Every twelve hours, hundreds of weather balloons all around the world are released at the same time as a part of the World Meteorological Organisation’s (WMO’s) Upper Air Program. The coordinated release of the weather balloons allows for a meteorological snapshot of the atmosphere to be taken globally. The information obtained from the weather balloons contributes to the climate record and is also ingested into computer models, which are interpreted by meteorologists who then forecast the weather from them.
The Australian Bureau of Meteorology participates in the WMO’s Upper Air Program by releasing weather balloons from 38 locations on continental Australia, some of our islands (Cocos, Willis, Norfolk and Lord Howe) and in Antarctica (at Casey, Davis, Mawson and Macquarie Island).
Here at Casey one of the Met Team members (Craig, Samuel or Tanya) release a weather balloon at 23:15Z and 11:15Z everyday of the year. Because weather balloon releases are an international effort, release times are based on one time zone, Zulu. For us here at Casey, this means 7:15am and 7:15pm in the winter and when we change time zones in the summer, 10:15am and 10:15pm.
So how do we get information about the atmosphere from a rubber balloon? Well, we don’t. We get the information from a radiosonde, which is suspended underneath the rubber balloon. The rubber balloon is simply a launch vehicle to take the radiosonde to the edge of space.
A radiosonde is a small electronic device that records temperature and humidity. It also has a GPS tracker so it’s position in three dimensions is always known at any given point in time. These three pieces of information are transmitted back to a receiver at Casey every second, where a computer processes the temperature and humidity data and also calculates the pressure, wind speed and wind direction.
The launch routine begins in the Meteorology Office at T minus thirty minutes when the radiosonde is removed from it’s packaging and placed in a special “Ground Check Set” which tests it’s performance. When it passes it is taken to the Balloon Building where a natural rubber latex balloon is tied to a gas nozzle and filled with hydrogen. Hydrogen is used instead of helium because it is cheap and easy to make which we do here on station using reverse fuel cell technology instead of passing an electric current through water. The balloon is filled with a specific volume of hydrogen so when it is released it ascends up through the atmosphere at 300 meters per minute. When the balloon is filled with the correct volume of hydrogen the radiosonde is tied to it, the giant balloon shed doors are opened, the balloon and radiosonde are walked outside and released.
That all sounds simple and straight forward but during a blizzard things become challenging. The balloon shed doors face downwind of the predominant strong wind direction and as the air flows over the building a horizontal spinning column of air (an eddy) forms against the downwind wall, which is where the giant balloon shed doors are. This eddy tries to blow the balloon back into the building when you walk it outside to release it. So the trick during a blizzard is to walk the balloon (which is being pulled horizontally backwards over your shoulder) about twenty meters past the eddy until it lunges over your shoulder and is then being pulled horizontally forwards, before letting go. Then you hope the balloon can quickly ascend up through the downdraft flowing over the balloon building and eddy before the radiosonde hits the ground.
You certainly earn your money for the day when you do that at night in winds greater than 100km/hr with visibility reduced down to twenty meters in heavy blowing snow…but it’s fun!
So, how high do the balloons go I hear you ask? It varies with the seasons and whether the balloon has been treated by soaking it Aviation Turbine Kerosene or not, but generally somewhere between 25km and 30km. As the balloon ascends up through the atmosphere the pressure decreases and the balloon slowly expands. Eventually it expands so much it bursts and falls back to earth.