Montana Earth Science Picture of the Week

The National Weather Service (NWS) has 4 weather forecast centers in Montana. They are located in Billings, Missoula, Great Falls, and Glasgow. Every day at 5 a.m. and 5 p.m. MDT, weather balloons are released from the centers at Great Falls and Glasgow. At the same time that balloons are launched from the two Montana sites, balloons are also being released from about 900 other sites across the world (including 92 in USA).

CLICK HERE to watch a 4-minute YouTube video, showing a weather balloon launch.

A small box of sensors, called a radiosonde, is tethered to each balloon. As the balloon rises into the atmosphere, data is sent back to the weather center by radio signal. The sensors give meteorologists the temperature, pressure, and humidity of the air, and tracking the radiosonde by the radio signals it sends out provides information about wind speed and direction at higher altitudes (see photo below). Data from the hundreds of balloons is collected to determine the location and speed of jet streams. Knowing what's going on with jet streams helps the NWS forecast the movement of storms and predict other aspects of weather.

In order to make the balloons rise, they must be filled with a gas that is lighter than air. At Great Falls, the balloons are filled with hydrogen inside a building designed especially for balloon inflation. Once the balloon is released and begins to rise, it gradually expands to the size of a small house as the pressure around it decreases. Typically the balloons burst at an elevation between 80,000 and 120,000 feet within two hours. A small parachute carries the radiosonde back to earth. Instructions on the box tell finders how to send it back to the NWS. About 10% of the radiosondes launched from Great Falls are returned, refurbished, and reused.

Why 5 a.m. and 5 p.m. MDT? . . . Weather observations must be taken at the same time everywhere to accurately represent the state of the atmosphere. So, all weather stations use "coordinated universal time" (UTC); sometimes called Greenwich Mean Time (GMT; the time along the prime meridian). The prime meridian runs through the Old Royal Observatory in Greenwich (near London). Using UTC helps avoid confusion caused by various time zones, and the use of daylight saving time. Worldwide, weather balloons are released an hour before it is noon in Greenwich (1100 UTC) and again an hour before midnight (2300 UTC). When it is 11 a.m. (1100) in Greenwich, it is 5 a.m. MDT in Montana, and when it is 11 p.m. (2300), it is 5 p.m. MDT in Montana. During the months when we are on "standard time" (MST), balloons are released at 4 p.m. MST and 4 p.m. MST from Great Falls and Glasgow.

Below: Meteorologist, David Williamson inflates the balloon with hydrogen. The dome on top of the building houses the device that receives radio waves from the radiosonde. The radio waves include the data and they also reveal the balloon's altitude and speed

Term: radiosonde