Wind is moving air. The Sun heats up some parts of the Earth more than others and the wind spreads this heat more evenly around the world. The direction of wind is expressed as the point of the compass from where the wind is blowing. Air moving from the northeast to the southwest is called a northeast wind. It may also be expressed in degrees from true north. A northeast wind would be 45°. A southwest wind would be 235°. The wind speed can be expressed in miles or kilometres per hour, meters per second, knots or as a force on the Beaufort scale.

HOW DOES IT BLOW ?

Winds begin with differences in air pressures. Pressure that's higher at one place than another sets up a force pushing from the high toward the low pressure. The greater the difference in pressures, the stronger the force. The distance between the area of high pressure and the area of low pressure also determines how fast the moving air is accelerated. Meteorologists refer to the force that starts the wind flowing as the "pressure gradient force." High and low pressure are relative. There's no set number that divides high and low pressure.

Once the wind begins blowing the Earth's rotation changes its direction. This is known as the Coriolis effect. In simple terms, as air begins flowing from high to low pressure, the Earth rotates under it, making the wind follow a curved path. In the Northern Hemisphere, the wind turns to the right of its direction of motion. In the Southern Hemisphere, it turns to the left. The Coriolis force is zero at the equator.

MEASUREMENT OF WIND

The two most important things about the wind are its speed and direction in which it is blowing. We use a weather vane or a windsock (a kind of long cloth tube through which the wind is funnelled) to see wind direction. It is expressed in compass points. Wind speed is measured by the Beaufort Scale, windsocks or by special scientific instruments called anemometers. The unit of measurement is kilometres per hour (km/h) or knots.

The Beaufort Scale The Beaufort Scale was invented in 1805 by Admiral Beaufort to estimate wind speed through observations of objects. The original scale was for use at sea but it has been adapted for use on land.

windsock

anemogarph

The sea breeze. A sunny, warm April morning with temperatures in the 70s begins a beautiful spring day in New Jersey. As you and the family go on a picnic to take advantage of the great weather, a sudden chill roars through the region and a stiff easterly wind quickly drops temperatures into the 40s. This often occurs near the coast during early spring warm ups. It is commonly known as a sea breeze.

1. Warm air over land rises

2. Sea Breeze moves inland

3. Cumuli develop aloft and move seaward

4. Upper level return land breeze

5. Cool air aloft sinks over water

6. Sea Breeze (meso-cold) Front

The Land Breeze. As the sun sets, cooling begins along the surface of the land and sea. Like daytime heating, cooling occurs at different rates over water and land. The rapidly cooling land soon has a higher air pressure over it relative to that over the sea, and the air begins to flow down the pressure gradient seaward. This is the land breeze. It too is influenced by the roughness of the coastline, strength of the large-scale winds, and coastal configuration. Unlike the sea breeze, the land breeze is usually weaker in velocity and less common. The land breeze is often dominant for only a few hours and its direction is more variable. Nevertheless, the land breeze can penetrate the marine atmosphere for 10 kilometres (6 miles) seaward.

1. Cool air over land sinks

2. Land Breeze moves out over water

3. Relatively warmer water heats air which then rises

4. Upper level return sea breeze

UPPER WINDS

Upper winds can be found at least 10 km above the ground, such as jet streams.

Jet Streams. Jet streams are very strong winds blowing about 10km above the Earth. They can be up to 4, 000 km long but no more than 500km wide. They were not discovered until World War II when pilots found their air speed reduced when they were flying against the jet stream. Wind velocity is highest in the core of the polar jet stream where speeds can be as high as 300 kilometers per hour. The jet stream core is surrounded by slower moving air that has an average velocity of 130 kilometers per hour in winter and 65 kilometers per hour in summer.

1. River of air forms high above cold and warm air boundary

2. Jet stream has core of fastest winds surrounded by slower winds

3. Air planes tht fly near jet stream often experience turbulece

Winds blowing at different speeds at different altitudes create turbulent eddies.

Winds blowing at different speeds at different altitudes create turbulent eddies.