Weather

Watching your favorite local television meteorologist in the evening is a good way to begin thinking about the weather for a flight you plan to make the next day.

You need to wait until an hour or so before taking off to obtain the required aviation weather briefing, but you can learn the night before about the next day’s weather. Will it be so unruly that you don’t even want to think about flying, or is a good day in store?

The broadcast meteorologist is likely to show a simplified upper air map while pointing to a “jet stream” running generally west to east across the United States. Meteorologists define a jet stream as “a relatively narrow river of very strong horizontal winds, usually 50 knots or faster, embedded in the winds that circle the Earth aloft.” These winds flow generally from west to east above both the Northern and Southern hemispheres.

Most general aviation pilots don’t fly high enough to encounter jet-stream winds. Nevertheless, jet streams high above you can have major or indirect effects on the weather at lower altitudes down to the surface. And at times, bands of wind blowing faster than 50 mph and less than 300 feet above the ground—called low-level jet streams—roar across the night sky, especially over the Great Plains in the summer.

THE JET STREAM WEATHER WALTZ. Broadcast meteorologists often relate the jet stream to surface weather. For example, if tomorrow’s forecast shows the jet stream curving to flow toward the south over your location, the meteorologist is likely to point at the jet stream’s dip on a national map and say something such as, “This dip in the jet steam will bring us cold weather.”

This is true enough, but it’s a kind of shorthand to describe the relationship between air temperatures below jet stream altitudes and the paths of jet streams. Air temperatures at all levels of the atmosphere, and jet stream winds, are engaged in a complex waltz, and who’s leading is hard to pin down.

Figure 1 starts to show how this “waltz” begins. The figure shows air temperatures and atmospheric pressures at a few representative altitudes from a U.S. weather station near the Gulf of Mexico, on the left, and one near the Canadian border, on the right. At each altitude, the pressure over the southern station on the left is higher than the pressure at the northern station on the right. Since cold air is denser than warm air, the column of air contracts just as a cold piece of metal would. But the air in the warm column expands. This creates pressure differences between the two columns of air, which in turn create pressure-gradient forces at each level that start pushing air from the high to the low pressure—that is, from the south toward the north.

In Figure 2 we see how forces come into play to create a jet stream, shown by the wavy green crossing the United States. The short, blue arrows south of the jet stream represent the pressure gradient forces that are pushing air at the 500-millibar level toward the north.

Once the air begins moving, the Coriolis force—caused by Earth’s rotation—causes the moving air to turn toward the east. The coldest air is over the central United States, which pushes the jet stream farthest south over Texas. The jet stream turns to blow toward the northeast in the East, where the contrast between cold and warm air below is not as great as over the south-central states.

If the jet stream you see on television is running generally west to east, the weather should remain generally calm for the next day or two.

As the winds of a jet stream are roaring along, the entire pattern is moving generally toward the east. For example, the jet stream in Figure 2 has its southernmost point over southern Texas. In a day or two this point could be over South Florida. If you were watching the evening news in, say, Atlanta, the meteorologist might say the jet stream is bringing cold air to northern Georgia.

HOW JET STREAMS AFFECT THE WEATHER BELOW. Areas of winds that are faster than the surrounding winds are called jet streaks. Both a jet stream’s curving winds and its jet streaks affect the weather at the surface and altitudes where most general aviation aircraft fly.

Where a jet streak begins—such as off the coast of Baja California, Mexico, in Figure 3—the air is speeding up. At the end of a streak, the air is slowing down. Where the wind is speeding up, the air is spreading out as fast-moving air leaves slower-moving air behind. Meteorologists say it’s diverging. Such divergence encourages air to rise from the ground, somewhat like air rushing to fill a vacuum. As air rises, it creates or deepens areas of low atmospheric pressure at the surface.

At the end of a jet streak where the wind is slowing down, the air is piling up—converging—which forces air down to create or strengthen an area of high pressure at the surface. Such convergence also can weaken an area of surface low pressure. The curving paths of jet streams also cause air to diverge or converge with similar effects on atmospheric pressures at the surface.

The formation and strengthening or weakening of surface areas of high and low air pressure help storms form and increase in strength or, in other cases, weaken. Storms in turn push warm air to the north, in the Northern Hemisphere, and cold air to the south. Here the weather waltz takes another turn: Movements of warm and cold air at or near the surface affect the locations and strength of jet streams.

You can think of all of this as weather’s classic chicken-or-egg question: Which came first, jet streams—or areas of warm and cold air masses?