Dew is the condensation of water vapour onto surfaces — grass, leaves, cars, spider webs — that have cooled to or below the dew point temperature through radiative cooling at night. The process requires clear or mostly clear skies (to allow the surface to radiate heat to space), light winds (to prevent mixing with warmer air aloft), and sufficient moisture in the lowest layers of the atmosphere.
Dew is most abundant in maritime and coastal environments where nighttime humidity is high. In arid regions, dew can be a biologically significant water source — some desert organisms depend entirely on dew for moisture, and traditional agriculture in parts of the Mediterranean and Middle East has used dew-harvesting techniques. Modern research into fog and dew collectors (mesh structures that condense atmospheric moisture) is being explored for water supply in dry regions.
Meteorologically, dew serves as a practical indicator: its presence confirms that the surface temperature reached the dew point overnight, and its abundance reflects the relative humidity of the air. If the dew point is below 0 °C, the deposit is frost (technically called hoar frost) rather than liquid dew. The daily cycle of dew formation (night) and evaporation (morning) is captured in the saying "dew on the grass, no rain will come to pass", reflecting the clear-sky conditions that favour dew.