Wind velocity data measurements, to determine whether the assumption of forced convection was valid, were taken from the closest automated weather station of the Royal Netherlands Meteorological Institute. This station is situated at the airport of Rotterdam (51–57033.6600 N, 4–26032.6600 E), 6 km from the experiment location. There can be important differences between the wind measured in Rotterdam and the wind at our site, but these data only served as a check to see whether there were no periods without wind (< 0.1 m s−1).