Check out the web page in the source. It says "Dew forms when a surface cools (through loss of infrared radiation) to a temperature which is colder than the dewpoint of the air next to the surface. Dew is made of liquid water that has condensed from some of the water vapor contained in the air. If the layer of air next to the ground were actually cooling, then fog would form. Instead, it is just the surface (for instance, grass) that is cooling, and a very thin layer of air next to the grass deposits some of its water vapor in the form of liquid water on the grass.
DEW FORMS FIRST ON... One of the most efficient surfaces for the formation of dew is the top of a car. On a cool evening just after the sun sets, you will often see dew form on the top of cars before it forms on grass or other surfaces. "