A Popular Weather Myth Down the Drain
Does water spin down the drain one way in the Northern Hemisphere and the opposite way in the Southern Hemisphere? Despite the popularity of this myth, it isn't true. The volume of water in a sink or toilet is too small to be significantly influenced by the Coriolis effect
OK, let's stretch the imagination a little and take a field trip. Picture yourself shrunk to the size of an air molecule, blowing on the wind. You start out in Ohio on a day with clear, sunny weather and high atmospheric pressure. Low pressure exists to the east, just off the coast of Maine. Since wind wants to go from high to low, you are carried east toward the center of the low pressure system. That's caused by something we call "pressure gradient force."
Hold on! You can't just go in a straight line. Suddenly the Coriolis effect kicks in. That bends your motion to the right. Also, if you get close the ground, you start bouncing off of trees and buildings; you're slowed down by friction, and that weakens the Coriolis effect slightly. (The Coriolis effect increases as speed increases.) Suddenly, pressure gradient force has a slight edge in this tug of war. You lurch back toward the low. But the Coriolis effect hasn't given up -- it tugs you back to the right.
Picture it as a series of small steps: First you take a step toward the low (due to pressure gradient force), then a step to the right (Coriolis effect), then another step toward the low, and so forth. After a while, you'll see that you're dancing in a counter-clockwise spiral around the center of the low.
In the Northern Hemisphere, wind blows counter-clockwise around low pressure and clockwise around high pressure. In the Southern Hemisphere, the directions are reversed.
In the next lesson we'll look at fronts and the jet stream.
Buys Ballot's Law
In 1857, a Dutch meteorologist named Buys Ballot discovered what we now know as Buys Ballot's Law: In the Northern Hemisphere, if you stand outside with the wind at your back, high pressure is on your right and low pressure is on your left.
Assignment 1: Twists and Turns
Try to figure out where high and low pressure centers are from your own back yard, using Buys Ballot’s law. Stand with the wind at your back. Hold your arms outstretched. Which direction is your right hand pointing? That’s the direction to high pressure and (usually) sunny skies. Which way is your left hand pointing? From that direction you will probably see increasing clouds and stormy weather. Check a current weather map for your area, either in the newspaper or online, and see if your backyard observations match up.
You can also try to predict the prevailing wind direction in your neighborhood even before stepping outside. While drinking your morning coffee, look at a national weather map. There’s probably a big H for high pressure or L for low pressure stamped somewhere on the page near your home state. With your finger, trace a counterclockwise swirl around the L (or a clockwise swirl around the H). In which direction is your finger moving when you cross over your own town or county? If your finger is moving from north to south on the map, chances are the prevailing wind outside is northerly -- and chilly!
Wind direction always refers to the direction the wind is blowing from, not the direction it’s blowing to. So a northwest wind blows from the northwest to the southeast