Moisture appearing on surfaces in the morning is often referred to as dew. Dew is not precipitation that falls from the sky; rather, it is condensed atmospheric moisture that forms directly on a surface. This condensation happens in place when the air next to the ground or an object cools sufficiently. Understanding the conditions for this localized cooling reveals the mechanism behind the water droplets found on grass and cars.
The Role of Dew Point
The science governing dew formation centers on the dew point, which is the temperature air must be cooled to become saturated. Air contains water vapor, and the maximum amount it can hold is related to its temperature; warmer air holds more moisture than colder air. When the air temperature drops to match the dew point, relative humidity reaches 100%, meaning the air is fully saturated.
Once saturated, any further cooling forces the water vapor to transition into liquid droplets, a process called condensation. Dew forms specifically when a surface, such as a blade of grass or a car roof, cools to a temperature at or below the air’s dew point. The surface acts as the condenser, drawing moisture out of the air immediately touching it. A higher dew point indicates more moisture is present, making dew formation more likely even with a smaller temperature drop.
The Conditions for Nighttime Formation
Dew formation is most common during the late-night hours, peaking just before sunrise. This timing is tied to radiative cooling, the mechanism by which the Earth’s surface cools most effectively after sunset. Surfaces radiate the heat absorbed during the day outward toward space.
The most intense cooling occurs on clear nights because clouds act like a blanket, reflecting radiated heat back toward the ground and preventing the surface temperature from dropping. With a clear sky, the surface temperature can plunge below the temperature of the air a few feet above the ground. This localized cooling of the surface below the air’s dew point triggers condensation. Calm air conditions, meaning low wind speeds, are also necessary for efficient formation. A strong breeze mixes warmer air from higher up with the colder surface layer, disrupting the cooling and preventing the air next to the surface from reaching saturation.
Environmental Factors Affecting Intensity
Several local environmental factors influence the intensity and amount of dew that forms. Ambient humidity is a factor, as a higher dew point indicates more water vapor is available to condense. When the difference between the air temperature and the dew point is small, surfaces require less cooling to initiate heavy dew formation.
The specific material of the surface also plays a role because different materials radiate heat and cool at varying rates. Poor heat conductors, such as grass, leaves, and metal, cool faster than materials like pavement or soil, making them preferential sites for dew accumulation. The presence of moisture in the soil, perhaps from recent rain, can contribute by increasing local humidity near the ground through evaporation throughout the night. Strong winds prevent significant dew by continually replacing the cooled, saturated air layer at the surface with warmer, drier air.
Distinguishing Dew From Frost and Guttation
While dew is a result of atmospheric condensation, it is often confused with other forms of moisture, such as frost and guttation. True frost forms through deposition, which occurs when the surface temperature is below the freezing point of water. In deposition, water vapor transitions directly into a solid state as ice crystals without first becoming liquid water. If liquid dew forms and the temperature later drops below freezing, the result is frozen dew, which appears as a slick, solid layer of ice.
Guttation is a purely biological process unrelated to atmospheric moisture. This phenomenon occurs when plants exude water droplets from specialized pores, known as hydathodes, typically located at the tips or edges of leaves. Guttation is driven by root pressure when the soil is saturated and the plant’s normal transpiration process has stopped, usually at night. The water droplets are precisely positioned at the tips of the grass blades, distinguishing them from the more evenly distributed condensation of atmospheric dew.