What are Weather Balloons?

Weather balloons are the most direct way in which meteorologists measure the atmosphere. Weather balloons are made up of a balloon filled with either hydrogen or helium attached by a string to an instrument that measures meteorological variables such as temperature, pressure, humidity, and wind. The National Weather Service launches weather balloons twice a day at nearly 100 locations across the United States. They will often launch even more balloons ahead of hazardous weather events such as hurricanes and blizzards. Below is a video of a special weather balloon launch by the NWS office in Corpus Christi, Texas ahead of a potential tropical cyclone in the Gulf of Mexico.

The weather balloon will expand as it rises due to lower air pressure high up in the atmosphere. Eventually the balloon will expand so much that it pops. At this point, the balloon is about the size of a large house. The weather balloon is fitted with a parachute so that the instrument safely floats back down to the Earth’s surface. This prevents damage to the weather instrument and also to people and property on the ground. If you ever find one of these weather instruments (often called a radiosonde), you can drop it off at the United States Post Office and it will be mailed back to the National Weather Service. Only about 10-20% of radiosondes get returned back to the National Weather Service.

There is a very specific reason the radiosonde is separated from the balloon by a long string of twine. This is because the balloon itself can actually affect some of the measurements the radiosonde is trying to take. For example, ice can form on the balloon as it rises and this could affect some of the temperature and humidity measurements by altering the environment the radiosonde is trying to sample.