Morning dew consists of liquid water droplets that form on exposed surfaces, such as grass blades and car roofs, when atmospheric moisture condenses. This phenomenon is a form of condensation, distinct from precipitation like rain, as the water vapor changes directly from a gas back into a liquid state on the object. It is a common occurrence in the early morning hours, creating a glistening layer of moisture across the landscape.
How Dew Condenses
The formation of dew is controlled by the physics of condensation, involving the air’s water vapor content and temperature. Air holds a maximum amount of water vapor related to its temperature; as air cools, its capacity to retain vapor decreases, causing relative humidity to rise.
The most important factor is the dew point, the specific temperature at which the air becomes completely saturated (100% relative humidity). When an object’s surface temperature drops to or below the air’s dew point, water vapor immediately adjacent to that surface begins to condense. This transition from gas to liquid releases latent heat, but cooling continues, allowing for sustained droplet formation.
Condensation happens most readily on surfaces that are poor conductors of heat, such as grass and thin metal. These surfaces cool rapidly because they cannot easily draw warmth from the ground below. When the cold surface chills the nearby air below the dew point, the resultant droplets form the visible dew.
Why Dew Forms at Night
Dew formation is a nighttime event because the necessary temperature drop occurs through radiative cooling. During the day, solar radiation keeps surface temperatures above the dew point. Once the sun sets, surfaces lose heat by emitting longwave infrared radiation directly into the atmosphere and space.
This continuous heat loss causes the surface temperature to fall rapidly, especially on clear nights that lack a cloud layer to trap outgoing radiation. Calm conditions are also favorable for dew. A strong wind would mix the cooled, saturated air near the ground with warmer, drier air from higher elevations, preventing the surface from reaching the dew point.
The greatest amount of cooling happens just before sunrise, which is why the heaviest dew is often observed in the early morning. As the sun rises, the surfaces warm, causing the condensed water droplets to evaporate back into the atmosphere, often within a few hours.
Distinguishing Dew from Other Moisture
While dew results from atmospheric condensation, other forms of morning moisture found on plants have different origins. One phenomenon is frost, which is not frozen dew. Frost occurs when the surface temperature drops below the freezing point of water. It forms directly as ice crystals through deposition, where water vapor turns straight into a solid without first becoming liquid water.
Another common source of morning moisture, especially on plants, is guttation, a biological process entirely unrelated to atmospheric conditions. Guttation occurs when a plant’s roots absorb more water than the leaves can transpire, often during periods of high soil moisture and humidity. The excess water pressure forces liquid water, which may contain organic compounds, to exude from specialized pores called hydathodes, typically found along the edges or tips of leaves. The resulting droplets look similar to dew, but the water source is internal to the plant.