By Airwaves writer Lucas Masiello
As sailors, we participate in a sport that is uniquely dependent on the weather. Everything from wind shifts to puffs and lulls is all products of both large-scale and local meteorological factors. However, aside from offshore racers and elite level course racers, not too many sailors know what is going on behind the scenes in the atmosphere that makes the conditions what they are. We learn very early on in our careers how to spot puffs and lulls and what to do on a lift or a knock, yet have no idea what causes these crucial factors in our sailing. This article will provide you with the basic tenants of how wind interacts with land and water to give you the information necessary to not only react to but understand and predict what is happening with the weather on a racecourse.
The first important factor to consider before pushing off the dock is the type of breeze you are in. While there are many different types of wind, the two that are important to your sailing are gradient and thermal breeze. Gradient wind is the wind caused by systems in the atmosphere that is unimpeded by anything happening on the Earth’s surface. It is very stable in direction and is identical to what a pilot would experience 500 feet above sea level. You know you are sailing in gradient breeze if it is windy in the morning, forecasted to be stable(ish) in direction all day, or if there is not a large temperature difference between the air and the water. Thermal breeze is wind caused by the difference in temperature between the land and the water. While you can get a land breeze late at night during the winter, the only type of thermal breeze that affects course racing is the sea breeze. A sea breeze happens when the land heats up over the course of the day, and the water stays cool. The hot land and cold water affect the air around them and cause a pressure difference between the air on land and on water (remember, heat rises) and the cold air moves to land to fill the vacuum that the hot air left by going up. You can predict a sea breeze if there is light or no wind in the morning and at least 5 degrees of temperature difference between the air and water. If you are sailing in an area with a predicted sea breeze, the shift to the sea breeze direction (ask a local) is going to be the driving factor in your racing strategy throughout the course of the day. If you are sailing with a predicted gradient breeze throughout the day, other factors will likely be more important. (Note: sea breezes are only possible in areas where large bodies of water meet land, so they do not occur on small lakes or offshore.)
The second factor to consider when strategizing before a race is the geography of the course. In the northern hemisphere, breeze shifts left when travelling over land. This is caused by greater friction between the wind and the buildings, cars and other objects wind has to travel through that it would not have to if over water. Because of this, wind on a course if affected by land parallel to the wind direction. If there is land to the left of the course, the breeze travelling left over land converges with the wind on the left side of the course; causing there to be more breeze out left. If there is land to the right of the course, the left shift over land pulls breeze away from the course; creating less breeze out right. This is why the left always works when there is a northerly in Annapolis or any direction except a westerly in Miami.
A third important factor to consider when developing a race strategy is the clouds. While there are many types of clouds that all have effects on the breeze or can be used to predict breeze, the most important type to analyze is low-level cumulus clouds. Cumulus clouds are the big, white puffy clouds that are usually very low to the surface that are often found in coastal areas. They are created by an updraft of moisture in the air, so can be telltales of a developing or developed thermal breeze. On a sunny day, there is high pressure under the cumulus cloud pushing it up and low pressure on the sides filling the air vacuum, meaning that there are lulls under low-level cumulus clouds and puffs on the edges of the clouds. Because of this, if you follow the edges of cumulus clouds, you will always be in a better breeze than boats that sail through the cloud or avoid it altogether.
Meteorology is the branch of science that plays the biggest role in sailing, and to understand what is happening above us is to understand what is happening and what will on the course. While this article provides extremely basic information that can help you on the course, there is an incredible depth of information in the field that can help you develop not only strategy but your own weather forecast. While apps like SailFlow can help you get a general idea of what the wind is going to do, very few of the weather buoys that they have posted are actually monitored by real people and can be incredibly inaccurate. If you want to further your understanding of the wind, I would recommend learning how to read a surface weather map. SWM’s are daily maps produced by NOAA that every meteorologist uses to predict weather on land and at sea and can often lead you to far more accurate predictions than simply trusting your sailing weather app. To not understand the weather conditions around you is to sail blind, and a sailor who can analyze and predict weather conditions both before and during racing will always have an advantage on sailors that simply sail towards choppy waters and tack on knocks.