Lesson 4: Let the Winds Blow

Just like the currents in the ocean, the atmosphere has currents that flow from one place to another. Air is constantly moving around the earth, making the wind.

Winds are created when there are differences in air pressure from one area to another. In an area where there is low-pressure (rising air), air at ground level comes in to replace the air that is rising. In areas of high-pressure (sinking air), air at ground level spreads out.

If a high and low pressure area are close to each other a strong wind will develop, because a natural circulation of air will occur (see diagram below.) The greater the difference in pressure is, the stronger the wind.

Winds can be produced in a localized area, to expanses of several hundred miles. An example of a local wind is a land-sea breeze. During the day, land will heat up faster than water, which makes air rise over the land – a low-pressure area is formed. Since the water is cooler, there is higher air pressure over the water. Air from over the water comes inland to replace the rising air, making a sea breeze. At night, things are reversed – the land cools down quickly, while the water stays warmer. High pressure is formed over the land and low-pressure forms over the water, so air flows offshore. This is called a land breeze.

A monsoon is a land-sea breeze on a larger scale. As large areas of land heat up during the summer (like on the Asian continent), a large area of low pressure will be formed. Moisture packed air from oceans will come on land, producing very heavy rains that may last for several months. During the winter, the airflow will be reversed, and the winds will blow from land to the warmer oceans.

Activity One – Whatever Way the Wind Blows

Materials: paper, pencil

Setting Up the Activity: For a three to seven day period, observe which direction the wind is blowing at ground level. Also look at which way lower clouds and higher clouds are moving. Make observations once or twice daily. If possible, try to do observations right before and right after a cold or warm front passes through (you'll need to watch the weather forecast to know if front passage is expected.)

Questions:

Q. Does the wind on the ground always blow the same direction as the wind at higher elevations?

A. No – winds at higher levels may be blowing in much different directions.

Q. Why does this occur?

A. Winds at ground level are produced from low and high-pressure areas at or near the ground. Winds at higher levels are associated with the jet stream or other wind system.

Q. How does the wind change with passage of a cold or warm front?

A. The wind will change direction and strength. For cold fronts, the wind will likely come from the north or northwest, while warm fronts will likely have winds coming from the south or southwest.

Activity Two – Measuring the Wind

Materials: paper, pencil

Setting Up the Activity: Wind speed can be estimated by observing the effect of the wind on different things like trees. A scale called the Beaufort scale give a range of air speeds, as shown in the table on this page.

Measure the wind speed by observing how the wind affects trees, flags or other things. This can be done in conjunction with activity one.

Questions:

• What was wind speed based on your observations?
• What did you use to make your decision?