You've seen dew many times, even if you live in a high rise apartment building. Just take a can of soda out of the refrigerator and you'll see dew start to form in minutes. Nature has its on way of producing dew though. There is always some amount of water vapor in the air even though we cannot see it. The amount of water vapor the air can hold depends mainly on the temperature of the air.
Warm air can hold more water vapor than cold air.
The air is saturated when it cannot hold additional water. As the Sun sets, the air begins to cool. If it cools enough, it will reach dew point. Dew point is the temperature to which the air must cool to in order to be saturated. During the day, the Sun heats up the ground and objects such as cars and buildings.