INTRODUCTION: WIND IS AIR IN MOTION

The air we breathe and the air that surrounds us is made up of atoms, which also form the building blocks of our physical world. The wind touches every continent on the planet.

A complex vocabulary exists in many languages, describing the wind and its effects. The words for wind carry meanings related to both human existence and philosophy. “Kona,” for example, is the name for a stormy, rain-bearing wind from the southwest in Hawaii. Kona is a Polynesian word meaning “leeward.” In Hebrew, the word for wind, or "ruach" (roo-ahck), means both breath and spirit. And in fact, wind is breath. We create a tiny gust of wind every time we inhale and exhale. We could even say that the wind blowing around the world is the planet’s way of breathing.

The wind’s behavior changes every hour of every day because of temperature variations, the earth’s rotation, the landscape and the seasons. In order to understand how wind is captured to make electricity, it helps to know about what makes the wind work.

Wind is actually a form of solar energy. As our planet rotates around the sun, the sun’s energy heats great amounts of air, and then the absence of the sun’s energy in other places cools other masses of air. Temperature dictates in large part the wind’s movement. Hot air rises. Think, for instance, of a hot air balloon. Balloonists use special heaters to fill the balloon with hot air, causing the balloon to rise, carrying the basket and its passengers with it. The same thing happens when the sun heats the air. Sometimes the difference in air temperature is even visible. Swirling waves of air can move around on the exterior of a car on a hot day, for example. These effects are actually created when small areas of hot air rise through colder air. Different temperatures move the air around and can even change how thick it is.

Temperature differences in the air produce air circulation. Air can circulate in a small place, like on the top of a car, or in a large space, like the entire planet. Hot air is lighter and rises into the sky. When it reaches an altitude of about six miles, it spreads to the north and south. As the air cools, it sinks down again. This constant motion forms huge movements of air in large circular patterns all over the earth’s surface. We experience that moving air as wind.

Some places on the planet’s surface are windier than others. If you’ve ever walked in a city on a windy day, you might have noticed how the walls of the buildings allow the wind to blow as if it were going through a tunnel, speeding up the movement of the air into quick gusts. That’s because in those alleyways formed by the streets, sidewalks and walls, there isn’t anything to slow the wind down or stop it. Similar effects are found where the wind can blow across large bodies of water unhindered, or over very flat landscapes.

On the other hand, obstacles can affect the wind’s direction and force. Mountains, buildings and trees, for example, can be large enough to redirect the wind and change the potential amount of energy we collect from the wind using windmills. Some obstacles affect the wind more than others. For example, when trees are covered in leaves, less space is available for the wind to flow through the branches. Instead the wind must go around the tree, losing some force. But bare-branched trees do not block as much wind as trees with leaves on them. The spaces between a tree’s branches allow the wind to flow through, although the branches do slow some of it down.

Many aspects of the wind—such as its seasonality, primary direction and strength—can be measured and accurately predicted over specific time periods. For example, in most areas of the world, wind comes from a prevailing direction most of the time. But subtle changes in the atmosphere and on the earth cause the wind to vary on a short-term basis. That’s why sometimes your hat blows away in a sudden gust and other times you can fly a kite in a constant wind for an entire afternoon in the very same spot. Nighttime winds are generally different from daytime winds. This is because temperature differences (between the sea surface and the land surface, for example) tend to be larger during the day than at night, which means more air movement. Location makes a big difference as well. In California, daytime winds are stronger, but in Iowa they are weaker.