Wind is a body of air that is in motion relative to the surface of the earth. It is what causes the weather and without it our model yachts would not move. What causes the air to move are the results from several factors, some global, and some local. In this article we will go over some interesting topics of wind and its effects. Without getting too deep in the reasons that causes air to move, there are some basic things we need to know.

The global cause of wind is the Coriollis force. In basic terms, the Coriollis force is the heating of the air over the equatorial regions of the earth, and at the same time, the cooling of air over the polar regions. This heating and cooling over the different parts of the earth causes the air to move from the warm equator to the poles, and then back to the equator once the air cools. Because of the rotation of the earth, the movement of air is displaced due to the coriollis force, (see Example 1). The end results are the prevailing westerly winds in the Northern Hemisphere and the easterly winds in the Southern Hemisphere, (see Example2). The coriollis force is not really a force per se, it is used to describe an apparent force experienced by the observer within a rotating frame of reference. To better understand the coriollis force, take a ride on a merry-go-around and try to throw a ball to someone across from you. You can't; the ball will travel a curved path.

The primary local factors that causes wind is Pressure Systems and Frontal Systems. For the most part, these two factors work together to generate wind on the local level. There are two types of pressure systems, low and high. Air flows from a high pressure system to a low pressure system. Because of the Coriollis force, it causes a circular motion of air around a pressure system. In the northern hemisphere, air circulates in a clockwise manner around a high pressure system, and counter-clockwise around a low pressure system. This is why the wind, at times, changes from the normal west-to-east direction and blows in a different direction.

In the event of a strong pressure gradient between high and low pressure systems, the wind can be quite strong and steady. This is called a gradient wind. Gradient wind blows parallel to isobars (isobars are lines drawn on a weather map that connect places of equal atmospheric pressure; isobars close together indicate a steeper gradient of pressure and stronger winds, see Example 3). Geostrophic winds exist in locations where there are no frictional forces and the isobars are straight. However, such locations are quite rare. Isobars are almost always curved and are very rarely evenly spaced. This changes the geostrophic winds so that they are no longer geostrophic but are instead gradient wind. They still blow parallel to the isobars, but are no longer balanced by only the pressure gradient and Coriolis forces, and do not have the same velocity as geostrophic winds.

Frontal Systems are air masses of a warm or cool temperature, and when there is an abrupt difference between two such air masses, a front exists. When a cold air mass advances against a warm air mass, thunderstorms and high winds are along the cold front. Warm fronts are somewhat different in that the storms are more widespread and winds are steady. With either a cold front or a warm front, the winds can be unpredictable and strong, and greatly effect the local wind conditions.

Additional local weather patterns include the Sea Breeze and Land Breeze. In simple terms a sea breeze is when warm air rises off the land, flows aloft toward the sea where it is cooled, and wind flows back toward land. Land Breeze is opposite, warm air rises off the water, and flows aloft toward the land where it is cooled, and wind blows back toward the sea. You will notice this effect if you live and sail around the seashore or large inland lakes.

A final effect to local wind condition is the Local Terrain. Local terrain has a dramatic effect on the wind conditions in an area. Hills, buildings, and trees have a great effect on the wind's strength and direction. As the wind moves around and is blocked by the local terrain, the effects are felt upon the water. This will require the skipper to pay more attention when sailing to keep their model yachts on course.

Local terrain will also cause the wind to veer in different directions without warning. Gusts and lulls are also caused when the wind increases or decreases because of hills or trees around the pond. The worst example of oscillating wind is small rotating cells of wind. These cells of air which rotate across the water are caused by local terrain or rising or falling air pressure. When oscillating wind occurs, the wind can change in direction up to 90 or more degrees.

I hope this explains some basic principles on how wind is created. Next time we will go into some detail on the effects of wind on the water. Fair winds!