After nightfall, the objects begin to cool. As the objects cool, they chill the air just above them and on calm, clear nights, can make the air above them reach dew point. When this happens, dew will condense out of the air onto surfaces such as grass, plants and other objects. Winds move the layers of air and disperse the cooling effect. Cloud cover tend to keep the heat from escaping and so the temperature does not fall as much and dew point is less likely to be reached.
Some surfaces cool faster than others.
Some objects hold heat well enough they may not get below dew point temperature. Metal loses heat quickly and this is why a car might be covered with dew within an hour after sunset while the concrete driveway it sits on might be dry throughout the night. Grass and plants are usually the first to collect dew because they lose water vapor themselves and so the air surrounding the plant becomes saturated more quickly than air a few feet above it. Contrary to popular belief, frost is not frozen dew. White frost, also called hoar frost, forms when the dew point of the air is below freezing.