Please select your home edition
Edition
Allen Dynamic 40 Leaderboard

The Coriolis Effect

by Brian Hancock on 4 Feb 2003
Bruce Schwab from the Around Alone competitor Ocean Planet explains the very tricky Coriolis Effect, a phenomenon that contributes to winds rotating around pressure systems rather than blowing directly from a region of high pressure to a region of low pressure.

It’s time to put the parties and shore news aside and take a science lesson, so with great thanks to Bruce Schwab who has saved me from having to writing a story today, here is Bruce’s explanation of the Coriolis effect:

'Ever wonder why things like weather systems rotate one way in the Southern Hemisphere and the other in the Northern Hemisphere? Or why do they rotate at all? Me too.'

Let's start with a rough description of how weather and wind works. Wind flows from areas of high pressure to areas of low pressure trying to fill up the low. How soon a low fills up depends on what is happening to the air in the upper atmosphere above it, but that's another subject. What concerns us is that if you look at a map or chart from above it will show which way the wind is blowing and it appears to be going in a circle around the low. It is also going in a circle around the high and sometimes it looks like two wheels or gears spinning together. Why doesn't the air go straight at the low? What is the Coriolis effect? The answer, which seems unbelievable, is that the air IS going pretty much straight, or starts out that way, but there is something else at work....let me explain.

As you know, the earth is rotating once every 24 hrs or 1000 miles per hour at the equator (the earth is roughly 24,000 miles in circumference there). Imagine that you are standing at the north pole and a friend of yours, let's call him Bob, is standing on the equator. Let's also say the two of you are giants and are tall enough to see each other over the curvature of the earth. If you are facing Bob and can see him it seems like the both of you are not moving, but he is actually going 1000 miles an hour faster than you! You are not moving, but are slowly rotating with the earth beneath your feet so that you keep facing him.

Now let's say that you and Bob have this thing going with paper airplanes and like to throw them at each other. The both of you are really good and can throw them far enough to hit each other and at 1000 miles per hour (just bear with me here....), you smile, take aim with your super paper airplane and fire one off directly at Bob at 1000 mph. Since Bob is on the equator, which is half way to the south pole (or 1/4 of the way around the world) he is 6000 miles away so it should take six hours for your airplane to get to him on the equator.

Now here's the weird thing: you watch in amazement as your airplane zooms off to the right and misses Bob by thousand of miles!! The reason is that Bob is not where he was when you threw the airplane, but has moved 6000 miles in the 6 hours due to the rotation of the earth.

Your airplane is going straight, BUT THE EARTH IS TURNING UNDERNEATH IT making your airplane look like it is going in a curved line. The earth is rotating to the east so anything that moves towards the equator looks or acts like it turns west, since the earth is going faster there than wherever the moving object started.

Conversely, anything moving towards the north or south pole has more rotational speed than where it is going, so it looks like it turns to the east.

Are you with me?

This is what happens to the air as it moves from high to low pressure. In the Northern Hemisphere the air flowing south from a high goes to the west, and air flowing north from the high goes to the east starting the circular flow that you see on the map. This is what we call the Coriolis effect.

Draw two high pressure areas out on a globe, one on the north side of the equator, and one on the south side, and you'll see why the two highs turn in opposite directions to each other.

Keep in mind that the wind doesn't really start out curving, but the earth is actually rotating underneath it.

In reality the wind eventually does get drug along by the friction of the earth but we only need to get so far into this....this is about as much as I have figured out so far anyway.

I hope this makes sense! If you get it you can now explain the Coriolis effect to amaze your friends with your knowledge of this earth, our Ocean Planet.

By the way, Gustave-Gaspard Coriolis was an 18th century math major in France.'
sMRT AIS Man Overboard Beacons AUS / NZVetus-Maxwell 2021 v2 FOOTERNorth Sails Loft 57 Podcast

Related Articles

Expanding horizons with West System Epoxy
Working on D-Class boats over the years sparked Jonathan Minnebo's love of sailing Returning to his roots, Jonathan Minnebo has broadened his sights from solely shipwright work to creating bespoke furniture and staircase design and installation.
Posted today at 6:54 am
2025 Santa Maria Cup in Annapolis, USA Day 2
Final four skippers emerge from a competitive qualifying double-round robin stage A thrilling second day of racing at the 2025 Santa Maria Cup saw the final four skippers emerge from a competitive qualifying double-round robin stage.
Posted today at 4:47 am
SailGP to Kick Off Summer in New York
With this Weekend's Mubadala New York Sail Grand Prix An unforgettable weekend of high-speed, close-to-shore racing and world-class entertainment will take place this weekend when the Mubadala New York Sail Grand Prix takes place in front of the most famous skyline in the world.
Posted on 5 Jun
Scandinavian Gold Cup at Sopot, Poland Day 1
A slow start with very little wind The Scandinavian Gold Cup opened Thursday in Sopot, Poland, with seven 5.5 Metres competing for this historic and iconic trophy that dates back to 1922. Unfortunately, there was not enough wind to even complete one race.
Posted on 5 Jun
49er, 49erFX and Nacra 17 Europeans Day 3
A six-hour bake on the water Veteran and multi-Olympic race officer Sulis was clear at the morning briefing, "Our priority is to get three races for the 49er so they can progress to the gold fleet."
Posted on 5 Jun
Galicia 52 Super Series Royal Cup Day 3
Platoon Aviation turn up the heat but don't dent American Magic Quantum Racing's lead After a disappointing first regatta of the season when they finished in an uncharacteristic last position in Saint-Tropez, the fortunes of the German flagged 2023 52 SUPER SERIES overall champions Platoon Aviation seem to have turned for the better.
Posted on 5 Jun
Diverse international fleet for 5th AEGEAN 600
The race start is a little over 1 month away In a sign that this newest of the world's 600-mile classic offshore races has reached maturity, organizers from the Hellenic Offshore Racing Club are pleased to announce yet another large international fleet will be assembled for the 2025 AEGEAN 600.
Posted on 5 Jun
America's Cup: Another sponsor signs for Italians
Luna Rossa has announced that Unipol has renewed its trust in the team Luna Rossa has announced that Unipol has renewed its trust in the team and will accompany it as an official sponsor for the duration of the 38th America's Cup.
Posted on 5 Jun
SailGP: Media Conference concludes in New York
The media conference ahead of the Mubadala New York Sail Grand Prix has concluded, without any speci The media conference ahead of the Mubadala New York Sail Grand Prix has concluded, without any special points being revealed, save for Brazil lending a crew member to Germany.
Posted on 5 Jun
A decade of pushing the boundaries at A+T
Celebrating 10 years of the world's best yacht instruments 2025 marks a decade of pushing the boundaries of quality and customer service in the global yachting industry for A+T Instruments.
Posted on 5 Jun