Weather

When you’re watching the weather on local television—which helps you see the big picture before you obtain a preflight weather briefing—the meteorologist is likely to show a simplified upper air map to help explain the current or expected weather while pointing to the “jet stream” on the map.

Often the map’s “jet stream” doesn’t meet the strict definition: “a relatively narrow river of very strong horizontal winds (usually 50 knots or greater) embedded in the winds that circle Earth aloft.” That’s OK. For your purposes, all you need to know is generally which way winds are blowing high in the atmosphere.

This is because high-altitude winds follow the boundaries between deep layers of warm and cold air. These boundaries are fronts at the surface and are usually locations of potentially dangerous weather.

If the “jet stream” you see on television is running generally west to east with maybe a few shallow waves, the weather should remain generally calm for at least the next day or two.

Since temperature of the air between the surface and any particular level in the upper atmosphere determines patterns of winds aloft, high-altitude winds that turn toward the south indicate cold air is moving south.

The jet stream is above the warm-cold air boundary below. When a jet stream that’s been heading toward the south turns to head north, it shows that warm air below is moving toward the north.

On the other hand, upper-level winds blowing generally west to east indicate that large masses of cold air are not moving toward the south and masses of warm air aren’t moving north. The absence of such movements stifles storms because they form when masses of cold and warm air come into conflict.

A closer look at jet streams. Figure 1 is a simplified 300-millibar map of North America showing only wind speeds and directions roughly 39,000 feet above sea level. The slightly darker black lines with numbers at the ends are isotachs, which connect equal wind speeds. The numbers at the end of the line show wind speeds in knots. If you look closely at these lines you don’t see any numbers below 50.

The lighter red areas are winds between 75 and 100 knots and the dark red indicates winds faster than 100 knots.

The map’s slightly lighter lines—such as those making circles off the Southern California coast and over the Gulf of Mexico—are contours of equal pressure heights.

The map shows a “polar jet” along and mostly north of the United States-Canada border; and a “subtropical jet” across the south and running up the Atlantic Coast. Such a pattern is common across North America during the winter.

You’ll see that an area of 50 knots plus connects the parts of the subtropical jet winds that drop below 78 knots above the Southeast and resume off the North Carolina coast. The polar jet’s dip over the Great Lakes shows that cold air has moved into this area.

The map clearly shows that jet stream wind speeds vary with “jet streaks” of faster winds embedded in slower winds.

Figure 2 is a three-dimensional view of a jet stream. As you see in Figure 2, jet streams are somewhat like tubes within tubes—although the “tubes” in real jet streams aren’t as perfectly round as those in the drawing. The tube at the center in Figure 2 could represent a core of winds as fast as 300 knots, although highest speeds are more often in the 150-knot range. The outer tube in the figure represents 50-knot winds.

The weather below. Jet streaks are one important way in which jet streams affect the weather down to the surface. At the beginning of a jet streak, such as off the coast of Baja California, Mexico (Figure 1), the air is speeding up. At the end of a streak, such as over the Southeast, 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 forms 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 form or strengthen an area of high pressure at the surface or to weaken an area of low pressure on the surface below.

The curving paths of jet streams also cause air to diverge or converge with similar effects on atmospheric pressures at the surface. These effects of jet streams are one of the biggest ways in which winds aloft influence the weather at the surface and the lower levels of the atmosphere where most general aviation aircraft fly.

Surface weather. If you look at a surface weather map that shows winds, you’ll see air is moving in counterclockwise swirls (in the Northern Hemisphere) as it flows into low-pressure areas. Air flowing into a low-pressure area would eventually increase the pressure inside the area to match pressures outside and wind would stop blowing. Meteorologists say the low has filled.

On the other hand, if air is flowing up from the surface low faster than surface air is arriving, the pressure in the low will drop. Meteorologists say the low has deepened; it is stronger and winds flowing into it will blow faster.

If converging air aloft is forcing air down into the surface low, the low will fill and winds flowing into it will slow down. Converging air aloft can also strengthen an area of high pressure at the surface if it more than makes up for air that’s flowing out of the surface high at the surface in a clockwise swirl (in the Northern Hemisphere).

Figure 3 shows a typical relation between jet stream winds and a surface storm system with a low pressure center, marked with the small “L” over the area where Oklahoma, Kansas, Missouri, and Arkansas meet. The curving black lines are pressure contours at the 300-millibar level.

The large “L” at the top of the page represents the lowest altitude of the 300-millibar pressure on this map. The dashed line from it down over Texas is the axis of a trough aloft, an elongated area of low pressure.

As shown here such high-altitude troughs often help set the stage for storms.

If you look at the three contour lines centered on the surface low, you see they are diverging. In other words, the 300-millibar winds are spreading out here. The surface low formed and strengthened as air rose to replace the diverging air aloft.

The relations among upper air and surface weather features, like most other aspects of meteorology are complex, but being a weatherwise pilot doesn’t require mastering meteorology’s complexities.

Nevertheless, you should go beyond what most self-study, pilot-training materials offer. Consider a good, college-level beginning text. One of the best is Meteorology Today: An Introduction to Weather, Climate, and the Environment by C. Donald Ahrens.