Ventilators at night

Around the place I am staying, so around Havelock there are quite a lot of fields with apples for example. At the moment the days are getting hotter, spring is coming so flower start growing out of these apple will come later. But the nights are sometimes cold, around 1 degree, so it is still freezing. To prevend the loss of all these flowers, the farmers rent helicopters with ventilators to make the hot air go down so that there is no frost. This is very expencive, it costes 15.000 $ per hour but it's the only way to save their crop. So sometimes during the night you can hear loud noises which are these helicopters.