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Effect of Changes in Air Pressure on Wind Velocity

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Author: Janice VanCleave

What You Need to Know

Force is a push or pull on an object. Air pressure is the force exerted on any surface by air molecules. Air pressure gradient is the difference in air pressure between two neighboring areas. Pressure gradient force is the force that causes wind. Density is a measure of mass or the number of particles per unit volume.

How Does the Pressure Gradient Force Work?

The pushing force of air is called air pressure. As the air pressure increases, the greater is its pushing force. Air pressure varies from one area to another. Winds begin with differences in air pressures. This difference in the pressure between the two neighboring areas is the air pressure gradient, which causes wind. Meteorologists (scientists who study the weather) refer to the force that starts the wind flowing as the pressure gradient force. Wind moves from a high air pressure area toward an area with lower air pressure.

What Does This Have to Do with Wind Velocity?

Velocity is more than just how fast an object is moving. It also includes the direction of the object. The velocity of wind is its speed and the direction from which it comes. A 5-mile (8 km) per hour south wind means the wind is coming from the south and blowing toward the north. Meteorologists use compass direction to identify the exact direction of wind. Thus a wind direction of 0° means that the wind is blowing from due north. A wind direction of 90° is blowing from due east. A wind direction of 180° is blowing from due south, and 270° is a wind blowing from due west. The arrow in the figure represents a wind with a 45° direction. This means that the wind is blowing from the northeast.

Real-Life Science Challenge

Air masses of different temperatures have different pressures. When such air masses meet, the resulting pressure difference (which causes wind) is highest in the upper atmosphere. Upper atmosphere wind speeds average 35 miles (56 km) per hour in the summer and 75 miles (120km) per hour in the winter, although speeds of over 250 miles (402 km) per hour are known. If the wind speed in the upper atmosphere is higher than 55 miles (88 km) per hour, the wind is called a jet stream. Jet streams are usually found somewhere between 6 and 9 miles (10–15 km) above Earth's surface and flow eastward. Since the progress of an airplane is aided by tailwinds and hindered by headwinds, eastbound aircraft try to fly with the jet stream in order to gain speed and save fuel. Westbound aircraft try to avoid the jet stream. The time it takes to fly across the United States from west to east is decreased by about 30 minutes if the aircraft flies with the jet stream or increased by the same amount if it avoids the jet stream.

Experiment

Now, start experimenting with air pressure and velocity.

Hints

• A glass container such as an aquarium, covered by a lid that has two holes in it, can be used to observe smoke moving in and out of the container due to differences in air pressure.
• A burning mosquito coil or punt can produce smoke to help you see air movement.
• Changes in air pressure can be created by changing air density or temperature in a container.

Caution: Have an adult supervise and assist you in heating something to create smoke.

Fun Fact

Air pressure affects the temperature at which water boils. At sea level, the temperature at which water boils is generally 212° F (100° C). But at high elevations, such as in the mountains, where the air pressure is lower, water boils at a lower temperature. This means that at higher elevations food does not heat as quickly, and it takes longer to cook it.