Chinooks blow

The Front Range of Colorado is under a high wind warning today — we’re having a Chinook!

I’ve talked about chinook winds before at https://amylaw.blog/2014/02/15/snow-eaters-melt-the-snow/ But in the intervening six years, I’ve learned a bit more about them, as well as upgrading my graphics and getting some better photos of what I’m talking about.

Chinook winds blow when warm moist air crosses a range of mountains. As the air mass crosses the mountains, it rises; as it rises, it dries.

As it crosses the ridge of the mountains, the now warm very dry air mass is held down by a strong jet stream. The warm dry air is trapped between the ground below and the jet stream above.

Lenticular clouds above the Flatirons west of Boulder Colorado. Fohn wall is behind them, to the left.

As the air moves from cold to warm, the clouds form and evaporate.

Fohn arch.

When the now warm very dry air hits the ground, we can have winds from 30-90 mph (48-145 km per hour). Chinooks blowing at these speeds can blow over semi-trucks. In the 1970s, extremely violent chinooks hit the National Center for Atmospheric Research, just west of Boulder, and pegged their anemometer (wind meter) at 120 mph (193 km per hour).

Broader, longer Fohn arch. The mountains are behind the trees.

Chinooks can raise the air temperature 40o (4o C) in minutes. Because they are dry winds, they can evaporate and sublimate (evaporate directly from snow or ice, without melting into water) snow at an astonishing rate. This is nice for getting ice off sidewalks, but chinooks have caused us to go from having a comfortable amount of snow in the mountain snowpack to needing water restrictions within the space of a day. This is why the Blackfeet Indians of Montana call it “snow eater.”

Chinooks blow on the lee side of mountains all over the world. In Europe, they are called Fohn winds; in California, Santa Anas. Loo winds blow off the Kirthar Range in Pakistan east onto the Indo-Ganges Plains in summer, making a hot season hotter.

Windy Days

We’ve had some windy days lately. Two days ago (October 20, 2019) we had gusts up to 40 mph (miles per hour) — it was hard to walk in that wind!

As we battled the blustery weather while walking the dogs, I happened to look to the west, where I saw lens-shaped clouds hovering over the tops of the mountains.

Once inside, I decided to clean up some photos on the computer. I happened across this shot of Longs Peak from near Estes Park from fifteen years ago.

North Face of Longs Peak

North Face of Longs Peak, October 2006

What caught my eye initially was the odd shaped cloud over the east face of the mountain top — just like what I’d seen while walking the dogs. This is called a ‘lenticular cloud’, meaning lens-shaped. Lenticular clouds indicate that the wind is really ripping, pulling relatively moister air up to the top of the mountain, where it forms a cloud as it crosses over. Although these clouds seem to stand still, in reality, they are constantly forming on the near side, then evaporating on the far.

Estes Longs 6..JPG

Notice the snow blowing off the ridge to the right (west) and dropping into the basin below the summit. This extra snow helps build glaciers.

According to the Rocky Mountain National Park Service Wind page, in the winter, the average daily wind speeds on Longs Peak are 65 mph, so the average is higher than our peak wind speed on Sunday. It often blows at over 100 mph, and the maximum wind speed recorded was in excess of 200 mph!

Suddenly, I’m more appreciative of our relatively calm air.

Snow-eaters melt the snow

Where did all the snow go? Last week at this time, the Front Range was entirely blanketed in several inches of the white stuff. Today, it’s almost gone. Where did it go?
The Answer: Chinooks.
Chinooks are warm dry winds. They get their name from a Pacific Northwest Indian word for “snow eater,” because when the chinook blows, the snow goes. With gusts that blast up to 120 miles per hour, pushing the temperature up 40 degrees F in minutes, chinooks along the eastern side of the Rockies are the most violent in the world. You always suspected that, didn’t you?
What causes a chinook? Chinooks are cool air pushed down by the weight of a high pressure front above them. Squeezing the cool air warms it up, to become warm dry, air. In winter and spring, chinook winds often buffet the areas just east of the mountains from Canada all the way to Mexico.14-Chinook
At ground level, the now-warm dry air is no longer under pressure. Like any compressed gas, it expands when released, and rips along the surface at 50 to 120 miles per hour as a chinook, melting and evaporating snow as it goes. By definition, chinook winds are warm and dry, and they suck the moisture out of plants, soils and people as they blow.

Foehn wall hanging above the Front Range (not visible in this photo).

Foehn wall hanging above the Front Range (not visible in this photo).

When a chinook blows, look toward the mountains. A bank of smooth clouds often hangs above the highest peaks. This is known as a foehn wall; it is created as the cool, wet, air crosses the mountains and is then forced down by high air pressure above that causes the chinook as well.
As the cool wet air containing the foehn wall is pushed down, it is compressed by the air above it; as it is compressed, it heats; as it heats, the amount of water it can hold increases (dew point rises) and the foehn wall cloud evaporates.
Chinooks also have the odd habit of “bouncing” over the land. They swoop down, blast some poor place with hurricane-force winds and then bounce back up, totally skipping over a locale just a few miles down the road.
In California chinooks are called Santa Aña winds, and in Europe, foehn (German for “warm”, pronounced “phone”. Actually, I heard a native German-speaker pronounce this word and it was closer to the way Peter Sellers and Steve Martin did it in the “Pink Panther” movies: “fun” as in “Your fun is ringing.”).

Beautiful, mesmerizing, educational.

My mom sent me this website a couple of weeks ago. http://hint.fm/wind/index.html  It shows the direction and speed of surface winds for the entire country. This screen shot doesn’t do it justice, because on the webpage, the wind lines flow. Beautiful.Image

In addition to being mesmerizing and beautiful, the map is very educational. When you look at it closely, you’ll see that the winds are flowing from blank areas to areas where they curl and swirl. The blank areas are areas of high air pressure, pushing extra air out; the swirls are areas of low pressure, where the air is sucked in.Image

Even more interesting is that the swirling lows often represent bad weather with high winds. The lows pull in moisture, in this case from the Gulf of Mexico. This is the moisture coming in for our storm tonight.

The Wind Map was created, not by the National Weather Service, or similar group, but as an art project from Point B. Studio. http://memory.org/point.b/windmap.html