How Weather Balloons Work

Weather Balloon Launches

In an isolated field in the middle of Australia, NASA officials slowly inflated a massive helium balloon that would carry a $2 million gamma ray telescope into the upper atmosphere. The location was perfect for a balloon launch: flat, dry and clear. Before the balloon was fully inflated, however, a sudden gust of wind caught the balloon and sent it hurtling across the countryside. Crew members ran for their lives as the telescope smashed into a nearby SUV and ripped through a fence before crumpling into a heap more than 492 feet (150 meters) away.

Of the many things that can go wrong during a balloon launch, leaving a trail of destruction is obviously one of the worst. Most weather balloons, on the other hand, are launched without a hitch. In the United States, weather stations will typically have an onsite shed built especially for the purpose of balloon inflation. To prepare a balloon for launch, a technician will first secure the balloon to a nozzle and begin filling it with helium or hydrogen. As it fills, he tests the radiosonde's battery, tunes the radio equipment and attaches the whole assembly together with a length of nylon cord.

Once the balloon has inflated to about the size of a yoga ball, the technician ties it off and ushers it outside. Walking the balloon a short distance clear of trees, power lines and other obstacles, he'll simply give it a gentle push upward.

As soon as the balloon begins to float, the radiosonde gets to work, beaming data to weather computers on the ground. In real time, these computers plot the data into three-dimensional weather models and send them to weather stations across the country. Ground technicians, meanwhile, track the rising balloon with radar equipment. By noting the sideways movement of the ascending balloon, they can calculate wind speed and direction at different altitudes.

There's a reason weather balloons don't just float into space. As the balloon moves farther away from Earth, there's less air to push against the outside of the balloon. With less air pressure to rein it in, the gas inside the balloon expands as its altitude rises. The balloon can only expand so much, however, and it will typically burst at altitudes above 15 miles (24.1 kilometers) -- about three times higher than Mount Everest.

If the radiosonde was simply allowed to plummet to earth, it could wreak deadly havoc on human settlements below. That's why each weather balloon has a small parachute connected to the cord joining the radiosonde to the balloon. As the balloon ascends, the parachute remains folded by the downward rush of air. When the assembly starts to descend, however, the parachute is blown open, slowing the balloon to a manageable 22 miles per hour (9.8 meters per second).

Much of the time, weather balloons simply become litter after a trip into near-space. If balloons catch a particularly strong gust of wind, they can travel several hundred miles -- touching down anywhere from a marshy bog to the snowy peaks of the Rocky Mountains. Sending helicopters to pick up almost 200 weather balloons launched in the United States each day simply isn't in the budget.

However, inside each radiosonde is a large postage-paid envelope. If you ever come across an old weather balloon, simply place it inside the envelope and pop it into a mailbox, and days later it'll be returned to the National Weather Service to fly again.

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