Jet stream cirrus, looking WSW

Jet stream cirrus, looking WSW

Ever seen the jet stream? Not on a weather map, or talked about on a weather show, but overhead in the sky?

Most of the time the jet stream, being air, is invisible. If you see clear sky looking up, or an overcast, you really can’t tell the jet stream’s up there. That’s one of the reasons it took so long to discover it was there. There were hints, starting with the way the ash from Krakatoa spread. Planes prior to WWII generally didn’t fly high enough to encounter it, though a few planes during WWII went high enough to find

Jet stream cirrus, looking north

Jet stream cirrus, looking north

their air speed and their ground speed were vastly different. Today the jet stream can make a difference of 100 mph or more in how fast a jet travels, and company meteorologists generally try to route planes away from a headwind in the jet stream.

Jet streams form where there is a large horizontal difference in temperature, and are very likely over fronts near the

surface. They also generally have a high-speed core with lower speeds to the sides, and this wind shear tends to drag out tendrils of cloud so they form lines parallel to the jet stream. If you look up and see long streaks of filmy cloud, all in the same direction from horizon to horizon, chances are good you are seeing the jet stream.

Jet stream cirrus, looking NE

Jet stream cirrus, looking NE

We don’t see that very often here in Alaska, because the jet stream is generally south of us, over the Pacific Ocean. But large waves do form in the jet stream, and last Wednesday a wave formed that put the jet stream moving from south to north over our heads. The warm air it brought aloft was reflected in a lovely warm day (77°F) at the ground, and the cirrus streaks were conspicuous. Thanks to perspective, they fanned out from the northern and southern horizons, and looked horizontal when you gazed east or west. All of the photos were taken at the Farmers Market, around 1 pm ADT.

In the lower 48, jet streams are more likely to blow roughly from west to east. Look for them!