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.

Night-glowing Clouds

I saw these clouds outside my back door last night, just as the sun was setting.

 

I think they are noctilucent clouds — nocti means nigh and lucent means glowing or shining.

Noctilucent clouds form when there is a lot of ice particles from water vapor extremely high in the atmosphere.Mysterious Glowing Clouds Appear Across America’s Night Skys In fact, noctilucent clouds are the highest kind of clouds that form.

 

Photo courtesy NASA http://spaceflight.nasa.gov/gallery/images/station/crew-17/html/iss017e011632.html. The noctiluscent clouds glow right at the edge of the atmosphere.

 

If you are thinking that you’ve never seen clouds like this before, you are not alone — noctiluscent clouds have only been reported since 1883. They are, however, becoming more common and brighter. They are also showing up in the mid-latitudes — as far south as Colorado. Nobody knows why.NASA: Strange Clouds

What makes this shot so interesting is that the sun is so low on the horizon and the ice crystals that form the clouds so high that other clouds block the sun’s rays, giving the noctilucent clouds a streaked appearance.

Minature Upslope Storm

My husband and I took a quick trip to the San Luis Valley of southern Colorado last week to see the largest migration left in North America — The Sandhill Cranes! Up to 20,000 Cranes and 20,000 other ducks, geese, coots, plovers and any other type of waterbird that you can imagine migrate through the San Luis Valley from February to April. They stop to fuel up in the rich salt marshes for their trek to Canada, Alaska and even Siberia. When they move on depends on the weather.

Unfortunately, by the time we got there, the Cranes had left the Valley.

But the San Luis Valley is such an interesting place that we found plenty of other things to keep us interested. One of the most interesting that we saw was purely by chance — a cloud getting caught on the top of Blanca Peak.

 

Warm moist air runs into 14,345 foot Blanca Peak. See how the cloud gets higher as it gets closer to the Peak? It is probably snowing at the summit. Then, once it’s over the top, POOOF! The cloud is gone.

 

This is actually a small upslope storm created as warm moist air coming up from the south (right) is forced to rise to get over the mountain. As the air rises, it cools. As it cools, it forms a cloud. Once the moist air is over the top, it can expand again and it evaporates. Climate people call this orthographic lift.

March through May is the season of upslope storms in Colorado, as warmer temperatures begin to pump air up from the Gulf of Mexico onto the Great Plains and from the Pacific into the Four Corners and San Juan Mountains. The process is exactly the same as what you see here, in miniature. These spring storms bring Colorado most of our moisture, and account for most of our snowpack.