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.

If air just followed the pressure gradient force, you would see surface winds flowing out of high pressure centers and rushing toward low pressure centers in all directions. Clearly, this is not the case with large-scale systems (greater than 100 kilometers). Winds in the Northern Hemisphere blow counterclockwise around low pressure centers (cyclonic flow) and clockwise around high pressure centers (anti-cyclonic flow). Why? There is a second force acting on the air called the Coriolis force.

The Coriolis force is an "apparent" force caused by the rotation of the Earth underneath the air flow. The Coriolis force is a very weak force and only becomes significant on a very large scale. One common myth is the belief that toilet water flows in an opposite direction in the Southern Hemisphere because of the Coriolis effect. Not true. At scales of just a foot, the Coriolis force is negligible.

However, artillery must take the Coriolis force into account when firing shells over large distances. The Earth's rotation causes the shell to "appear" to move to the right in the Northern Hemisphere. The same thing happens to air. As winds flow from high pressure to low pressure, the rotation of the Earth "pushes" the air to the right. When the pressure gradient force and Coriolis force become balanced, air flows cyclonically and anticyclonically around lows and highs, respectively.

FORCE OF FRICTIONThe Atmosphere, 8th edition, Lutgens and Tarbuck, 8th edition, 2001Friction slows surface wind speeds (which in turn reduces the Coriolis force) causing the wind to shift toward the low pressure center.

Friction is an "opposing" force that decreases the magnitude of air flow. Friction also causes winds to blow slightly toward a low pressure center and away from high pressure centers. With just pressure gradient force and the Coriolis force, winds blow parallel to isobars on a map (lines of equal pressure), as the pressure gradient force is balanced by the Coriolis force. With friction added, winds blow slightly across them. This can be explained by the fact that the Coriolis force is dependent on wind speed. As winds are slowed by friction, the Coriolis force decreases, and the wind is pulled toward the pressure gradient force, meaning toward low pressure and away from high pressure.

Look closely at today's surface weather map. The winds are flowing around lows cyclonically and slightly across the isobars toward the area of low pressure, while the opposite will hold true around high pressure centers. Winds blowing over water are subject to less friction than winds blowing over land. Buildings, trees and terrain all inhibit air flow. As a result, the wind frequently blows stronger on the water than ashore. The friction between water and air, though much less than over land, creates waves. That's why high winds and high waves go together. It also explains why wind right at the surface is slower than even a short distance aloft–thus the need for sails have some twist from bottom to top for maximum efficiency.

Forecasters can predict wind speed and direction by analyzing the formation and movement of large scale low and high pressure systems. Of course, weather prediction would be a lot easier if there were not complications everywhere from local effects, interactions between land and water and weather phenomena like thunderstorms. But by understanding the forces involved in determining wind speed and direction, you can learn what to look for when gathering weather information for a day out on the water.

NOAAA weather map from Nov. 7, 2007 showing the Atlantic Ocean Basin.