March 2010Departments

The Weather Never Sleeps

Rise up

Atmospheric stability is key

Twice each day the U.S. National Weather Service (NWS) launches weather balloons carrying packages of instruments aloft from 92 stations in North America and the U.S. Pacific islands. As these balloons rise to 100,000 feet or higher, the attached package of instruments—called a radiosonde—collects air pressure, temperature, and humidity readings, as well as the package’s geographic location, and radios the data to the NWS. Computers use the location data to calculate wind speeds and directions at the altitudes the balloon is rising through.

The U.S. stations are part of a global network of approximately 800 stations that launch weather balloons at midnight and noon Zulu time each day. (Zulu time is the standard time at Greenwich, England, which is why some call it Greenwich Mean Time.)

Pilots, including student pilots who’ve learned navigation, understand the need for wind data for flight planning. The air pressure, temperature, and humidity data do more than satisfy idle curiosity of meteor-
ologists. Pressure measurements enable meteorologists to map areas of high and low pressure at altitude, which determine wind strength and direction. The temperature and humidity data enable them to calculate the atmosphere’s stability, which is used to predict the weather over the next several hours.

When meteorologists say the atmosphere is stable, they mean that if a bubble of air is given a shove upward, it will stop rising as soon as the force pushing it is removed. When the atmosphere is unstable, air that’s given an upward push will continue to rise after the upward force stops.

The “shove” that causes the air to begin rising could be from wind flowing over a hill or a mountain, warm air flowing over cooler air as a warm front advances, or a cold front pushing warm air up as it advances. The sun’s heating of the Earth can cause air to begin rising after the ground warms the air right above it.

To understand what happens after something gives air an initial shove upwards, we need to look at what happens when a parcel of air rises. Meteorologists talk about parcels of air to simplify what goes on in the atmosphere. Think of a parcel as a bubble of air in a stream of rising bubbles. As long as the water vapor in a parcel of rising air is not condensing into cloud drops, the rising air cools at the rate of 5.5 degrees Fahrenheit for each 1,000 feet it rises. This rate is always the same because the temperature of the surrounding air doesn’t affect it. This temperature change is called the dry adiabatic lapse rate.

Eventually, if the air rises high enough, the air will grow cold enough for the water vapor in it to begin condensing into tiny cloud drops—or turn into ice crystals. When this happens the condensing or freezing water vapor releases the heat, called latent heat, that it gained when it evaporated from water to become water vapor. This added heat slows the cooling rate of the rising air. The new cooling rate is called the moist adiabatic lapse rate. While it varies, a good average figure is about 3.3 degrees per 1,000 feet.

Since warm air is less dense than cold air, if both are at the same air pressure, an air parcel will continue rising as long as it’s warmer than the surrounding air.

We see, then, that the atmosphere is stable when rising air—cooling at 5.5 degrees for each 1,000 feet it rises—grows cooler than the surrounding air. If the rising air stays warmer than the surrounding air, the atmosphere is unstable. Because the rate of cooling by rising air doesn’t change, differences in temperatures of the air above a location determine whether the atmosphere there is stable or unstable.

Weather balloons are a major source of temperature readings from right above the ground to 100,000 feet up. The change in temperature with altitude is the environmental lapse rate.

Using the term lapse rate in these two senses can be confusing. Remember, the adiabatic lapse rate describes changes related to the process of air rising, and cooling because it’s rising. The environmental lapse rate, on the other hand, represents a condition—the temperatures are those that the air at various altitudes just happens to be.

The examples shown in the figures on this page will help you understand stable and unstable atmospheres.

In general, a stable atmosphere is characterized by air that cools relatively slowly as you go aloft. Very warm temperatures at the ground combined with very cold temperatures aloft make the atmosphere unstable.

The atmosphere changes its stability in many ways. One of the most common occurs on clear nights when heat from the earth radiates out into space, cooling the ground. As the ground cools, it chills the air next to it, which makes the air more stable. In fact, the air in the first few hundred feet above the ground can actually grow colder than the air above it. This creates a temperature inversion with warm air atop cold air. An inversion is the most stable atmos-pheric condition because rising air, which is becoming colder as it rises, is moving up into warmer air. The inversion acts like a lid on rising air.

As the sun comes up on a clear day, a stable atmosphere begins to grow unstable as the sun heats the ground. As the ground warms, it heats the air next to it while the air aloft does not warm. In addition to daytime heating, the air can become more unstable as winds bring in cold air aloft. High humidity makes the air less stable because as very humid air rises, its water vapor begins condensing sooner to cool the rising air at the slower moist adiabatic lapse rate.

As with most aspects of the atmosphere, there are gray areas. The atmosphere in a particular location doesn’t have to be all stable or unstable. It’s not unusual for the atmosphere to be unstable at some altitudes and stable at others.

On a day when the atmosphere is stable, pilots can expect a smooth ride because currents of rising and sinking air are suppressed. But the smoothness might come with poor visibility. That’s especially true if the stable air has been around a few days, because clean air from aloft is not mixing with polluted air at lower levels.

When the air is very unstable violent thunderstorms are likely. These are characterized by rising air currents that climb above 60,000 feet. When the air is only slightly unstable showers are likely, but not steady rain over a wide area.

Ever wonder what happens to a weather balloon after it collects its measurements of the air’s stability? The balloon, which was approximately six feet in diameter at launch, expands as it rises into lower air pressure. By the time the balloon has reached 100,000 feet or so, and has expanded into a 25-foot diameter, it bursts. When this happens the radiosonde drifts down to earth under a small parachute. Since it’s small—only about 8 by 3 by 3 inches, and weighing maybe half a pound—it poses no danger. Most are lost, but they do have instructions on the side for mailing them back to the Weather Service to be reconditioned for reuse.

Jack Williams, a freelance science writer specializing in weather and climate, is an instrument-rated private pilot. The latest of his six books is The AMS Weather Book: The Ultimate Guide to America’s Weather. He answers questions about weather on his Web site.