By Airwaves writer Eric Tobias
(Editors note: To start at the beginning of this great series, check out Part 1 here: https://www.sail1design.com/using-weather-dominate-competition/)
Sea Breeze I was once racing a distance race, and we were probably about 30 hours into the race at this point. It was extremely light wind and we were beating to windward with all of the crew hiking on the low side rail. Less than a mile abeam of our boat, our rival competition was riding the shoreline just flying past us with (get this) a full spinnaker. I thought to myself, “How is that possible?” The answer: sea breeze.
If you can understand the fundamentals of sea breezes, it can help you immensely any time you’re racing near a coast or shoreline. It’s important to note that a sea breeze isn’t as basic as a “warm air moves towards cold air” situation. Additionally, any sailor who has experienced or utilized a sea breeze will tell you that timing is very important. A sea breeze is a day-to-night vertical wind circulation resulting from the temperature differences between the land and water. If you have a cold body of water and a hot summer day, around mid-day/early afternoon the air over the land will heat up and rise into the atmosphere. At the same time, the air over the water will cool down and sink. In order to complete the circulation, the air up high will move from land towards water, and the air down low, where we sail, will have wind moving from the water towards the land. The opposite is true for a warm body of water and cooling land around nightfall (technically called a land breeze). A good way to tell how strong a sea breeze is going to be is to look at the air temperature reading on a NOAA buoy compared to the air temperature on shore: bigger difference means a stronger sea breeze close to the shoreline. Just remember that sea breeze is a vertical circulation, and not just a surface wind.
It’s also important to note that the sea breeze is not necessarily going to be 100% of the true wind speed and direction at the time. Sea breeze works in addition to whatever the current wind conditions are in the area. This also means that if the true wind is blowing the opposite direction of the sea breeze effect, the sea breeze will work to suppress the wind speed. When there’s otherwise really light or no wind is when sea breeze has the largest influence. Let’s check out a sailing example:
Because of the effect of the sea breeze, the true wind near shore shifts from north to northwest. This allows the red boat to catch a nice lefty to pull ahead of yellow. Both boats are close hauled on port tack, and if their destination to the next mark is straight north, red has the better VMG thanks to the huge sea breeze lift.
Depending on the location, the water temperature and the time of day, a sea or land breeze can provide a lift, a knock, an increase in wind speed or a decrease in wind speed. A sailor can know which one it is going to be just by knowing the water temperature versus the land temperature and visualizing the effect of the vertical circulation on the wind in the area. Overall, sea breeze can be a dynamic tool to pass the competition. In part 3 of this weather series, we’ll check out the effects of precipitation on sailing, as well as provide a weather checklist to be ready for any given regatta.
For Part 3, go here: https://www.sail1design.com/using-weather-dominate-competition-part-3-precipitation/
By Airwaves writer Eric Tobias