Can You Feel the Pressure?

You have probably noticed that water swirls as it drains from a sink. This swirling is caused by the Coriolis effect, a force that deflects moving objects as a result of the earth's rotation and is responsible for tornadoes, hurricanes, and cyclones. Let's examine this force and how it relates to high- and low-pressure systems.

To illustrate the Coriolis effect, imagine a child on one side of a merry-go-round throwing a ball to a child on the other. Because of the carousel's rotation, the ball appears to be deflected away from the straight-line path in which it was thrown. The direction of the apparent deflection depends on whether the rotation is clockwise or counterclockwise. Due to the earth's rotation, the path of the wind is deflected in a similar manner. In the northern hemisphere the induced deflection is to the right, while in the southern hemisphere the deflection is to the left.

What causes the wind to begin moving in the first place is the pressure gradient. This pressure gradient is the difference between the atmospheric pressures at two different locations. Air pressure results from the weight of the air above us. To understand why air pressure changes, think of the earth's surface as existing at the bottom of an ocean of air. As the waves of the atmosphere pass over us, the air pressure rises and falls. Warm tropical areas of the planet have rising air patterns and create low-pressure systems. Colder arctic areas create descending air and high-pressure systems.

The downward trend of the air in an area of high pressure stifles upward cloud development. This results in generally good flying conditions. If an area of high pressure exists over a given area for a long period of time, the visibility can be diminished by trapped haze and dust. The Bermuda high has a semipermanent influence on the southeastern United States from about May to October. This long duration of high pressure results in hazy skies in the late summer months. It also repels the advance of weather fronts into the Southeast in the summer. In an area of high pressure the air flow is outward from the center. This outward flow is deflected to the right, creating a clockwise circulation. This results in an overall high-pressure circulation that is downward, clockwise, and outward.

The characteristics of low-pressure systems are just the opposite. As air is pulled inward from all directions there is nowhere to go but up. The upward tendency of the air can encourage the development of convective thunderstorms. A hurricane is little more than an intense low. As forecasters monitor the development of a hurricane, a decreasing central pressure is an indication of a strengthening storm. In an area of low pressure the inward flow is deflected to the right, causing a counterclockwise circulation. The overall flow pattern in a low is inward, counterclockwise, and upward.

The Coriolis force is cumulative in that as the air is pulled into the center of the low, the rotational speed increases with each revolution. Picture a merry-go-round being pushed faster and faster each time it revolves. For this reason the strongest winds associated with a hurricane are found in the center near the eye wall.

Isobars, or lines of equal pressure, depict the pressure gradient. Isobars are represented on weather charts by lines, much like elevation lines on a topographical map. The closer these lines are spaced, the steeper their gradient. Isobars can be used to approximate wind. Wind circulates parallel to the isobars, clockwise in a high and counterclockwise in a low. The closer the isobars are spaced, the stronger the wind.

Have you ever heard that a toilet will flush backward in the southern hemisphere? Now you know it is because of an effect called Coriolis.

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