If you have ever tried to hit a 7-iron into a stiff breeze, or struggled to keep your umbrella from being torn apart by a gust, or attempted to get a kite airborne in the dog days of summer, you know the importance of the wind on everyday activities. But, it can be argued that no activity is more affected by the wind–or lack thereof–than boating.
Wind impacts everyone and everything on the water. The sailor depends on it for power. Surfers decide whether to sleep-in or hit the beach based on the wind's direction and speed. Even motorboats are affected, as any trawler captain who has tried to dock in 20 knots will tell you. That's because winds cause waves–and the size of those waves is predicated by how hard the wind is blowing, how long it's been blowing and how long the "fetch" is.
Any boat is more nimble working with the wind rather than fighting it. Spend a day slamming dead to windward and you may wonder why you thought boating would be so much fun. Spend a day skipping across the sea with a perfect breeze abeam or abaft and you know why.
Ultimately, winds are created by the unequal heating of the Earth's surface. Because the Earth is a sphere, the sun's rays shine more directly on the tropics than at the poles. Weather is the by-product of the atmosphere attempting to reach equilibrium as heat from the tropics is transported toward the poles. Hurricanes, mid-latitude cyclones, high pressure systems and low pressure systems all create winds that redistribute heat across the globe.
The reality is that this is no-win battle, because the sun never ceases to favor the tropics, but it does produce some spectacular weather events. And wind plays a role in all of them. For boaters who want to learn more about weather, understanding how wind works is a great place to start. It means dipping into the sciences a bit, but it is a worthwhile exercise.
SUM OF ALL FORCES
Wind speed and direction are determined by the sum of three forces that act upon air: Pressure gradient force, Coriolis force, and the force of friction. To illustrate Pressure gradient force, we can use a simple example. Everyone is familiar with pumping up a bicycle or car tire. A pump uses energy to force air into the tire. As more and more air is pumped into the tire, the pressure rises. When the valve of the tire is opened, the air rushes out. The rate at which air escapes is related to the pressure difference between the inside and outside of the tire.
The same thing holds true for pressure differences at the surface of the Earth. Air flows from areas of higher pressure to areas of lower pressure. The difference in pressure determines whether that air flows as a gentle breeze or a stormy gale. This force, caused by the difference in pressure, is called the pressure gradient force. Strong winds are the product of the difference in pressure over a geographic area. However, unlike the tire analogy, the atmosphere is much more complex: Winds cover a much larger area and flow over a rotating Earth.
The Atmosphere, 8th edition, Lutgens and Tarbuck, 8th edition, 2001Cyclonic flow around a low pressure center and anticyclonic flow around a high pressure center. Pressure gradient force and Coriolis force are balanced.
