Most people are interested in the weather at ground level because that’s where they spend most of their time. This is why most television, Web, and newspaper forecasts and maps focus on what is happening—or forecast to happen—at ground level.
Because pilots leave the ground, they must understand the weather aloft. The text and images on these pages introduce you to the part of the atmosphere where those who fly go.
Meteorologists need upper atmosphere data to measure and forecast weather both at the surface and aloft. They use soundings, which are measurements from the surface up through the atmosphere, and also measurements along routes of flight from cruising aircraft, and over wide areas from satellites.
A weather balloon rises pulling a small parachute and, at the bottom of the cord, a box—called a radiosonde—containing weather instruments and a radio to transmit readings to the weather station that launched the balloon. As the balloon rises into the lower air pressures aloft it expands and eventually bursts, often above 100,000 feet. The parachute lowers the half-pound radiosonde to the ground.
From the 1940s though the 1970s weather balloons, which are usually launched only twice a day in developed countries, less often elsewhere, collected most upper-air data with radio reports from pilots adding a little more. Since the 1970s automated reports from airplanes have become an important, but far from complete, source of data.
Let’s explore four of the ways we measure and consume that data from aloft.
1. The standard atmosphere
The standard atmosphere is a basic principle of how things work in the atmosphere. It is a predetermined set of average temperatures, atmospheric pressures, and air densities at different levels of the atmosphere. The actual values at any location change with the weather. Aircraft performance charts are based on the standard atmosphere. Pilots’ performance calculations and altimeter settings correct for differences between the standard atmosphere and the actual atmosphere at the time and place of a flight.
2. Super thunderstorms need big CAPE
The weather at any particular time and place depends on the atmosphere’s stability. In general, a stable atmosphere will be calm with flat clouds (if there are clouds) and smooth air. Precipitation will tend to cover a wide area.
Thunderstorms occur in an unstable atmosphere, with the strongest thunderstorms in the most unstable conditions. Meteorologists use upper-air data to calculate various measures of stability, including the Convective Available Potential Energy (CAPE), a good measure of how strong any thunderstorms that form could become. The National Weather Service’s Storm Prediction Center and local weather offices often mention CAPE in their discussions of the thunderstorm potential. The chart gives you a good idea of what you could face if thunderstorms occur.
3. Jet streams
As shown in this image, a jet stream has a core of very fast winds. A jet stream can be more than 100 miles wide and a few thousand feet deep. Although they usually occur higher than most piston-engine general aviation airplanes fly, low-level jets can be found as low as 300 feet above the ground.
Pilots who fly at jet-stream altitudes can sometimes catch a ride with a jet stream that’s going in the desired direction. At other times a pilot may have to detour 100 miles or more to avoid jet-stream headwinds.
4. Pilot reports
The National Weather Service urges pilots to radio in weather reports, known as pilot reports, or pireps. That’s because it is the only immediate source of directly observed weather aloft. A report of beautiful weather with no turbulence, icing, or poor visibility can be as important as one of terrible conditions. The easiest way to file a pirep is by calling Flight Watch on 122.0 MHz. Several airlines file automated pireps, providing temperature and wind reports every three minutes in cruise flight and at each 300 feet during climb or descent. These reports have greatly improved computer forecasts. However, they report soundings from the ground to the higher flight levels from only the few airports served by large jets. And, they don’t measure humidity, the raw material for aircraft hazards such as icing, low ceilings, and reduced visibilities.
The Tropospheric Airborne Meteorological Data Reporting (TAMDAR) sensor, which is mounted on commuter airplanes, is addressing these shortcomings. These airplanes use smaller airports, which increases the number of soundings through the first few thousand feet of the atmosphere. These sensors also measure humidity.
