Dew is liquid water that condenses onto exposed surfaces when those surfaces cool to a temperature at or below the local dew point and atmospheric moisture is sufficient for condensation. It commonly forms during clear, calm nights when surfaces lose thermal energy through infrared radiation and become colder than the air immediately above them. When the surface temperature falls to the dew point, air adjacent to the surface reaches saturation and water vapour begins to condense into liquid droplets.
Dew can form on grass, leaves, soil, vehicles, roofs, windows and many other surfaces, with the amount depending on surface properties, humidity, wind, cloud cover and radiative conditions. Dew is therefore a surface condensation phenomenon rather than precipitation falling from clouds. It is an important component of the local hydrological cycle, especially in arid and semi-arid environments where small quantities of atmospheric moisture can have ecological significance. The phenomenon also illustrates the relationship among temperature, humidity, saturation vapour pressure and phase changes of water.
The dew point is central to understanding dew formation. At a given atmospheric pressure, saturation vapour pressure increases with temperature, meaning that warm air can contain more water vapour before reaching saturation than cold air. When a surface cools, the air immediately adjacent to it may reach saturation even though the broader atmosphere remains unsaturated. The temperature at which saturation occurs is called the dew point. If the surface continues cooling below the dew point, condensation generally becomes increasingly favourable.
The actual formation of droplets also depends on the availability and characteristics of microscopic surfaces that allow water molecules to accumulate. Hydrophilic surfaces can encourage spreading or droplet formation, whereas hydrophobic surfaces may cause water to form more discrete beads. The latent heat released during condensation partially offsets radiative cooling, but this does not usually prevent dew formation under favourable conditions. Dew therefore provides a visible manifestation of atmospheric moisture reaching a thermodynamic saturation threshold at a cooled surface.
Clear skies and light winds are classic conditions for dew formation because they allow surfaces to cool efficiently without being strongly replenished by warmer air from above. Clouds reduce net radiative cooling by emitting infrared radiation downward, while strong winds promote mixing and can replace cooled near-surface air with warmer air. Nevertheless, dew can occur under cloudy or moderately windy conditions when other factors permit the surface to reach its dew point.
Surface characteristics also matter because different materials cool at different rates and exchange heat differently with the underlying environment. Grass can become colder than nearby air and therefore collect substantial dew, while elevated leaves can have distinctive thermal properties. Vehicles and roofs may also develop visible droplets because their surfaces radiate energy efficiently. Soil moisture, vegetation and local humidity influence the available water vapour. Valleys and sheltered areas can be particularly favourable because local air can remain humid while wind speeds are low.
Dew has ecological significance because it supplies small amounts of liquid water directly to vegetation, microorganisms and animals. In dry climates, dew can provide a meaningful supplementary moisture source, particularly for organisms adapted to exploit frequent overnight condensation. Some insects and small animals obtain water from dew droplets, while plants can interact with deposited water through leaf surfaces.
Dew can also contribute to the wetness duration of leaves, which is relevant to plant disease because many fungal pathogens require a sufficiently long period of surface moisture for spore germination and infection. Agricultural models sometimes incorporate leaf wetness duration as an indicator of disease risk. At the same time, dew can reduce overnight water stress for certain plants, although its contribution is usually much smaller than rainfall in humid environments. Dew therefore occupies a subtle position within ecosystem water budgets, being locally important even though its total global contribution is modest compared with precipitation and evaporation.
Dew is distinct from fog, frost and rain despite involving the same fundamental water substance. Fog consists of suspended droplets within air near the surface, whereas dew consists of condensed water deposited directly onto surfaces. Rain falls from clouds and therefore represents atmospheric precipitation rather than surface condensation. Frost forms when water vapour deposits as ice or when liquid water freezes on a sufficiently cold surface, producing a solid rather than liquid deposit.
Dew can sometimes freeze after forming, resulting in a combination of dew and ice, but the initial formation mechanism remains condensation. Dew point should also not be confused with relative humidity. Relative humidity expresses the amount of water vapour relative to the saturation amount at the current temperature, whereas dew point expresses the temperature to which air must be cooled for saturation to occur. These distinctions are important in meteorological observation and in understanding why two locations with identical relative humidity can have very different amounts of atmospheric moisture if their temperatures differ.
Human societies have observed dew for thousands of years because it is visually conspicuous and occurs regularly under suitable conditions. Historical agricultural traditions sometimes regarded heavy dew as beneficial because it moistened vegetation and could provide a small supplementary source of water. Modern meteorology treats dew as an indicator of near-surface humidity and surface energy exchange. Dew gauges and other instruments can measure deposition, although estimating total dewfall is difficult because droplets can evaporate, spread, drip or be absorbed.
Remote sensing can sometimes infer surface moisture conditions but generally cannot directly replace local measurements of dew deposition. Dew is also relevant to engineering because repeated condensation can contribute to corrosion, material degradation and moisture-related damage. Buildings are designed partly with consideration of surface temperatures and condensation risk, particularly where warm humid air can contact colder materials.
Dew formation is governed by a balance between radiative cooling, sensible heat transfer, turbulent mixing, latent heat release and moisture supply. A surface loses energy through longwave radiation, while heat is supplied from the underlying soil or structure and from the surrounding air. If radiative losses dominate sufficiently, the surface temperature declines. Once the temperature reaches the dew point, condensation begins and releases latent heat, slowing further cooling.
The magnitude of dew deposition depends on how long conditions remain favourable and on the rate at which water vapour is transported to the surface. Calm conditions can promote strong cooling but may eventually limit turbulent moisture transport, while gentle turbulence can replenish water vapour without completely eliminating surface cooling. This interaction means that maximum dew deposition does not necessarily occur under perfectly calm conditions. The physical process is therefore more complex than the simple statement that dew forms whenever a surface becomes cold.
Dew is also relevant to atmospheric observation because it provides information about the relationship between air moisture and surface temperature. A small difference between air temperature and dew point indicates relatively humid conditions, while a large difference generally indicates drier air and makes dew formation less likely unless substantial surface cooling occurs. The spread between these temperatures is therefore useful in forecasting overnight condensation.
However, dew forecasts must consider surface temperature separately because air temperature measured at standard screen height may not equal the temperature of grass, soil, metal or other exposed surfaces. In some conditions, frost can occur even when the measured air temperature is slightly above freezing because exposed surfaces may cool more strongly. This illustrates why near-surface micrometeorology is essential for understanding dew and frost. Dew thus connects basic thermodynamics with practical weather observation, agriculture, building science and ecological processes.
