Wind occurred, from the west or north quadrants, in the previous days;
Cold polar mass entered, with the cloudy and rainy day;
In the afternoon the wind stops and the night remains calm, with the starry sky;
Air temperature is below 7 ° C to 7:00 p.m. or below 4 ° C to 22:00 p.m.
Relative humidity is below 70% at 10:00 p.m.
Frost should not occur if
Southeastern wind, continuous, occurred in the two previous days;
It was a sunny day the day before;
It is cloudy or windy all night;
Air temperature above 12ēC to 19:00 hours or above 8ēC to 22:00 hours;
Relative air humidity above 90% at 10:00 p.m.