There is a danger but experience shows it is extremely small.
In answer to my question,...., e-mailed that, “We don’t have any record of a weather balloon striking an airplane.”
Also, I searched the Safety Board’s Accident Database & Synopses for the term “weather balloon.” The database covers all accidents investigated.
Only a couple of dozen reports included the term “weather balloon” in the text and in all but one of these cases, the term was used to identify the source of weather data investigators used.
BOM MET complies “with all CASA regulations. Before launching a balloon from forecast offices near airports, and generally coordinate each launch.”
In fact, regulations require that before launching a balloon within five miles of a controlled airport, the weather observer launching the balloon has to coordinate with the airport’s ATC. Also, at all sites, “the observer will check the horizon to ensure there are no aircraft or other activity that the balloon might interfere or endanger. If aircraft or other activities are present, the observer will postpone the launch until it is safe to launch,” the manual says.
The instrument packages the balloons carry aloft weigh from approximately half a kg to a little more than a kg.
This is well under the weight for which the Regulations requires that a NOTAM (Notice to Airmen) be issued before the launch of a free balloon.
While the odds of a collision with a weather balloon are small, there is some danger, wrote Mitch Mitchell in the May 1, 2009 issue of IFR magazine. Writing about the six-kg rule, Mitchell wrote: “The thinking must have been that we could survive a collision with six kg in flight. But I’m not so sure.”
He also notes that the balloons rise at the rate of approximately 1,000 feet a minute–faster than general aviation airplanes can–and are not easy to see visually or with radar. On the other hand, he notes that the data the twice-daily launches from are vital for weather forecasts that pilots and others need.
As for the question about a calculation of the odds of an aircraft hitting a weather balloon, I could find no one who’s heard of anyone doing this.
Pilots could do one thing to lower the already low odds. If you are planning to take off or land at an airport near where weather balloons are launched, you could be extra careful around the noon and midnight Zulu time regular balloon launches; maybe wait until ATC tells you the balloon has been launched–remember they climb at approximately 1,000 feet a minute.
By the way, balloons are also launched at times other than noon and midnight Zulu when they are needed to collect additional data to predict threatening weather such as tornadoes or an approaching hurricane. But these are times you might not want to be flying.
