The dew point is defined as the temperature at which the water vapour present in the air is just sufficient to saturate it.
If the temperature falls below the dew point, condensation will form.
If the temperature stays above the dew point, condensation will not form.
Air consists in part of water vapour. The amount of water vapour present in the air varies, depending on temperature, the amount of evaporation from surface water, whether or not it has rained... The air can only hold so much water. The amount of water the air can hold depends on temperature.
In general the temperature increases by day and decreases by night. Water evaporates during the day, when the temperature is above the dew point and the air can absorb the water, and falls during the night. If the temperature falls below the dew point, condensation will form. Dew is common – in the desert some animals get most or all of the water they need from dew.
Dew can be a very localized phenomenon. A person entering a warm room from the cold outside may have condensation form on their glasses. Because their glasses are cold, they will lower the temperature in the region of their glasses, maybe below the dew point, in which case condensation will form.
Every liquid has a dew point for a specific pressure of vapour. The phenomenon has a wide range of uses:
In cloud chambers. The chamber is filled with vapour from 'dry ice' – frozen CO 2. The chamber is expanded with a piston, which lowers the temperature below the dew point, then vapour condensates on the ionized particles left by radiation to expose the path of the radiation.
To determine the humidity of the air – how much water it holds. The temperature is lowered until condensation begins to form. The temperature at which this happens is the dew point. The humidity can then be determined.
To extract substances from the air. The air is a mixture of substances, all with differing dew points. We can extract them all one by one by cooling the air. As the temperature passes each dew point, the corresponding substance will condense.