cloudy or overcast days indicate lighter sea breezes. In our example, you observe a cloudless sky, so you should expect a strong easterly sea breeze to fill in by early afternoon.
3. Combine directions and speeds
In the afternoon, the prevailing wind and sea breeze will combine to produce an average direction--northeasterly in the example. The strong sea breeze could increase the prevailing wind speed by 50%, or up to 22 1/2 knots.
Whether the winds increase that much depends on meteorological conditions, but you and your sailing crew will be ready in any case.
John Jamieson (Captain John) with 25+ years of experience shows you the no-nonsense cruising skills you need for safer sailing worldwide. Visit his website at www.skippertips.com and sign up for his FREE highly popular Sailing Tip of the Week. Discover how you can gain instant access to hundreds of sailing articles, videos, e-Books and more!
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