Trajectories from commercial hot-air balloon flights can provide interesting wind information of the lower atmosphere.
Hot-air balloons float in the air and travel with the wind. Global Navigation Satellite System (GNSS) navigation data acquired during the flight, provide a displacement during a time interval which is a measure of the airspeed. This wind information is obtained in the Atmospheric Boundary Layer (ABL) where in general only few observations are present.
During the flight the balloon feels the drag as soon as the relative speed is not zero anymore, for instance when the balloon enters a layer with vertical wind shear. In the beginning the response is fast and levels off as the speed difference becomes less. From the theory we know that the balloon reacts with a response length . Due to the inertia of the hot-air balloon, its displacement does not capture the small scale variations in the wind pattern. The wind observations represent an average in space and time.
In Figure 2 we compare the balloon wind data during the last 20 minutes of the flight of 18 June 2013 with Numerical Weather Prediction (NWP) model data and with the observations at the Cabauw observatory. The NWP model data consist of the +01 and +02 forecasts starting from the analysis at 18:00 UTC. The wind-profiler is located at Cabauw and the data is available as a 30 min average. In the observations the height-dependence of the wind is stronger than in the NWP model.
Low level jet
A jet is observed in the 28-Sep-2013 flight and is located in a small vertical zone not higher than 500 m (see Figure 3). The sharp gradient in the wind speed occurs both in the model and in the wind data from the hot-air balloon track. These data apply to the first 10 minutes of the flight. The model and observations are in good accordance.
Hot-air balloon flights can provide useful wind information. The first validation results are encouraging. Using smartphones, data will be collected in a simple way. An experimental flight has been conducted with a SONIC anemometer attached to the gondola, to measure the relative wind speed around the balloon . We will validate the response time function and improve the algorithm for the processing of the data.
 Evert I.F. de Bruijn, Siebren de Haan, Fred C. Bosveld, Ben Wichers Schreur, Albert A. M. Holtslag, 2016: Observing Boundary-Layer Winds from Hot-air Balloon lights, Weather and Forecasting, DOI:10.1175/WAF-D-16-0028.1