Weather Imagery

At some point, most of us have heard that water spins down a drain in different directions depending on which hemisphere we happen to be in. The fact is, the Coriolis force (an apparent force as a result of the Earth’s spin) has virtually nothing to do with which direction water spins as it empties down a drain. Although this force is “real” and does have an affect on other large, long-lived systems that travel great distances (such as hurricanes, high and low pressure systems, and long range artillery shells), water draining from toilets, sinks, and bathtubs are rendered virtually immune from its affects. The Coriolis force is much too weak to have any affect on such small, short-lived rotating bodies of water under normal conditions.

rotation and has different magnitudes depending on how close or far you are away from the poles and the equator. As you move closer to the equator, the Coriolis force weakens. Keep in mind that at the equator the Coriolis force has no affect on any object. So what about water going down a drain? Is it affected or not? Let’s look at some other things first.

As a demonstration, imagine that a merry-go-round represents the entire surface area of either the northern or southern hemisphere of the Earth. If you start in the center of the merry-go-round and walk towards the edge as it’s spinning, you will start to feel a deflection and will find yourself drifting to one side or the other (depending on which way you spun the merry-go-round). Now, if something moved a comparably equal distance in one of the Earth’s hemispheres (such as a hurricane does), it too will be deflected. The merry-go-round is an exaggeration because nothing moves over a comparable distance on the Earth in the same amount of time. But you can sort of get the picture that the Earth is huge and a little toilet of water very small in comparison.

On the Earth, hurricanes and pressure systems move hundreds of miles over a period of days and they are most certainly affected by the Coriolis force. In a low pressure system, air flows towards the center and as it does so, it is deflected to the right in the northern hemisphere and to the left in the southern hemisphere. If the Earth were to stop rotating, there would be no deflection and the low pressure system would quickly equalize as the air rushed towards the center. However, on a rotating Earth the air keeps getting deflected away from the center which causes the low pressure system to spin counter-clockwise in the northern hemisphere and clockwise in the southern hemisphere. In a high pressure system, air flows out from the center. In the northern hemisphere, the deflection is to the right which causes the system to spin clockwise. In the southern hemisphere, the deflection is to the left which is why a high pressure system spins counter-clockwise. The fact that the Coriolis force is “zero” at the equator is the reason why no hurricanes cross from one hemisphere to the other. The low pressure would simply equalize.

As for water emptying down a drain, the amount of deflection produced by the Coriolis force is a couple orders of magnitude too small compared to other influences. It’s not that the Coriolis force doesn’t exist, it is just too weak to have any influence on such a small amount of water in an 18″ toilet bowl or sink as compared to a hurricane which is hundreds of miles across and moves thousands of miles. The only things that affect which way the water empties down a drain is the shape of the container it’s draining from, the way the water was introduced into the container, irregular eddies, and any other objects which may affect currents in the bowl. So in the case of a toilet, take a look and observe which way the water jets into the bowl and see if your toilet bowl is perfectly round. Chances are the water enters at an angle and the bowl itself is elliptical. All you need is a small rotation at the start and as the water empties towards the bottom of the toilet/sink, the water will begin to rotate faster due to the narrowing of the drain (same principal as when an ice skater pulls their arms in towards their body … they spin faster). So the next time a scam artist in another country offers to demonstrate this “water down a drain phenomenon” by stepping on the other side of the equator where the Coriolis force has zero affect, save your money!

Some things affected by the Coriolis force are Long range artillery shells that “fly” 20 or more miles. The affect is much smaller than a hurricane, but still needs to be taken into consideration so the shells hit the right target. A 50-yard difference could have disastrous results. Ballistic missiles must really take the Coriolis force into consideration because they will travel thousands of miles at great speeds. Ocean currents span the entire globe and as a result they too are affected by the Coriolis force. Interestingly, a tornado is not affected by the Coriolis force. In fact, about 10% of tornadoes spin clockwise while about 90% spin counter-clockwise in the northern hemisphere. However, the meso-cyclones, supercells, and low pressure systems which are responsible for spawning tornadoes are affected by the Coriolis force, which in turn tend to bias tornado genesis.

Even though the Earth does pass closer to the sun during part of its orbit, this is not what’s responsible for our hot summers or cold winter months. If this were the case, then the northern and southern hemispheres would both have their summers and winters at the same time. However as you’ll see below, exactly the opposite is true. When it’s summer in the northern hemisphere, it’s winter in the southern hemisphere.

The Earth rotates about its axis once per day. What you might not of known is that the Earth’s axis is tilted, 23.5° from vertical (Actually, the tilt varies from near 22° to 24.5°. The Earth wobbles a bit). This tilt virtually never changes and for half the Earth’s orbit around the sun, the north pole is tilted towards the sun while the south pole is tilted away from the sun. During this time the northern hemisphere has its summer and the southern hemisphere (which is tilted way from the sun) has its winter. For the other half of the orbit, the south pole is tilted towards the sun while the north pole is tilted away. This is when the northern hemisphere has its winter and the southern hemisphere has it’s summer. The tilt of the Earth has two primary affects which create our seasons.

When the north pole is tilted away from the sun (winter), sunlight must pass through more atmosphere before striking the Earth’s surface. Therefore, less of the sun’s energy is absorbed by the ground and converted to heat which results in cooler temperatures. At the same time, the southern hemisphere is tilted towards the sun (summer). During this time, there is less atmosphere the sun’s energy must pass through and as a result, more of its energy reaches the Earth’s surface which in turn is converted to heat. This results in warmer temperatures. See the image below for an illustration. Notice how the top arrow representing the sun’s energy must pass through twice as much atmosphere as compared to an arrow closer to the equator. This is due to the Earth’s axis being tilted away from the sun.

The second affect the Earth’s tilt has on our seasons in the angle at which the sunlight strikes the surface of the Earth. The greater the angle, the more dispersed and less concentrated the sunlight is on that particular point. As an example, take a flashlight and shine it straight down. Most of the light is focused in a circle on the floor. Now, slowly rotate you wrist so the light shines at an angle. The same amount of light now covers a larger area and is a little less intense in that same spot. This is what happens to the sun’s energy as the Earth is tilted away or towards the sun. When the northern hemisphere is tilted away from the sun (winter) the sun is lower in the horizon resulting in a greater sun angle. The Image below illustrates this point.

Also important is your location on the Earth as it plays a big role in how much of the Sun’s energy you receive on average and how much your seasonal temperatures may vary from winter to summer. Obviously, the north and south poles average the coldest temperatures anywhere on Earth. This is because they receive the least amount of the sun’s energy due to the extreme sun angle and the amount of atmosphere the sun’s energy must penetrate. In fact, during the winter, the sun never rises! Conversely, in the summer, the sun never sets. You might think that if the sun never sets, it would be incredibly hot, but you must remember at the poles the amount of atmosphere the sun must pass through is at its greatest.

As you move away from the poles and get close to the equator, your seasonal temperatures will vary less and the average temperature will be warmer year round. This is because there is less variation in the sun angle as the Earth orbits the sun and the amount of atmosphere the sun’s energy must pass through is less. This is why Hawaii, Florida, Mexico hardly ever see harsh winters (excluding mountainous regions). However, areas further away from the equator such as Montana, New York, and Canada will see lots of very cold wintry days.

As you can see, the tilt of the Earth’s axis is the reason we have seasons. Not because the Earth orbits closer to the sun for one part of the year as opposed to the other. To eliminate the myth that the Earth’s distance to the sun causes the seasons, think about this. The Earth is closest to the sun on January 7th when it’s 91,399,726 miles away. The Earth is furthest from the sun on July 7th when it’s 94,508,727 miles away. The Earth is 3,109,000 miles closer to the sun during the Northern Hemisphere’s winter!

Significant Dates

• Summer Solstice: June 22nd. Longest day of the year.
• Winter Solstice: December 22nd. First day of Winter. Shortest day of the year.
• Vernal Equinox: Around March 21st. Marks the first day of Spring.
• Autumnal Equinox: Around September 21st. Marks the first day of fall.