Weather

Even if you rarely expect to climb above 10,000 feet in an airplane you’re flying, understanding high-altitude weather will help you cope with weather closer to the ground. Weather is three-dimensional. What happened aloft a few hours ago can affect the weather you’ll face as you line up on the runway for takeoff.

Thunderstorms and their potential violence offer a good example. College or university programs for pilots normally require semester-long meteorology courses that focus on the atmosphere. Most other student pilots learn mostly about surface and near-surface weather. For example, they learn that thunderstorms are most likely in warm air as an advancing cold front shoves the warm, humid air up to create thunderstorms. Pilots who have taken college-level meteorology know that what’s happening as high as 35,000 feet above the ground can make the difference between an afternoon with isolated “ordinary” thunderstorms that pilots can easily avoid and a tornado outbreak that can threaten airplanes parked in well-built hangars.

Knowing at least a little about how events high above can affect your low-level flight will help you make better decisions about when to fly and when to wait and go another day. Such knowledge could tell you when you should be extra alert to potential weather dangers arriving sooner than forecast.

Early in the twentieth century, meteorologists who used telescopes to track balloons measured high-altitude winds as fast at 150 mph, but these seemed to be nothing but curiosities. As airplanes begin flying higher in the 1930s, pilots began encountering what we now call “jet stream” winds, but no one could figure out how they fit with what scientists then knew about weather.

Pilots of World War II aircraft that routinely flew higher than 20,000 feet regularly told meteorologists about encountering 150-mph headwinds or tailwinds. After the war, the U.S. military funded university research into these then-mysterious winds. This research led to today’s knowledge of how jet streams fit into the global air circulation. It also led to regular observations of high-altitude winds, temperatures, and humidity that helped improve all weather forecasts, for both the weather aloft and at the surface.

Looking over weather maps showing various levels of the upper atmosphere is a good way to begin understanding weather aloft. These maps are different from surface weather maps you’re probably used to. Surface maps show the sea-level air pressures at the stations on the chart, even when the stations are not at sea level. They also show centers of high and low pressure and fronts.

Instead of showing the pressures at fixed altitudes, as you might expect, upper air charts show the altitudes of specified pressures measured by weather balloons. The charts show the pressure altitude of specific pressures in millibars. Pressure altitude is the altitude measured by a properly calibrated altimeter set at the standard sea-level pressure of 1013.15 millibars (mb), which is equivalent to 29.92 inches of mercury in the inches-of-mercury pressure system the NWS uses for public forecasts and pilots use for altimeter settings.

The 500-mb chart is a close-up view of the map at left of the eastern United States and Canada. It helps to illustrate how temperature differences create upper-atmosphere winds. The weather balloon launched by the Environment Canada weather station at Maniwaki, Quebec, northwest of Montreal, found the 500-mb pressure level at 5,120 meters above sea level. At roughly the same time the balloon launched by the NWS at Key West, Florida, measured 500-mb 5,880 meters above sea level.

We’ll consider only pressures aloft over Key West and Maniwaki and the pressures at only at 5,120 meters above sea level at each station. Since the 500-mb pressure over Key West was all the way up at 5,880 meters, the pressure there at 5,120 meters was higher than at Maniwaki. This pressure creates a force trying to push air from Key West to Maniwaki.

This force doesn’t create wind blowing directly from Key West toward the north because whenever air begins moving across the Earth either at the surface or aloft, the Coriolis force, which is caused by the Earth’s rotation, pushes the wind toward the right in the Northern Hemisphere and toward the left in the Southern Hemisphere.

If the Key West-Maniwaki wind were the only one on Earth, it would follow a path along one of the pressure lines over Virginia and North Carolina, where the winds are blowing faster than 120 knots.

The actual winds, of course, are the results of the complex set of pressure forces among all parts of the atmosphere.

In general, since temperatures throughout the depth of the atmosphere create upper-atmosphere winds, greater temperature contrasts create stronger winds. The speed and location of these winds depend on the locations of warm and cold air.

The chart above illustrates how the average temperature of a column of air determines the height of a particular pressure, such as 500 millibars. Atmospheric pressure depends on the amount of air above the place where you’re measuring the pressure. To keep it simple, we’ll assume that the same number of molecules of gases make up the air from the surface to the top of the atmosphere everywhere in the illustration.

In the cold air on the left, the column of air has contracted as the speed of its molecules slowed down as the air grew colder. The column of air below a pressure of 500 millibars has contracted just as a steel rod would. In the warm air on the right, the column of air has expanded as it warmed up, just as the steel rod would when heated, pushing the 500-millibar level higher.