It takes just one hour in one day when the temperature drops below 0C to turn a good crop that might yield 3t/ha to nothing - that's what frost do. Western Australia got hit by a big frost three week ago that has so far wiped 1.5m tonnes of grain off the expected harvest. It was a flowering frost where the wheat flowers are aborted due to the temperature dropping below 0C - which typically reduces yield by between 10-30%.