Understanding Wind

What is wind anyways? Quite simply it is due to moving air masses responding to pressure and temperature differences. As we discuss, keep in mind that air tends to move from areas of high pressure to low pressure, and heat tends to move from hot to cold.

If you’ve ever travelled close to the equator, you’d be right to realize that this part of the Earth gets significantly more solar heating than latitudes further north or south. At the equator, this surface heat is transferred to the atmosphere causing the air to rise. On a global scale, the warm air at the equator is trying to mix with the cool air at the poles.

The rising hot air at the equator creates a low-pressure area and the cooler air further north or south attempts to fill the void (nature generally doesn’t like vacuums). This is wind. But we’re not done yet!

Global air circulation. Source: “Oceanography” 6th Ed. Garrison, T.

Enter the Coriolis effect, stage right. Every 24 hours all points on the Earth complete a 360 degree rotation, marking one Earth day. However, not every point on the globe moves at the same speed.

Understanding the Coriolis effect; Source: Calvin and Hobbes by Bill Watterson

The Coriolis effect.

Points near equator are making a longer trip than those at the pole, meaning that points at the equator must move faster to complete their circuit within 24 hours. This effect causes an appeared deflection in objects that transit from one latitude to another. In the northern hemisphere this deflection is to the right, and in the southern hemisphere to the left.

As the air at the equator rises into the atmosphere and attempts to move poleward it is deflected due to the Coriolis effect. As the air travels it becomes cooler and denser as it radiates heat back to space. This cooler air sinks backs towards the Earth’s surface to start the process all over.

 

This circuit of moving air masses is referred to as a cell, and six of them are responsible for our planet’s atmospheric circulation (see image above). This is what generates tradewinds and westerlies, those dependable surface winds that sailors depend on.

These same basic principles also apply to more local air movements.

Let’s consider an island for example. On a sunny day the land and sea will heat at different rates, this time due to specific heat and thermal mass. The land will warm faster causing the air above it to rise. Cool air over the water then rushes in to replace the hot air rising over the land, creating a see breeze. At night the reverse can happen as the warm ocean water is now hotter than the land, generating an offshore breeze.

Onshore and Offshore breezes

Since wind is the result of uneven solar heating on the surface of the earth, some people refer to wind energy as a form of solar energy. However, a wind turbine is not harvesting solar energy, but the kinetic energy of the moving air.