Raven spreads a globe-shaped diagram across the lab table. Arrows loop from the equator toward the poles and back.
"Three loops in each half," she says. "What do you notice about where they meet?"
Rocket traces one arrow with his finger. "This air leaves the equator heading north, but it bends to the east. Why would it bend?"
Nova projects a slowly spinning disk onto the wall. "Try drawing a straight line on a spinning turntable," she says. "What shape do you get?"
"A curve!" Rocket says. "So the Earth spinning underneath makes the wind curve."
Raven points to a band around the globe at 30 degrees. "And look where the deserts are," she says quietly.
If the Earth did not rotate, global wind would be simple. Hot air would rise at the equator, flow toward the poles, cool, sink and flow back.
But the Earth does rotate. The National Weather Service explains that rotation splits the circulation into three cells in each hemisphere.
In the Hadley cell, near the equator, air rises where it is heated and moves toward the poles high up.
In the Ferrel cell, in the middle latitudes, surface air flows toward the poles and eastward. This makes the westerly winds between 35 and 60 degrees.
In the Polar cell, air sinks over the poles, forming high pressure, and spreads outward as the polar easterlies. It is the smallest and weakest cell.
| Cell | Where | Surface air moves | Named winds |
|---|---|---|---|
| Hadley | Low latitudes, near the equator | Toward the equator, then rises | Tropical circulation |
| Ferrel | Middle latitudes | Toward the poles and eastward | Westerlies, 35 to 60 degrees |
| Polar | High latitudes, near the poles | Outward from the poles | Polar easterlies |
Between the cells lie bands of high and low pressure at the surface. High pressure sits near 30 degrees north and south and at each pole.
Low pressure sits at the equator and near 50 to 60 degrees north and south.
Fair, dry weather usually goes with high pressure, and rainy, stormy weather with low pressure.
That is why many of the world's deserts line up near 30 degrees. It is also why west coasts near 50 to 60 degrees are so rainy.
This table is drawn from the JetStream school of the National Weather Service.
| Latitude band | Surface pressure | Usual weather |
|---|---|---|
| Equator | Low | Rainy and stormy |
| About 30 degrees N and S | High | Fair and dry; many deserts |
| 50 to 60 degrees N and S | Low | Rainy; many storms |
| The poles | High | Dry and cold |
Why does the air curve? Because the Earth rotates, air moving across its surface appears to turn. This is the Coriolis effect.
In the Northern Hemisphere, moving air turns to the right. In the Southern Hemisphere, it turns to the left.
Picture a spinning turntable. Drag a pencil straight across it, and the line on the turntable comes out curved.
At the equator, the Earth's surface moves east at more than 1,000 miles per hour. At the poles, it does not move east at all.
Air leaving the equator keeps its eastward speed, so it runs ahead of the slower ground near the poles. That is why it curves east.
Near the surface, a third force acts: friction with the rough ground. It slows the wind and lets it spiral out of highs and into lows.
| Statement | True or false? |
|---|---|
| Without rotation, air would flow straight from the equator to the poles and back. | ? |
| In the Northern Hemisphere, moving air turns to the left. | ? |
| High pressure sits near 30 degrees north and south. | ? |
| Friction with the ground speeds the wind up. | ? |
Excellent map reading. Tomorrow you will find these winds in everyday life and review the week.