I’ve been hearing a lot about the Roaring Forties and how rough the sea could be in our study area between 38 and 42 degrees south of the equator. What are the Roaring Forties? I asked a friend I met off the port bow, Ali the albatross. Ali spends her time soaring on air currents, so she’s practically an expert.
Here’s how I understand what she told me. We all know that the sun’s energy heats Earth’s surface unevenly. We also know that temperature differences cause density differences and density differences cause wind when less dense, warm air rises and more dense, cool air sinks. What’s interesting is how so much wind ends up at latitudes south of 40 degrees south.
In the southern hemisphere warm air rises at the equator and moves south while cold air moves north to replace it. Our atmosphere is too thin to carry the warm air high up for very far, so it sinks back toward the surface at around 30 degrees south. From there it travels along Earth’s surface southward to about 60 degrees where it rises above Antarctica. So, you’ve got this surface air moving southward between 30 and 60 degrees south, but that’s not all that’s happening because the Earth is rotating on its axis as well.
The Earth is roughly spherical, so different parts of the planet are moving at different speeds. The equator has a circumference of about 40,000 km, so it’s moving at about 1,700 km per hour. At 40 degrees south, Earth’s circumference is smaller and it’s traveling at about 1,300 km per hour. The speed of a point on the surface of the planet just gets slower the farther south you go. As air moves over the surface, the surface is slowing down dramatically and that makes the air’s movement relative to the surface much faster! Voila – high speed winds!
Of course, the winds aren’t always in exactly the same place or moving at the same speeds – there’s a lot going on down here. A big factor is the seasons. During southern summer, the area of the planet receiving the most sunlight is shifted toward the south pole and so is the area of highest wind. Likewise, during winter the sun energy is shifted away from the south pole and so is the area of highest wind. That’s a simplified version, but that’s how I understand it.
Roaring Forties sounds rough, but if you go ten degrees farther south, you’ll reach the Furious Fifties and another ten degrees will get you to the Screaming Sixties! I’m happy to avoid those extreme wind speeds!
I bet you’re wondering why you don’t hear about these outrageous wind speeds in the northern hemisphere. Well, it’s because there’s so much more land up there to break up the air masses that the wind just can’t get up a good head of steam. Down here, there’s only Tasmania, a slip of southern Australia and the south end of South America in the 40’s and only that tip of South America in the 50’s. The wind runs unhindered in the south!
All this excitement’s got me winded!