Whether it comes as a gentle, brow-cooling zephyr or a roaring gale, wind is a constant presence on Earth, ruffling the planet's surface with its unseen touch, driving the waves, moving the storms, and bringing relief or heat and parching dryness to the local weather. Winds can be pleasant or destructive, and they are intimately involved in the weather cycles of the world, as both the cause and result of changing atmospheric conditions.
Wind is the movement of air -- a lot of air, covering at least a few square miles and often blowing over much larger regions. Surface winds can be very localized -- the onshore breeze from a small lake at certain times of day, for example, or a wind blowing down from a mountainside as temperatures change over the course of the day. On a larger scale, enormous wind patterns such as the jet streams and other mighty air flows determine how the different air masses of the planet move and interact, cause or quell storms, and create rain forests or deserts.
What makes the wind blow
Winds blow over the Earth based mostly on differences in air pressure between regions. A high pressure area is an area where air is sinking towards the ground, while a low pressure area is one where air is rising. Since air cannot penetrate the ground, it spreads out from the edges of a high pressure area and is drawn towards areas of lower pressure. The stronger the high pressure or the low pressure areas, the faster the wind will blow as it is pushed out of one and sucked into the other.
The Coriolis effect, which is caused by the spin of the Earth, makes the path of winds curve over a distance, even when they are actually traveling in a straight line. This is because the Earth itself has changed position before the winds arrive at a point they would otherwise reach, so they tend to curve to the right relative to the Earth in the northern hemisphere, and to the left in the southern hemisphere.
Small, localized winds are caused by pressure differences due to local heating and cooling of the atmosphere. For instance, a breeze will blow off a lake onto the shore on a hot day because solar heating causes the air over the land to rise, sucking cooler air in off the lake.
The huge winds of the whole world, such as the jet streams and the trade winds, result from much the same forces as the light breeze off a summer lake, just on a much larger scale. Air is heated at the equator, meaning that it rises massively all around the waist of the world, and streams out to the north and south from this permanent low pressure area. The Arctic regions, bitterly cold, witness high pressure and sinking air, making air flow outwards from these areas as well. Many other zones exist, such as the Hadley cells above the world's deserts, which influence the wind patterns of the planet, but all operate on the basis of pressure, usually driven by temperature and moisture but sometimes by topographical features such as mountain ranges as well.
The strength of winds
Winds at the surface are slowed by friction -- the ground literally exerts drag on the air passing over it. This is fortunate, since the winds aloft are so powerful that if they could occur at ground level, it is unlikely that life could ever evolve in the perpetual planet-wide supertornado that would result. Winds can be steady or gusty, and can vary from the barest movement of the air to well over a hundred miles per hour, depending on conditions.
The speed of the wind is determined by the strength of the pressure difference that is causing it, as well as local topography. If there is a strong pressure difference -- as when a line of storms is approaching -- then the wind will grow strong, gusty, and possibly violent. Weaker pressure differences will result in soft, sometimes aimless winds, or steady but mild breezes.
There are several ways to measure wind -- with an instrument, or, more practically for most people, by observation. The Beaufort Wind Scale is a series of visible wind effects that allow you to judge to approximate force of the wind. The scale includes 16 steps, although 12 are enough for all but the most destructive hurricane or derecho winds. There are different lists of observed effects for land and sea. For example, wind force 3, which is a 'gentle breeze' of around 8 to 12 miles per hour, causes waves of 2 to 3 feet with a few whitecaps, while on land, leaves and twigs are in constant motion, but small branches are not yet moving.
The effects of wind
Winds have countless effects on the climate and weather of the Earth. At the biggest scale, they distribute air mass from their source regions, moving heat and cold, moisture and dryness across the globe and allowing climate as we know it to exist. These winds, including the Trade Winds and the Westerlies, are part of the air circulation system for the whole globe and are crucial to nearly every kind of weather we see on a daily basis.
On a more local scale, wind can be both beneficial and destructive. On the beneficial side, it disperses pollutants -- both natural and man-made -- and frees us from their effects by diluting them or carrying them away. It can be harnessed to move boats and ships by using a sail, and windmills can generate electric power in a clean, environmentally friendly way. It also ushers in weather systems that bring needed rain -- or sun to dry out the landscape and prevent plants from dying from oversaturation after prolonged deluges. On the destructive side, it can erode topsoil, snap trees and power lines, damage roofs, and even kill large numbers of people if it strengthens into a derecho, tornado, or hurricane.
Only in the deepest caverns are we free from the eternal, insistent presence of wind. Wind is a key part of our planet's climate cycles and its complicated patterns, arising from simple causes, are one of an essential handful of factors that make life possible on Earth.