Weather: Upper-Air Observations

Upper-Air Observations

So far we've looked at how weather is observed near the ground, but the atmosphere is like a layer cake. We must examine all the layers before we can determine a complete picture. The lowest layer is important because it's where we live, but what happens at ground level is really a result of the integrated behavior at all the different levels. So before we can put together a good forecast, we must figure out what is going on above the ground.

In the early days of upper air observations, kites were sent upward with instruments attached. In one of the earliest attempts to record high-level readings, eighteenth-century physician John Jeffries went up in a balloon and took weather instruments along with him. On November 30, 1784, he made the balloon voyage, which lasted an hour and 21 minutes. He took numerous readings of the pressure and temperature. Thomas Jefferson wrote about the meteorological utility of balloons in April 1784, when he said that balloons would be useful in "throwing new lights on the thermometer, barometer, hygrometer, rain, snow, hail, wind, and other phenomena of which the atmosphere is the theatre." During this century, balloons are still used daily, but they are self-contained with remote sensing instruments.

The simplest balloon is called a pilot balloon and is filled with gas. After being released, it's tracked with a telescope-like device called a theodolite. At equal intervals, such as once a minute, the balloon's position is noted in terms of its vertical and horizontal angles. These can be put into a formula to determine wind speed and direction.

Other balloons carry a special instrument package called a radiosonde, which measures the pressure, temperature, and humidity at the different heights. The balloon is tracked, often with radar, and the wind can be determined, just as it is with a pilot balloon. At the same time, the data is transmitted back to the tracking station at given intervals. For example, every few millibars of ascent, the switch goes on, and data is sent. The balloon's position is known, and its pressure given. The strength of the returning signal is proportional to the temperature and humidity.

Even during the era of space-age technology, these balloon observations remain the mainstay of upper-air weather observations. They are taken twice each day, at 12-hour intervals. The stations across land are spaced from 200 to 500 miles apart. Although there are more than 1,000 radiosonde launch sites globally, a dense collection of upper-air observations is not routinely available. Most of the sites are in populated areas. The balloons provide data through the troposphere, up to about 19 miles where they normally pop. The instrument package falls to the ground and used again if it's returned to the National Weather Service. The package contains a message asking that it be returned if found.

Above 19 miles, radar and rockets are used to determine weather conditions. The rocket drops an instrument package, and it's tracked by radar. Also infrared sensors are being used to examine the temperature as well as the motion of the atmosphere. These are called radiometers. They can detect sharp changes in temperature that also correspond to sharp changes in wind. Water vapor is also a good emitter of infrared radiation, and its variation can be measured with these radiometers. That variation can often be linked to turbulence. Such instruments are helpful in aviation to help pilots determine when they are moving into rough air.

Satellites are now being used to profile the various atmospheric variables all the way down to the lower troposphere. Microwave sounding units are used to measure the global temperature. Satellites have the advantage of monitoring more than 95 percent of the globe, and each satellite measures the temperature above most points every 12 hours.

(In the next section, we'll take a look at some of the advances in remote sensing from radar and satellites.)