In order to understand the dynamic processes that result in the weather we experience, we need to know what is happening through the entire atmosphere. These observations are primarily taken with the aid of radiosondes.
The radiosonde is a small instrument package that is suspended below balloon filled with either hydrogen or helium. As the radiosonde is carried aloft, it measures pressure, temperature, and relative humidity.
These sensors are linked to a battery-powered radio transmitter that sends the information to a ground receiver. By tracking the position of the radiosonde in flight via GPS (Global Positioning System), measurements of wind speed and direction aloft is also obtained.
Worldwide, most radiosonde observations are taken daily at 00Z and 12Z (6 a.m. and 6 p.m. EST) (See What is 'Z' time?). With worldwide coordination of these upper air observations, we can obtain a picture of the various pressure and wind patterns across the globe.
Radiosonde observations technically provide only pressure, temperature, and relative humidity data; the tracked position of a radiosonde is actually called a rawinsonde observation and is used to obtain wind speed and direction. However, meteorologists and other data users frequently refer to them as part of the radiosonde observation.
The radiosonde flight can last in excess of two hours, and during this time the radiosonde can ascend to over 115,000 feet (35,000 m) and drift more than 125 miles (200 km) from the release point. During the flight, the radiosonde is exposed to temperatures as cold as -130°F (-92°C) and air pressures of only a few hundredths of what is found on the Earth's surface.
When the balloon has expanded beyond its elastic limit (20-25 feet in diameter) and bursts, the radiosonde returns to Earth via a small parachute. This slows its descent, minimizing the danger to life and property.
Worldwide, there are about 1,300 upper-air stations. Observations are made by the NWS at 92 stations: 69 in the conterminous United States, 13 in Alaska, nine in the Pacific, and one in Puerto Rico.
NWS supports the operation of 10 other stations in the Caribbean. Through international agreements, data are exchanged between countries worldwide.
How Are Radiosonde Data Used?
Radiosonde observations are used over a broad spectrum of efforts including:
- Input for computer-based weather prediction models,
- Local severe storm, aviation, and marine forecasts,
- Weather and climate change research,
- Input for air pollution research, and
- Ground truth for satellite data.
Data from a radiosonde observation is plotted on a seemingly complicated chart called a "Skew-T" but provides a wealth of information concerning the state of the atmosphere.