Imagine the carousel spins counterclockwise. If a kid at the outside throws a ball toward the center, it misses because it carries the momentum of the rim. Seen from a vantage point above, the ball arcs to the right as the carousel swings to the left. Conversely if a kid at the center overcomes her nausea long enough to throw a ball toward the rim, it misses because the kid at the rim is carried around to the left by the spinning carousel. Either way, the ball is deflected to the right because the carousel is spinning counterclockwise.
Earth is just like the spinning carousel! It's so big that we don't notice it, but we're constantly spinning toward the East (counterclockwise as seen from "above" the North Pole). Warm air aloft that flows poleward gets deflected to the right just as the ball thrown from the rim of the carousel does when it approaches the center. The upper air in the midlatitudes carries the momentum of the huge Equatorial circumference with it, moving in the same direction as Earth's spin but faster. That's what causes the Prevailing Westerly winds aloft at midlatitudes. It's why the weather in most of the USA and Europe comes from the west over a period of days, and it explains why there's a jet stream aloft, spinning endlessly around the winter pole.
Conversely, the cooler return flow is also deflected to the right as it approaches the Equator just as a ball thrown from the carousel's center is deflected if thrown toward the rim. Tropical air moves form east to west as seen from the surface. These are the tropical Trade Winds, and this explains why tropical weather systems (think hurricanes) approach from the east.
In the Southern Hemisphere, the exact same logic applies but the carousel spins clockwise as seen from above the South Pole. Warm air aloft flows toward Antarctica but is deflected left (toward the east), leading to Prevailing Westerlies in the middle latitudes (think New Zealand). Cold return flow near the surface is deflected to the left as it approaches the Equator, leading to Easterly Trade Winds. We will experience the southern Trades as we sail from Kenya to South Africa to Ghana.
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| Gustave Gaspard Coriolis |
This tendency of air to be deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere is called the "Coriolis Effect" after a 19th Century scientist.
When I was a teenager, my high school physics teacher showed us a short movie about the Coriolis Effect called The Rotating Room, starring Dr. Ivey and Dr. Hume. The movie was old even then (in 1976). I'm guessing it was made int he mid-1960s. I find it kind of charming, in a skinny-tie black-and-white way.
I like to imagine the "Dead Hand of Doctor Coriolis" reaching down and pushing moving objects toward the right in the NH and to the left in the SH. On our 2020 voyage, my oceanography students made a spoof video about it because they found it amusing!
