The common sight of a dew-covered lawn on a cool morning is a temporary feature of the landscape. These water droplets, which coat surfaces like grass and car roofs, vanish completely as the day progresses. The disappearance is not due to the water being absorbed into the ground or swept away, but rather a reversal of the atmospheric process that created the droplets. This shift from liquid to gas is driven by a change in the energy balance of the immediate environment.
How Dew Forms
Dew formation is condensation, where atmospheric water vapor changes directly from an invisible gas into a liquid. This occurs when a surface cools down through radiative cooling, typically overnight, as it loses heat to the atmosphere. The surface temperature must drop to or below the dew point. The dew point is the temperature at which the air becomes saturated with water vapor. Once the surface is cooler than the surrounding air’s dew point, water molecules lose energy and aggregate onto the cold surface, forming the dew observed at sunrise.
The Primary Mechanism of Disappearance
Morning dew disappears through evaporation, a phase change where liquid water molecules escape into the atmosphere as water vapor. This process requires a significant input of energy, supplied by the rising air temperature and incoming solar radiation. The energy needed to convert a liquid into a gas without changing its temperature is known as the latent heat of vaporization.
To evaporate, water molecules must gain enough kinetic energy to break the hydrogen bonds that hold them together. When the sun rises, the absorbed light transfers this energy, causing the liquid water to transition back into an invisible gas phase and removing the droplets from the surface.
Environmental Factors Accelerating the Process
Several environmental variables accelerate the rate at which dew evaporates alongside rising temperature. The intensity and angle of the sun are direct energy sources, with a higher solar angle providing more rapid heating of the surface and a faster supply of latent heat of vaporization. This concentrated energy allows more water molecules to reach the kinetic energy threshold required to break their bonds and escape into the air.
Wind speed is also a major factor because evaporation creates a thin layer of highly saturated, humid air immediately above the dew-covered surface. This saturated layer acts as a barrier, slowing down further evaporation. Wind removes this moist air, replacing it with drier air from the surrounding environment.
The introduction of drier air increases the gradient, or difference, in water vapor concentration between the liquid surface and the atmosphere, which pulls moisture away more quickly. Relative humidity further influences this gradient; a lower initial relative humidity means the air is less saturated and can accept the escaping water vapor molecules much faster, leading to a quicker disappearance of the dew.