Windstorm '95

Kate Farquhar-Shirley, DO-IT Mentor

On Tuesday, December 12th, 1995, a windstorm hit the Northwest with enough destructive power in some places to be christened "Windstorm '95". While relating their various experiences with the storm, DO-IT Scholars and Mentors began an enlightening discussion over electronic mail on windstorms, hurricanes, and weather patterns. Atmospheric scientist and DO-IT Mentor Imke Durre shared her expertise on this subject.

The storm probably had its origins near Hawaii. It produced very heavy rains and strong winds (stronger than here) in San Francisco. It was not a hurricane though.

Hurricanes form over the warm ocean waters of the tropics, only when water is warmer than 82 degrees. The water that evaporates from the ocean condenses into clouds, thereby releasing the heat which drives the hurricane. When such a storm is in its beginning stage, it is called a tropical depression. When its winds exceed 39 mph, it becomes a tropical storm; when the wind speeds become greater than 74 mph, the storms are hurricanes.

A hurricane has an eye at its center, in which conditions are calm. The eye is surrounded by tall thunderclouds. Once a tropical storm has formed, it may move out of the tropics into mid-latitudes. Such storms usually travel from east to west across an ocean because the winds which steer them tend to come from the east in the tropics. This explains why hurricanes generally make landfall on the eastern edge of continents.

The storm that hit us on Tuesday did not form this way. It was an extratropical storm which formed due to strong north-to-south temperature differences near Hawaii, where cold air from the north tends to collide with warm air from the tropics at this time of year. Extratropical storms do not have eyes, and their clouds are low- to middle-level stratus clouds, not the deep thunderclouds. They travel from west to east because winds in the mid-latitudes generally come from the west. As we found out, these storms can get rather strong, too.

DO-IT Mentor Michael Herbert then wrote:

I find weather completely fascinating, and as a sailor I'm quite intrigued by it's implications. Here's a question to stimulate some though, as well as to initiate another great DO-IT email debate. Can anyone tell me how wind happens? And how come it was so strong last night?

I'm sure you have heard weather forecasters talk about lows and highs. In a low, the pressure is lower than average, and in a high, pressure is higher than average. Wind is a response to such differences in pressure. Near the surface, winds usually blow from high pressure to low pressure. Air tends to circulate around centers of high or low pressure. In the Northern hemisphere, air goes around lows in a counterclockwise direction, and around highs in a clockwise direction. As the pressure inside a low falls, the wind circulating around the low strengthens. Since the pressure inside yesterday's storm was really low, the winds were very strong. And because the low moved up along the coast west of us, and the air goes around it in a counterclockwise sense, we had south winds.

At this point in the discussion, Ed Pottharst, DO-IT Career Mentor, interjected the question:
Why does air go counterclockwise around low pressure centers and clockwise around high ones (in the Northern hemisphere)?

The reason for counterclockwise circulation around lows and clockwise circulation around highs is the Earth's rotation. Because of the Earth's rotation, things in motion are pushed to the right of their path in the Northern hemisphere and to the left in the Southern hemisphere. This force is called the Coriolis force. Its magnitude is rather small, so that it only becomes significant for large-scale motions like those in highs and lows.

If you picture a circle as a low, with the lowest pressure in the center, then air will want to move towards the center in an attempt to balance out the difference in the pressure between the area outside the low and the center of the low. But the Coriolis force pulls the inward-moving air to the right of its direction of motion. Since the force due to the pressure difference (called pressure gradient force) and the Coriolis force are acting constantly, the air ends up moving counterclockwise around the low. It may help you to draw a circle and an arrow pointing towards the center and then imagine what will happen when the tip of the arrow gets pushed to the right. Or, if you are into physics, you can think of it as a force balance between the inward-pulling pressure gradient force, the Coriolis force acting 90 degrees to the right of the pressure gradient force, and an outward-pushing centripetal force which starts to act once the air is circling around the center. The centripetal force is the force that pulls your outward when you sit on a merry-go-round.

This exchange is an example of how easy it is to carry on a conversation on the 'Net. And, you can learn something in the process!

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