The Coriolis Effect and Weather: How They Are Related

We know that gravity is real although we can’t see it, and it’s a force that affects everything in the universe. The Coriolis effect, often called the Coriolis force, is not a real force — it’s what we call an apparent force — but it seems real to us on Earth. It’s an important factor in the way both our air and the oceans circulate.

What Is the Coriolis Effect?

The Earth is a sphere spinning over 1,000 miles per hour at the equator. The rotational speed decreases as you move away from the equator because the closer you get to the poles, the less distance there is to cover in the same amount of time. This concept is key in understanding the way the Coriolis effect works.

How Does the Coriolis Effect Work?

Picture yourself on a stationary merry-go-round tossing a ball with a friend who’s on the other side. It’s going to be easy to catch the ball, so let’s raise the bar a notch: We’ll start to spin the merry-go-round at a moderate speed.

We’ve made it a lot more difficult! After the ball leaves your hand, you and your friend continue to rotate but the ball is moving straight ahead, so your friend won’t be there to catch it when it reaches your target point. To someone on the merry-go-round, the ball appears to turn to the right; but someone not on the ride will see it moving in a straight line. That’s the deal. The Coriolis effect happens because the Earth is rotating while air and water currents move in a straight line. In fact, all of the Earth’s oceans have large rotating currents, called gyres, that are strongly influenced by the Coriolis effect.

Here’s a deeper look into the basics of the Coriolis effect:

• In the Northern Hemisphere, moving objects tend to curve to the right of their expected path; in the Southern Hemisphere, they tend to veer to the left of their intended path. This is simply because of the way the Earth rotates.
• The Coriolis effect increases as you get farther from the equator or when the wind is faster.
• The Coriolis effect impacts both large air and water currents.
• The Coriolis effect will have no impact on an eastward- or westward-moving wind or ocean current at the equator, but it has an effect on weather everywhere else on the globe.

Understanding the Scale of the Coriolis Effect

You may know that 0.01 means one-hundredth and 0.001 means one-thousandth. Well, the Coriolis effect is on a scale of 0.0001, or one ten-thousandth! This is why it has a negligible impact within a short distance of a mile or two but does impact larger-scale weather systems.

Coriolis Myth Debunked!

You may have heard that toilet water spins one way when you flush it in the Northern Hemisphere and the opposite if you hop a plane to the Southern Hemisphere. Let’s hit the buzzer on that one — it’s just not true. Toilet flushes spin one way or the other based on the design of the toilet, nothing more. Why? A toilet is simply too small for the subtle Coriolis effect to come into play. Oh, and don’t forget to wash your hands, please.

How Does the Coriolis Effect Impact Weather?

The size of the system is the key: If we’re talking small-scale, then effects will be less; for larger systems, the effect is greater.

Hurricanes

A hurricane is a large weather system composed of low pressure, sometimes up to 1,000 miles across, with winds that may, in extreme cases, reach 180 mph. The small Coriolis effect becomes more noticeable the bigger the system gets and the faster the wind blows through it. For instance, a 100 mph hurricane will experience 100 meters of Coriolis wind deflection in an entire hour, but that’s enough to ensure that the system spins counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.

Hurricanes do not form within five degrees latitude of the equator because the Coriolis force is too small to create a deflection and get things spinning. Remember, the effect increases the farther you get from the equator.

Tornadoes

If you compare the size of a typical hurricane to that of an average tornado, hurricanes are about 500 times the size! The Coriolis force is too small to affect tornadoes; twisters develop spin based mostly on wind shear, not because of the Coriolis effect.

High and Low Pressure Systems

Large high pressure systems rotate clockwise, and low pressure systems rotate counterclockwise because the atmospheric pressure gradient is reversed, not because the Coriolis effect changes. These systems can be thousands of miles across and are always influenced by the Coriolis effect.

Tracking Winds and Pressures

You cannot track the Coriolis effect directly, but it’s easy to track changing winds and pressures with today’s sophisticated and affordable home weather stations. You might try comparing your readings to weather maps on the web or on your favorite TV weathercast. Are you close to the center of a high or low pressure cell? How does the wind differ when high pressure is overhead versus low pressure? What about before and after a cold front? A quality home weather station can make exploring our planet’s wind a lot more fun!