Seeing the Jet Stream

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

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

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!

January Wasn’t Warm in Alaska!

A warm January? Not in Alaska! Borrow, at the northernmost tip of the state, was only 7°F below normal, but the western and central parts of the state fared far worse: 22°F below normal at Nome, in the far west, 24°F below normal in Bethel in the southwest, a mere 14.5° below normal at Anchorage in the south, even 4°F below normal at Annette, in the far southeast.

Fairbanks, the area where I live, was 19°F below normal, for the fifth coldest January in a record that goes back over 100 years, and the coldest January since 1971. Snowfall was extremely heavy on the south coast, but lacking inland.

I remember 1971, as does anyone who lived through it. We had 100” of snowfall before the end of 1970, and the roads were well coated with ice. That was the Christmas my father visited from Kansas, and I still remember picking him up at the airport very late in the evening of December 23. The temperature was 36 above and it was raining. My father couldn’t understand why I was driving only about 5 miles an hour on a straight, level road that simply looked wet from the rain. I made sure there was no traffic in sight before I tapped the accelerator to show him why. It was a light tap, and I was able to let the fishtail correct itself, but there’s a lot of difference between wet pavement and wet ice.

By the time my father left, at the beginning of January, the temperature was 36 below and dropping fast at the start of the coldest January in the Fairbanks record. Waist-deep snow with an icy crust from the rain did not help!

But why was Alaska so cold this year when much of the rest of the country was warm?

The sun shines more directly and longer at the equator than at the poles, especially in winter. The air is radiating energy to space, so if there were no movement of air, the poles would get colder and colder while latitudes nearer the equator would get warmer and warmer. Since the earth rotates, direct flow of cold air south and warm air north forces a west-to-east current of air, the jet stream, which always has warmer air to its right side (in the northern hemisphere) and cold air to its left.

But this is an unstable situation – the cold air keeps getting colder and the warm air keeps getting warmer until the simple west to east flow breaks into waves which carry colder air south and warmer air north, though at different longitudes. In one extreme case (rare this winter) warm air from Hawaii flows north over Alaska, the so-called pineapple express, and the air turns back south over Northwestern Canada and continues south to produce the Siberian express. When this happens, temperatures in Fairbanks can be warmer than those in Miami.

The jet stream not only moves air, it steers storms. So storm tracks frequently follow the jet stream.

This January the jet stream in the western hemisphere didn’t have much in the way of waves. The cold air stayed north, while the warm air stayed south. The upper winds over Alaska mostly came from Siberia or the northernmost Pacific. Things are changing, though. Not quite as much as I might like, but probably a bit too much for those of you enjoying unseasonably balmy weather. For the jet stream forecasts, look here. The different colored lines are different forecast models. Where they cluster close together, the forecast is likely to be accurate; where they spread apart there is a lot of uncertainty. Next week? It might even warm up here, but there’s a lot of spread in the forecasts, so I’m not counting on it.