I snipped the above map from Joseph D’Aleo’s site at WeatherBELL. (It costs me fifty cents a day, and contains the most wonderful information.) It shows the Chinook’s Foehn Winds blowing down from the Rockies into western North Dakota a couple days ago, and the temperatures rising swiftly from the teens in the northeast corner of that state to the upper thirties in the west.
When such air makes it over the Rockies it often makes a mess of a forecast, for it injects kinder and gentler temperatures into a northwest flow. Rather than bitter air from Siberia sucked over the top of the globe by a cross-polar flow, you can get Pacific air that is warmed further by a process that occurs when air is lifted over mountains.
Such air has its moisture wrung out of it. As the moisture is wrung out the latent heat in water vapor, (the heat put into it turning it into steam,) is released. Even though the air is cooled as it is lifted, it is warmer in a relative sense. As it gets higher, and the water turns to snow, further latent heat is released. (The heat put into melting the ice in the first place.) By the time the air crests the mountaintop much moisture is gone, and much heat has been added. Then, as the air descends, it warms, and is much warmer than it was when it began, when it sinks back down to the same altitude it began at.
If the air was relatively mild pacific air to begin with, by the time it crosses the Rockies it is quite warm and bone dry, and “eats” the snow. (The word Chinook means “Snow Eater.”)
Then it crosses the entire country, becoming moister and cooler as it comes, but of a completely different character than the bitter arctic sting northwest winds usually have in January, in New Hampshire.
As I worked outside at our Childcare yesterday you could feel the change, even before the thermometer showed it. There was very little wind, but even as the sun sank into a ruddy twilight there was a mercy in the growing night. It made me stand still under the appearing stars, wondering if the deer were also standing and sniffing the change in the weather. The children by the campfire at the skating pond also seemed to note the change, in their intuitive manner, leaving the fire more, and spending more time constructing their mazes of paths on the snow-covered ice.
When I got home and checked the maps there was no warm front to see, no cloud shield indicating warmer air. There was even an early zero reading in a valley thirty miles to our northwest, however elsewhere temperatures were hesitating. After refusing to rise in the day’s brilliant sunshine, they were refusing to fall as night pressed down. The arctic shot had passed by, and the Chinook had arrived.
This daybreak’s temperatures are up into the twenties already, when they struggled to reach fifteen at noon yesterday.
I’ll take such mercy where I can find it, for the Chinook’s benevolence will pass by just as the arctic shot did, and there’s plenty of arctic upstream, waiting in the wings.