Global Winds: Pressure, Convection Cells, the Coriolis Effect and the Jet Stream

Let's start with a weather forecast

Watch a weather forecast for Ontario and you'll hear things like "a low-pressure system moving in from the Prairies" or "a cold front crossing from the west". Almost never do you hear "a storm arriving from the east". Why does our weather nearly always travel west to east?

The answer starts with uneven heating (Lesson 1) and convection (Lesson 2), then adds one more ingredient: Earth is spinning.

Step 1: Uneven heating makes pressure differences

Air has weight, and it presses on everything. This is air pressure. Where the surface is heated strongly, air warms, expands and rises, leaving lower pressure at the surface. Where air cools and sinks, it piles up and makes higher pressure at the surface.

Wind is air moving from high pressure toward low pressure. The bigger the pressure difference over a given distance (the pressure gradient), the stronger the wind. Weather maps show pressure with lines called isobars; where isobars are close together, the wind is strong.

Step 2: The three-cell model

If Earth didn't spin, hot air would rise at the equator, flow toward the poles high up, sink at the poles and return along the surface: one giant convection loop per hemisphere. Because Earth does spin, the circulation breaks into three cells in each hemisphere:

• Hadley cell (0° to 30°): Hot, moist air rises at the equator, cools, and drops heavy rain. That's why the tropical rainforests are near the equator. The now-dry air flows poleward high up, then sinks around 30°. Sinking air warms and dries, so many of the world's great deserts (Sahara, Arabian, Australian, Sonoran) are near 30° N or S.
• Ferrel cell (30° to 60°): a middle cell, driven by the two cells beside it. Surface air flows poleward.
• Polar cell (60° to 90°): cold, dense air sinks at the pole and flows toward 60°, where it meets warmer air and rises. Polar regions are actually very dry; much of the high Arctic gets less precipitation than some deserts.

Where air rises (around 0° and 60°), there is more cloud and precipitation. Where it sinks (around 30° and the poles), the air is drier.

Step 3: The Coriolis effect

Earth rotates from west to east. A point on the equator travels about 1670 km/h, while a point near the pole barely moves. Air (or water, or a long-range ball) moving across this spinning surface appears to curve: to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This is the Coriolis effect. It is strongest near the poles and zero at the equator.

Apply it to the surface winds from the three cells (Northern Hemisphere):

• Air flowing south toward the equator in the Hadley cell is turned right, toward the west. It becomes the northeast trade winds (they blow from the northeast).
• Air flowing north in the Ferrel cell is turned right, toward the east. It becomes the westerlies (blowing from the west).
• Air flowing south from the pole becomes the polar easterlies.

Ontario (roughly 42° N to 57° N) lies in the westerlies. That's why weather systems usually move from west to east across Canada, and why a storm in Manitoba today may reach Ontario tomorrow.

Highs, lows and air masses

The Coriolis effect also makes air spin around pressure centres. In the Northern Hemisphere, air flowing into a low is turned right, so it circles counterclockwise as it spirals in. The air in the middle rises, cools and forms clouds: lows bring unsettled, wet weather. Around a high, sinking air spirals out clockwise and usually brings clear skies.

Winds also move air masses: huge bodies of air that take on the temperature and humidity of where they formed. In Ontario, continental Arctic air from the North brings bitter winter cold, while maritime tropical air from the Gulf of Mexico brings hot, humid summer days. A front is the boundary where two air masses meet; warm air is forced up over the denser cold air, producing clouds and precipitation.

The jet stream

High above the boundary between cold polar air and warmer air to the south, about 9 to 12 km up, is a narrow river of fast wind called the polar jet stream. It blows from west to east, often at well over 100 km/h. The jet stream forms because of the big temperature difference between polar and mid-latitude air.

The jet stream doesn't flow in a straight line. It meanders north and south in big waves. Where it dips south, cold Arctic air can spill into southern Canada and the U.S.; where it bulges north, warm air reaches farther north. Low-pressure systems tend to form and travel along it, so the jet stream acts as a storm track. When a big wave in the jet stream stalls, the same weather can sit over a region for days: a heat wave, a long rainy spell or a deep freeze.

The Arctic is warming faster than the rest of the world, which reduces the temperature difference that drives the jet stream. Scientists are actively researching whether this is making the jet stream wavier or slower. This is an area of real scientific debate, which you'll revisit when you look at changes in wind patterns as an indicator in Lesson 13.