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E. Wind Speed and Direction
The sun's heat creates patterns of wind and differences in air pressure locally, as well as wind patterns that can be identified globally (which influence and are influenced by local air pressure). These wind patterns can be identified or described according to speed and direction.

What Causes Wind?
Land heats up and cools down more quickly than water. As the land absorbs the sun's rays during the day, it warms the air above it, and does this more quickly than does water. As a result, the air above the land is warmer, and that warm air rises, creating a lower level of pressure in that area since the air is rising. Air, just like water, will fill a space so that it is distributed evenly.

For example, picture what happens when you take a bucket of water out of a bathtub. There isn't less water in just that one spot. The remaining water distributes over the area of the tub with an even surface at the top. There is a lower level of water overall, but the water that remains is evenly distributed.

When air warms, it rises. The air above land heats faster than the air above water because land absorbs more heat than water (water reflects more of the sun's rays). As a result, the air from the cooler area above water moves into the low-pressure area created by the rising warm air to equalize the pressure of the air. This movement of air to the area of lower pressure causes a breeze from the ocean, especially on a warm day.

In the evening, the water is slow to lose the heat it stored up during the day (slower than the heat loss on land), so the opposite effect occurs. Air from above the land quickly loses its heat as the sun sets, and it is now cooler than the air from above the water. So, as the air above the water is warmed by the heat released from the water, it rises, creating an area of lower pressure than that of the air above the land. The land air moves in from the area of higher pressure to the lower pressure area causing an outgoing breeze from the land.

On a larger level, differences in air pressure are caused by differences in air temperatures, which causes air to move in patterns, which cause winds of various intensities. Breezes and winds do not only occur in places that are close to the water, but the basic principles are the same in terms of cold and warm air fronts rising, falling and moving. Cold and warm air fronts play a major role in temperatures and precipitation around the world. These world systems are in constant motion causing winds of various strengths and directions ranging from a very mild breeze to a dangerous hurricane.
JUMP TO: More on weather fronts

Measuring Wind Speed - Beaufort Scale
Winds have been rated according to the Beaufort Scale since the early 19th century, when a man named Sir Francis Beaufort, a British Admiral, introduced a set of descriptions for wind according to the types of things that the wind could move.

You can tell that he was British by the wording of the descriptions in the scale. We still use this scale to describe the wind, but we also more accurately measure the speed of the wind now, providing more detailed and useful information for scientists.

An instrument called an anemeter is the instrument used by weather scientists to measure wind speed. These are usually electronic now and computers convert electronic signals from instruments into digital displays.

 

Beaufort Scale
Force Range
Wind Name
Description
Speed
0-1
Calm to Light Chimney smoke rises upwards or drifts slightly 0 - 5 km/hr
2-3
Gentle Breeze Soft wind on your face. Leaves and twigs sway gently on the trees. 6 - 20 km/hr
4-5
Fresh Breeze Dust and rubbish blow along the ground. Small trees bend. 21 - 40 km/hr
6-7
Strong Breeze Big trees sway and branches toss. Washing on the line flaps. 41-60 km/hr
8-9
Gale Branches snap on trees. Tiles fall from roofs. Wind is hard to walk into. 61-85 km/hr
10-11
Storm People can be blown over and trees uprooted. Chimneys collapse and sheds blow over. 86-120 km/hr
12
Hurricane Houses blown down. Cars and trucks are thrown about. Over 120 km/hr

Measuring Wind Direction
Winds that are predictable, resulting from global air circulation patterns such as jet streams, are called prevailing winds. Regular patterns are also a result of local geography. In fact, in many areas people associate winds from a particular direction with a specific kind of weather.

To determine the direction of the wind, you can use a weather vane or a windsock. When describing the direction of the weather, you determine the direction that the wind is coming from and NOT the direction that it is blowing towards. So, if your weather vane's arrow is pointing north, the wind is a southerly wind (comes from the south and is blowing towards the north). If your windsock is pointing to the south east, then you are experiencing a north west wind.

References

Davies, Kay. (1992). The Super Science Book of Weather. England: Wayland Publishers Ltd.

Dickinson, Terence. (1988). Exploring the Sky by Day. Ontario: Camden House Publishing.

Ward, Alan. (1992). Project Science: Sky and Weather. New York: Franklin Watts Inc.

Mezger, Gabi. (1995). Weather Instruments: Teacher's Guide. New Hampshire: Delta Education Inc.

Kary, Diana. (1996). Ready, Set, Science: Weather Activity Guide. Calgary: Science Alberta Foundation.

 

 



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