Five conditions are necessary for frost formation:
1. Low temperature
2. High humidity
3. Low to medium cloud cover
4. Little to no wind
5. No recent rain

Dewpoint
The dew point is the temperature to which a given amount of air must be cooled, at a constant air pressure, to condense water vapor. The condensed water is called dew. The dew point is a saturation point. When the dew point temperature drops below freezing, no dew is formed but frost.
The dew point depends on the relative humidity (moisture content) of the air. A high relative humidity indicates that the dew point is closer to the air temperature. A relative humidity of 100% indicates that the dew point is equal to the prevailing air temperature and that the air is saturated with water. If the dew point remains constant and the air temperature increases, the relative humidity will decrease.
At a given air pressure and independent of the air temperature, the dew point is an indication of the mole fraction of water in the air and therefore it determines the specific moisture content of the air. The dew point is an important measure for meteorologists.
Dew point temperature in relation to frost is important: the higher the dew point temperature (above zero degrees) the more moisture the air holds. The more moisture there is in the air, the more heat capacity the air contains and therefore the slower the air will cool to fall below zero degrees.
A dew point of for example 10 degrees will therefore be difficult to turn below zero on a cold evening versus one of for example 1 degree Celsius, given all the other weather conditions are the same.

Dew point is consequently critical in frost, for several reasons. It firstly determines the type of frost that will occur and secondly it also tells you when condensation will occur - at what temperature.
Tmin is the minimum temperature for a particular morning.
TDew is the dew point temperature for a given hour.

The following scenarios may occur:
1. Dew Conditions = 0 ° C < T min < T Dew
The air temperature has cooled down to dew point (still above zero degrees), condensation then took place, but the minimum temperature remained above zero degrees (dropped below the dew point) and one then gets dew on crops.

2. Frozen dew conditions = T min < 0 ° C < T dew
The air temperature has initially cooled down to dew point (still above zero degrees), condensation then took place, the temperature then dropped further below zero degrees and the dew drops then started to freeze, hence frozen dew.

3. Frost conditions = T min < T dew < 0 ° C
The air temperature has cooled down to dew point, and the moment this happens the droplets immediately freeze as they form, with the temperature then dropping below the dew point temperature, and frost forms.

4. Black frost conditions = T dew < T min < 0 ° C
The air temperature has cooled down but never reached the dew point (very dry air), thus condensation never occurred and thus no frost or ice crystals occur. But plant cells’ cytoplasm freezes and it then tears the plants' tissues apart, due to the expansion of the water in the cytoplasm that freezes – this causes black frost.