Dew is moisture that forms on surfaces overnight, commonly seen on grass and vehicles in the early morning. It is not precipitation, but water vapor that changes form right at the surface. Dew typically forms late at night and accumulates until sunrise. Understanding when and why dew forms requires looking closely at the thermal mechanisms and atmospheric requirements that must align for this natural event to occur.
The Science of Condensation
Dew formation results from the atmosphere’s capacity to hold water vapor, quantified by relative humidity. Relative humidity describes the amount of water vapor currently in the air compared to the maximum amount it can hold at that temperature. Warmer air holds significantly more moisture than cooler air.
The physical threshold for dew is the dew point, the specific temperature at which the air becomes completely saturated (100% relative humidity). When a surface cools to this temperature, the surrounding air can no longer hold its water vapor. This triggers a phase change, causing water vapor to condense directly into liquid droplets on the surface.
This condensation is a thermal exchange where water vapor transfers latent heat to the cooler surface. Dew is classified as condensation, not precipitation, because the surface temperature must drop to the dew point. If the surface temperature drops further, more water vapor condenses, leading to heavier accumulation.
Optimal Environmental Conditions
Condensation requires specific external conditions to draw the surface temperature down to the dew point. One requirement is a clear, cloudless sky, which facilitates maximum surface cooling through radiative cooling. At night, the ground and objects release accumulated heat energy (longwave radiation) back into space.
Clouds act as a blanket, absorbing this outgoing radiation and re-emitting some heat back toward the surface, which slows the cooling rate. A clear sky allows heat to escape unimpeded, enabling the surface temperature to drop quickly below the air temperature. Dew is far less likely to form on cloudy nights.
Still air, meaning very little wind, is another requirement. Objects cool first, and the air immediately touching them also cools, becoming the saturated layer necessary for condensation. Wind mixes this cool layer with warmer, drier air from above, preventing the surface from reaching the dew point. Calm conditions maintain the thermal stratification required for cooling.
The Timing of Dew Formation
Dew formation begins the moment a surface’s temperature drops to the air’s dew point, usually several hours after sunset. As solar energy ceases, the surface begins radiative cooling, gradually lowering its temperature. The precise start time depends on the initial temperature, the dew point value, and the rate of heat loss.
Accumulation is continuous throughout the night, but the heaviest deposits occur much later. The lowest temperatures are typically recorded just before sunrise, representing maximum accumulated cooling. Since the surface is at its minimum temperature during these pre-dawn hours, the difference between the surface temperature and the dew point is greatest, resulting in substantial condensation. Peak accumulation occurs in the hour or two immediately preceding dawn.
Distinguishing Dew from Related Phenomena
Dew is often confused with other forms of surface moisture, but their origins differ significantly. Frost is the most closely related phenomenon, occurring when the dew point is below 32 degrees Fahrenheit (0 degrees Celsius). Water vapor then transitions directly into tiny ice crystals on the surface through deposition, bypassing the liquid phase entirely.
Another source of surface moisture is guttation, a biological process unique to plants, not atmospheric condensation. Guttation involves the excretion of liquid water through specialized pores at the tips or edges of leaves. The resulting droplets appear similar to dew, but they are composed of water mixed with mineral salts and sugars pushed out from the plant’s vascular system.