Dew forms when a surface cools enough for water vapor in the nearby air to condense into liquid droplets. Clear, relatively calm nights often favor the cooling that produces it. Dew is not rain falling from an unseen cloud: the droplets form directly on grass, leaves, cars and other surfaces.
A surface can cool faster than the air above it
After sunset, exposed objects lose energy by emitting infrared radiation. When the sky is clear, the balance can favor substantial cooling. Grass blades and a car roof may become cooler than the air measured a short distance above them, allowing condensation to start on the surfaces first.
Cloud cover changes that energy balance by emitting radiation back toward the surface. Wind can mix the shallow layer of cool air with warmer or drier air. That is why a change in cloud or wind can determine whether the same lawn is wet one morning and dry the next.
The dew point marks the relevant moisture threshold
The dew point is the temperature at which air would reach saturation if cooled at constant pressure without changing its vapor content. When a surface cools to the local dew point or below it, vapor can condense on that surface. The local part matters: moisture near damp soil and vegetation can differ from a measurement taken elsewhere.
A weather report’s temperature and dew point give clues, but they do not perfectly describe every leaf or roof. Exposure to the sky, material, shelter and heat stored in the object all affect surface temperature. A dry pavement beside wet grass is therefore not a contradiction.
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Where the water comes from
Dew draws on vapor in nearby air. That vapor can have come from evaporation from soil or water, plant transpiration, or air transported into the area. The droplets do not require rain to have fallen during the night, although earlier rain can leave a moist environment favorable to later dew.
Dew is part of the local water balance, but the amount varies greatly. A sparkling lawn is not evidence that a dry landscape has received the equivalent of a useful rainstorm. Visual brightness depends on the droplets and lighting, not simply on total water volume.
Dew, rain and guttation are different observations
Rain arrives from a cloud as falling drops. Dew forms by condensation on a surface. Plants can also release liquid water through structures near leaf edges or tips, a process called guttation. That liquid comes from within the plant and can contain dissolved substances.
Droplets at a leaf tip are therefore not automatically dew, particularly if they follow a regular pattern along the margins. Appearance alone may be inconclusive. Consider the whole surface, recent weather and whether similar moisture appears on nearby nonliving objects. Do not taste plant droplets to investigate them.
What changes when the surface falls below freezing
Liquid dew can freeze if the surface later becomes cold enough. That is frozen dew. Hoarfrost follows a different route in which vapor deposits directly as ice crystals. Both can leave a white or icy morning scene, but the path taken by the water is not identical.
A thermometer above the ground may still read slightly above freezing while an exposed surface is cold enough for ice. Pay attention to slippery paths and clear vehicle windows fully before driving. A thin deposit is still an obstruction or a footing hazard.
Watch the morning transition
Choose a safe location and compare an open patch of grass with a sheltered patch under a tree. Note the cloud cover, wind and the time sunlight reaches each patch. As the surfaces warm or drier air arrives, droplets evaporate; shaded areas may stay wet longer.
A photograph taken at a low angle can reveal many droplets without needing to disturb the plants. Record what is known, such as water on grass after a clear night, and keep uncertain explanations separate. One photograph cannot establish the overnight temperature or rule out every other source of water.
Explore related questions
How Does Fog Form? Cooling Air, Moisture and Visibility
How Does Rain Form? From Cloud Droplets to Falling Water
Sources and further reading
National Weather Service: dew and frost development
National Weather Service: humidity and condensation
University of Missouri: guttation versus dew
Optional tools for learning more
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