Around 8:00 AM Mountain Time yesterday morning, three hot air balloons collided near Teton Village, Wyoming, injuring 16 passengers. Crews, passengers, and witnesses reported that winds suddenly shifted during flight, resulting in a loss of control and the collision. The nearest surface observations from Jackson Hole Airport, about 6 miles away from the accident site in reported northerly winds at 4 knots (4.6 mph) under clear skies. Given the relatively light surface winds observed in the Jackson Hole area, and no other nearby reporting sites, at least one question will arise: Did a localized wind burst occur around the accident site, or was wind shear a contributing factor?
During my years as the Chief Meteorologist for the Albuquerque International Balloon Fiesta, I gained a thorough understanding and appreciation for how sensitive hot air balloons are to even slight changes in weather. Because safety is the number-one priority, there were many mass ascensions that were delayed or cancelled even though the weather looked perfect to the casual observer. Educating the audience on why winds the average person would consider "a slight breeze" would ground balloon flights was part of my job.
Unlike other aircraft, hot air and gas-filled balloons have no ability to steer on their own. The balloon envelopes are relatively light but have large surface areas, thus making them easily "steered" by the winds. To navigate to a particular location, balloon pilots will leverage the differences in wind speed and directions at various altitudes to reach their destination.
Because hot air and gas balloons are so sensitive to weather conditions, very light winds (about 8 mph or less) at the surface are required for a safe launch and landing. Wind shear--winds that change speed or direction over a short vertical or horizontal distance--is dangerous to balloonists because a sudden change in wind speed or direction can result in a loss of control of the balloon. Generally speaking, wind shear exceeding 15 mph at any level is unfavorable for hot air balloon flight.
Related to the problems with encountering wind shear is when the balloon pilot performs a rapid ascent or descent to get to an altitude with more favorable winds. Because the balloon is large, it has a significant amount of inertia, so the balloon does not immediately slow down while ascending or descending from a layer of stronger winds to an altitude with lighter winds. During past Balloon Fiestas, the Weather Team would receive reports from pilots of hard landings where the balloon touches down with ground speeds of 10 to 20 mph, although surface winds may have been only 5 mph.
Forecasting for balloon flights is especially challenging because hot air balloons are impacted by micro-scale weather phenomena that are on space and time scales that are too small to be accurately captured by our numerical weather models. Additionally, many of these micro-scale phenomena go undetected by the existing surface and upper-air observation networks. As the old saying goes, "You can't catch what you can't see."
Understanding the weather conditions present near the Teton Village accident site will contribute greatly to determining the cause of the accident. Was it wind shear, or was it a localized wind burst, or some other phenomenon? Hopefully, the accident investigation will give us some answers.