Chinooks and Mountain Weather

A warm wind in the middle of winter

It is January. The morning is bitterly cold, the snow squeaks underfoot and the car groans when it starts. By mid-afternoon the eaves are dripping, the streets are running with meltwater and people are walking around with their jackets open. Nothing about the season has changed. The wind has.

That wind is a chinook: a warm, dry, often gusty west or southwest wind that comes down the eastern slopes of the Rockies. The name is generally traced to the Chinook people of the Pacific Northwest, and the wind is often nicknamed the “snow eater” because of how quickly it can make a snowpack vanish.

Chinooks are not unique to Alberta. The same thing happens downwind of mountain ranges around the world. In the Alps it is called the foehn, and scientists often use “foehn” as the general name for any warm, dry wind blowing down the lee side of mountains.

The rule that makes chinooks work

Everything about a chinook rests on one simple rule: rising air cools, and sinking air warms, even when no heat is added or taken away.

Think of a bicycle pump. Push the handle down hard a few times and the barrel gets warm, because squeezing air packs its energy into a smaller space. Now think of a spray can: let the gas out quickly and the can feels cold, because the gas expands and spreads its energy out.

The atmosphere does the same thing on a huge scale. Air pressure drops as you go up (remember Lesson 3), so a pocket of rising air expands and cools. A pocket of sinking air gets squeezed by the higher pressure below and warms up. Meteorologists call this adiabatic warming and cooling, a fancy word meaning “no heat in or out.”

Two speeds of cooling

Dry air (air that is not forming cloud) changes temperature by about 1°C for every 100 m it rises or sinks. Once air is cooled enough to form cloud, it cools more slowly as it keeps rising, typically somewhere around half that rate. The reason is hidden heat. When water evaporated into that air, back over the ocean, it soaked up energy. When the vapour condenses into cloud droplets, that energy is released again, partly offsetting the cooling.

The trip over the mountains

Here is the classic chinook story, step by step:

1. Strong westerly winds aloft push mild Pacific air inland toward the mountains of British Columbia and the Rockies.
2. The air is forced up the western slopes. It cools, first at the clear-air rate, then more slowly once cloud forms.
3. Rain and snow fall on the windward side. This is why the western slopes are so much snowier than the prairie.
4. The air crosses the ridges having dropped much of its moisture.
5. It sinks down the eastern slopes, warming at the full clear-air rate the whole way, and arrives on the prairie warmer and much drier than it started.

Scientists have since learned that real chinooks are often a bit more complicated. In many events, cold air on the western side is partly blocked by the mountains, and the air that reaches the prairie actually comes from near or above ridge-top height. Simply sinking from that altitude warms it a great deal, even without much rain or snow on the western slopes. Either way, the key ingredient is the same: sinking air warms.

Reading the sky: the chinook arch

On a chinook day, look west. You may see a long, flat-bottomed sheet of cloud stretching across the sky, with a band of clear blue sky running underneath its western edge, parallel to the mountains. That is the chinook arch. At sunrise and sunset it can glow orange and pink, one of the great sights of the prairie sky.

The clear band is a clue to what is happening. Directly east of the mountains, air is sinking, and sinking air warms and dries out any cloud. Farther east, the air bounces back up in a wave, cools again and forms that sheet of cloud. The arch is the visible fingerprint of the mountain wave.

Mountain waves, rotors and wind storms

Watch a fast creek flowing over a large rock. Downstream of the rock, the water rises and falls in a series of ripples that stay in the same place even though the water rushes through them. Air flowing over the Rockies can do exactly the same thing. These standing ripples are called mountain waves, and they can stretch a long way downwind.

Where air rises in a wave crest, it cools and may form smooth, lens-shaped lenticular clouds. They look as if they are parked in the sky, even in a howling wind, because the air flows through them: cloud forms on the upwind edge and evaporates on the downwind edge. Underneath the waves, air near the ground can tumble in a rotor, a zone of violent turbulence that pilots take very seriously.

Under the right conditions, especially when a stable layer of air sits just above the mountain tops, the flow can surge down the eastern slopes as a downslope windstorm. That is when southern Alberta sees its most damaging chinook winds. The area around Pincher Creek and the Crowsnest Pass is famous for these winds, which is one reason wind turbines line the ridges there.

How a chinook ends, and the battle with Arctic air

A chinook lasts only as long as the strong westerly flow keeps pouring over the mountains. It can end in two main ways. The west winds aloft may simply weaken, or, more dramatically, a mass of cold Arctic air can slide back south along the foothills. Arctic air is dense and shallow, like cold water creeping along the bottom of a bathtub, and it can undercut the warm chinook air within hours.

Sometimes the two air masses fight back and forth for days. The boundary can sit right over southern Alberta, so one town is mild and breezy while another, not far away, stays locked in the cold. The temperature can yo-yo as the edge wobbles back and forth.

Cold air can also hide in low spots. The Oldman River valley and the coulees can hold a pool of cold, calm air even while the tablelands above are mild and windy, until the chinook finally scours it out.

Many Albertans also swear that chinooks bring on headaches. Researchers have looked into a possible link with migraines, and some people do seem more sensitive than others, but not everyone notices any effect.