Dew forms when the temperature of a surface cools down to a temperature that is cooler than the dew point of the air next to it. When this happens water vapour will condense into droplets depending on the temperature.

At what time does dew form?

At a frost point, water vapor does not condense. It sublimates, or changes directly from a gas to a solid. Moisture changes from water vapor to ice. Dew is most likely to form at night, as temperatures drop and objects cool.

How do I know if there will be dew in the morning?

Morning condensation (dew) is very common in some regions and can easily be forecasted. The favorable weather elements for dew include clear skies, light wind, decent soil moisture, and low night-time dewpoint depressions.