Dictionary Definition

Moving air, especially a natural and perceptible movement of air parallel to or along the ground.

Power Source

The wind gets its power from three major sources: solar radiation, the Earth's rotation, temperature differences, and landscape differences.

• Solar Radiation: The Sun powers the winds through its rays. The solar rays hit the planet at an angle, and a lot of the energy is deflected into space. The solar rays pass right through the upper atmosphere, not heating it, and strike the Earth to radiate the lower layers of the atmosphere. As this layer is heated, the air near the surface expands and becomes lighter than the colder air above it. So the warm air of the equatorial regions rises, creating an area of low pressure near the surface at the equator and increasing the air pressure above it. The rest of the earth isn't heated as greatly as the equator, and the pressure in the upper atmosphere isn't as great. The air pressure of the upper atmosphere is the lowest above the poles, so the high-altitude air pressure is in a slope with the highest over the equator and decreasing value up to the poles. Warm air slides down the slope, creating a global pattern of air circulation. Since cold air keeps close to the poles, a high-pressure area forms in the lower atmosphere over this region and the air circulating from the equator is constantly added to this patch of cold air. At middle latitudes, the air is warmed slightly, weakening the air pressure and allowing the colder air from the poles to flow toward the equator. This creates a second circulation pattern at the surface. Cool air flows from the poles to the equator, where it is heated and returned to the upper atmosphere to continue the cycle.
• Earth's Rotation: Since the Earth rotates, it slides any air that moves over its surface. The west-to-east rotation of the earth in space causes the Coriolis force, which in turn makes the global winds change course. For example, an airplane flying straight from the North Pole to where New York should be will end up over the Midwest because the earth has rotated. From earth, the route would look curved. In the northern hemisphere, winds are deflected to the right by the earth's rotation, while in the southern hemisphere they are deflected to the left.
• Temperature Differences: Land and sea absorb and retain the Sun's heat at different rates, which complicates the circulation of air. Water warms more slowly, so its surface reflects more of the Sun's rays than it absorbs. The rays that are absorbed go to a great depth, and the waves mix this warm water with colder layers beneath. The water's surface is also cooled by evaporation when the warmed water rises as vapor. Also, water retains the heat that it does absorb for a long time. Land, on the other hand, absorbs more rays than it reflects, but the rays don't go very deep. So, the the land heats rapidly during the day and cools quickly at night, making the land temperatures change more often than ocean temperatures. These differences change the direction of local winds because the warm air of the land rises rapidly and is replaced by a layer of colder air flowing from the sea (called sea breezes). Then, when the sun sets the warm air of the ocean rises and is replaced by the colder air from the land (called land breezes).
• Landscape Differences: The landscape can power the wind, as well. Building and paved surfaces of cities and towns radiate more heat than the forests and meadows of the countryside. So warm air rises over the city and the cooler breezes from the countryside replace it, creating a local cycle of winds. Irregularities in the landscape also affect the wind. A level plane without any obstructions allows winds to sweep across it regularly, but the trees of a forest or the buildings of a city break the winds into smaller winds, called turbulence.

Direction

Explanation

Winds can blow from many different directions, and sometimes its helpful to know which way its coming from and headed. The experiment below will help you determine the direction the wind is blowing. Also, you can read more about the different types of winds (and the directions they come from) in the Belts section.

Experiment #1: Wind Vane

What You'll Need:

• empty thread spool
• paper clip
• straw
• paper
• glue

Directions:

1. Cut an arrow out of the paper about eight inches long.
2. Cut a paper disk with a hole in the middle to fit around the straw and decorate with the directions (see picture at right).
3. Stretch out the paper clip and put it down the empty thread spool.
4. Put the drinking straw over the pin, making it easy for the straw to move.
5. Slip the disk down the straw and glue it to the top of the spool of thread.
6. Glue the arrow at the top of the straw
7. Set your wind vane outside where it will be in contact with the wind, setting it so that the "North" direction on the dial is pointing to the north. To do this, you can use a compass. Turn your body (with the compass pointing the same direction as you) until the moving hand of the compass is pointing to the north.

What's Happening?

With the bottom disc lined up correctly, you know which way is north. The wind will move the arrow on the weather vane, and you can compare the direction the arrow is pointing with the disc at the bottom. If the arrow is pointing the same way as the "N", the wind is blowing north, if the arrow is pointing the same way as the "S", the wind is blowing south, and so on.

Speed

Explanation

The wind can change quickly, gradually, or not change at all. Therefore, there are many different levels of wind speed, from extremely fast and devastating to rather slow and calm. Use the scale below to decide the level of wind, or build your own speed measurer using the experiment below.

Scale

There is a scale that helps you figure out the speed of the wind by using everyday objects and how they are affected by the wind. It was created by Sir Francis Beaufort, a British admiral, in 1806 to help him measure the speed of the wind at sea. This system of his was adapted to use on land. Below is the chart that you can use to determine the strength of the wind. Beaufort used the numbers 1 to 12, but the numbers of 13 to 17 were later added to measure the force winds of different speeds of hurricanes. Below is the scale Beaufort created.

Experiment #2: Speed

What You'll Need:

• paper plate
• paper cups with handles
• long pin
• thin stick or long pencil
• stapler

Directions:

1. Measure the diameter (distance across) of the plate using a ruler, and determine the halfway mark. Draw light, straight lines across the plate that go through the middle out to the edges.
2. Color one of the paper cups any color and let it dry.
3. Staple the handles of the four paper cups where the lines you drew in step one hit the edge of the plate. They should be at equal intervals.
4. Poke the pin through the paper plate to make a hole in it.
5. Push the pin into the thin stick or long pencil to attach the plate to the stick.
6. Put it in the wind and watch the cups spin.
7. Calculate the speed by counting how many times the colored cup goes around in ten seconds. You can tell how fast the wind is going by the speed of the cups. Compare throughout the day how fast the wind is going by counting the rotation periodically.

What's Happening?

When the wind blows, it flows into the cups and spins this contraption around. It is called an anemometer, and is helpful for determining the speed of the wind. More wind will make the anemometer spin faster, and you will count more spins in the ten seconds. You can tell when there is a lot of wind and when there isn't very much by watching the anemometer spin.

Types

• Sea Breezes: The warm air above the land rises rapidly and is replaced by a layer of colder air flowing from the sea.
• Land Breezes: When the sun sets, the warm air of the ocean rises and is replaced by the colder air from the land.
• Turbulence: Land that has obstructions causes the wind to break down into eddies.
• Microburst: An intense but short-lived downdraft of air.
• Wind Shear: A rapid change in wind and direction that occurs when the downdraft of a microburst hits the ground and fans out. This is dangerous for pilots when they're taking off or landing, and can cause the plane to crash. Using Doppler radar, pilots can detect these innocent-looking, and dangerous, conditions.
• Prevailing Winds: Winds that blow mostly from one direction.

Belts

There are many belts of wind that follow the same general direction for several months at a time. Below are some descriptions of the world's major wind belts.

• Intertropical Convergence Zone: The air over the equator is constantly warming and rising to higher altitudes, causing low pressure along the equator and periods of calm alternating with variable winds. This low-pressure belt is also called the doldrums, meaning a state of listlessness or boredom. Sailing ships could be caught for weeks in the doldrums, hoping for a wind to get them out.
• Trades: To the north and south of the doldrums are the trade winds, strong and steady currents of air that flow from the poles to the equator. The Earth's rotation deflects this flow to the right in the Northern Hemisphere, and to the left in the Southern Hemisphere (the Coriolis effect). In the Northern Hemisphere, these winds blow from the northeast to the southeast, so they are called northeast trades after the direction from which they blow. In the Southern Hemisphere, the trade winds blow from the southeast to the northwest, so are called southeast trades. These winds are steadier than any other wind and get their name from an older meaning of the word trade, "path" or "course", because they keep a pretty straight course. However, the trade winds do shift sometimes for a short period, especially when they're blowing over land.
• Antitrades: The prefix "anti" means opposite, so the antitrades blow in the opposite direction from the trades. These winds occur above the trade winds as the upward outflow of warmed air from the equator to the poles. They blow from the southwest in the Northern Hemisphere, and from the northwest in the Southern Hemisphere.
• Horse Latitudes: Beyond the trade winds are another set of winds that alternate calm and irregular winds. These are subtropical high-pressure belts that are centered at about thirty degrees at the north and south latitudes. Surface winds are often absent here because the air descends in this high-pressure area. When the wind does blow, it is often irregular and weak. One idea for the origin of the name for these winds is the trip vessels carrying horses from Europe to the West Indies made through these waters, when the winds stalled so long that the horses died.
• Westerlies: These winds tend to blow towards the poles, but are turned by the Earth's rotation and become southwest or west winds in the Northern Hemisphere and northwest or west winds in the Southern Hemisphere. They are particularly strong over the ocean. Since there is a lot of ocean area in the Southern Hemisphere, these winds can become so strong that the region that they blow over (between latitudes 40 degrees south and 50 degrees south) is known as the "roaring forties."
• Polar Winds: These winds tend to blow from the cold regions of the Arctic and Antarctic toward the equator, but because of the Coriolis effect they become northeast winds in the Northern Hemisphere and southeast winds in the Southern Hemisphere. So, they are sometimes called northeasterlies (in the Northern Hemisphere) or southeasterlies (in the Southern Hemisphere).

Jet Streams

There are some swift westerly winds that flow in a band several hundred miles wide at heights of four to eighteen miles above the Earth's surface. These jet streams move faster than the air on either side or above and below them. The center of a jet stream is called the core, and averages at about 100 miles (160 kilometers) per hour in the winter and 50 miles (80 kilometers) per hour in the summer. The speed in the currents of the jet stream decreases out from the core, and the number and paths vary from week to week. The strongest jet streams are in the Northern Hemisphere, across Japan and the United States.

Jet streams move with the seasons, heading north in summer and south in winter. Also, they have a big role in weather changes because when they move northward they pull masses of warm air with them. As they flow south, they pull masses of cold air from the Arctic regions. Airplanes like flying into jet streams traveling eastward because of the favorable winds that add to their speed. But if the airplanes are headed west, they avoid the jets because they'll just slow them down.

Local

There are also local winds that blow steadily for a long time. Below is a few examples of these winds.

• Foehn: This is the wind that crosses the northern side of the Alps from the south in the winter and early spring. It loses most of its water vapor as it goes up the slope, so it descends on the other side warm and dry.
• Chinook: Blowing from west to east over the Rocky Mountains, it warms and dries the prairies at the east of the Rockies.
• Santa Ana: In Southern California, this hot, dry wind blows down from the coastal mountains.
• Bora: Cold and violent, this northwesterly wind blows down on the shores of the Adriatic Sea from the mountains.
• Mistral: This wind blows down on the Rhone Valley of France and lasts for more than a hundred days, making the skies cloudless, the atmosphere dry, and the weather extremely cold.
• Simoom: Also known as the simoon, this wind blows hot weather and dust onto Saudi Arabia and the lands of the eastern Mediterranean.
• Harmattan: Another Saharan wind (along with the simoom), this wind blows along the Atlantic coast of Africa.
• Sirocco: Coming from the Libyan deserts, this hot wind blows over Malta, Sicily, and Italy.

Air Masses

There are movements of cold and warm air masses that form over different areas of the Earth's surface and travel great distances, causes strange movements in many of the winds. These air masses are different depending on where they form. Below are some examples of different air masses.

• Tropical Maritime: Air masses in the tropical and subtropical regions of the world become warm and moist because atmospheric circulation causes air to linger longer there.
• Tropical Continental: These are air masses that form over land in the tropics.
• Polar Continental: Air masses that form over polar land regions.
• Polar Maritime: These form over the polar oceans. They usually keep their temperature and moisture for a long time, and over great distances, so they can affect the weather when they get to warm places.

The boundaries between air masses are called fronts. A cold front is the boundary of cold air from the poles that moves into an area with warmer air, usually from the tropics. A warm front is formed when warm air moves into an area with colder air. Sometimes a cold front takes over a warm front and forces it upward. The two fronts then combine into an occluded front. If two air masses remain in contact for some time without advancing upon each other, the boundary is called a stationary front. When two contrasting air masses meet, the air currents flowing along their front can create tremendous storms and spiraling winds (tornadoes and hurricanes).

Changes

There are many changes that happen from year to year and decade to decade that affect the wind. For example, there is more air in the Northern Hemisphere than in the Southern for several years in a row. Then, there is an more air in the Southern Hemisphere than normal. It is believed that this is associated somehow with small, but widespread, temperature differences over the oceans.

And then of course there is El Niņo. There is a relaxation of the winds that causes the water to flow eastward to the coast of South America, affecting weather all over the world. Read more about how El Niņo is caused by going to the Introduction.