Gary Archibald, Weathercaster

For the seventh installment in my series about weather devices and instruments, we turn our attention to weather balloons. My uncle worked for NASA in radio tracking and engineering, stationed in the Caribbean at a USA military base tracking the Apollo missions in the 60s. An exciting time to be sure! He was involved in the launch of weather balloons for flight data and forecasting as it pertained to launches and recoveries. Let’s take a closer look at these valuable weather data collecting instruments…

A weather or sounding balloon is a balloon which carries instruments aloft to send back information on atmospheric pressure, temperature, and humidity by means of a small, expendable measuring device called a radiosonde. A radiosonde (Sonde is French for probe) is a unit for use in weather balloons that measures various atmospheric parameters and transmits them to a fixed receiver. Radiosondes may operate at a radio frequency of 403 MHz or 1680 MHz and both types may be adjusted slightly higher or lower as required.

A rawinsonde is a radiosonde that is designed to also measure wind speed and direction. Colloquially, rawinsondes are usually referred to as radiosondes.To obtain wind data, they can be tracked by radar, radio direction finding, or navigation systems (such as the satellite based Global Positioning System).

The balloon itself produces the lift, and is usually made of a highly flexible latex material (though Chloroprene may also be used). The unit that performs the actual measurements and radio transmissions hangs at the lower end of the string, and is called a radiosonde. Specialized radiosondes are used for measuring particular parameters, such as determining the ozone concentration.

In North America prior to release the balloon is usually filled with hydrogen gas due to lower cost, though helium can be used as a substitute. The ascent rate can be controlled by the amount of gas the balloon is filled with.

Dave Decker holds a weather balloon filled with hydrogen at the National Weather Service in Boise, Idaho, Thursday, March 15, 2007.

Weather balloons may reach altitudes of 40 km (25 miles) or more, limited by diminishing pressures causing the balloon to expand to such a degree (typically by a 100:1 factor) that it disintegrates. The instrument package is usually lost. Above that altitude sounding rockets may be used. After sounding rockets, satellites are used for even higher altitudes.

Weather balloons are launched around the world for observations used to diagnose current conditions as well as by human forecasters and computer models for weather forecasting. About 800 locations around the globe do routine releases, twice daily, usually at 0000 UTC and 1200 UTC. Some facilities will also do occasional supplementary "special" releases when meteorologists determine there is a need for additional data between the 12 hour routine launches in which time much can change in the atmosphere. Military and civilian government meteorological agencies such as the National Weather Service in the US typically launch balloons, and by international agreements almost all the data is shared with all nations.

Specialized uses also exist, such as for aviation interests, pollution monitoring, and research. Examples include pilot balloons (Pibal). Field research programs often use mobile launchers from land vehicles as well as ships and aircraft (usually dropsondes in this case).

Is your interest in weather instrumentation further piqued? I hope so – I’ll have another posting in my series coming up in the not-so-distant future!

Cheers!

GA