In order to understand dew we should understand a few of the things thrown at us in every weather report, like humidity, relative humidity and dewpoint. In our atmosphere there is always present a quantity of invisible water vapor mixed with other gases. The absolute humidity is the mass of water vapor present per unit volume at any given temperature. Warm air can hold more water vapor than cold air. As the air temperature is lowered the relative humidity increases.
Some moisture is squeezed out of the air, falling to the ground or condensing on cold surfaces.
The relative humidity may be found by measuring the temperature difference between simultaneous readings taken with an ordinary thermometer and a thermometer having a wetted and amply ventilated bulb, then using a lookup table or applying a rather complicated formula. This is what causes rain to fall, fog to form and dew to spoil our telescope viewing. We set up our scopes and wait for darkness to set in. After the sun sets in the west we marvel at how clear the sky could be tonight. When the sun drops below the horizon it no longer warms our ambient air and things begin to cool down.