Dew is the moisture that forms as a result of condensation. Condensation is the process a material undergoes as it changes from a gas to a liquid. Dew is the result of water changing from a vapor to a liquid.

Dew forms as temperatures drop and objects cool down. If the object becomes cool enough, the air around the object will also cool. Colder air is less able to hold water vapor than warm air. This forces water vapor in the air around cooling objects to condense. When condensation happens, small water droplets form—dew.

The temperature at which dew forms is called the dew point. The dew point varies widely, depending on location, weather, and time of day.

Humid locations, such as the warm, coastal tropics, are more likely to experience dew than arid areas. Humidity measures the amount of water vapor in the air. Warm, humid air is full of moisture that can condense during calm, cool nights.

Weather conditions can also influence an area's dew point. Strong winds, for instance, mix different layers of air, containing different amounts of water vapor. This reduces the atmosphere's ability to form dew.

Cold weather can also prevent the formation of dew. As temperatures drop below freezing (0° Celsius, 32° Fahrenheit), a region may reach its frost point. At a frost point, water vapor does not condense. It sublimates, or changes directly from a gas to a solid. Moisture changes from water vapor to ice.

Dew is most likely to form at night, as temperatures drop and objects cool. However, dew can form whenever a dew point is reached. 

Although warm, humid areas commonly experience heavy dew, dew does not form in amounts people could collect as a water source.

Still, several organizations are working to develop technology that will allow them to harvest dew as a freshwater resource. They hope that foil condensers will help meet the water needs of arid areas. The most successful of these condensers have been put to work in Kutch, India, where hundreds of liters of water can be harvested every night at certain times of the year.