It is a common morning observation: surfaces like a car windshield, a lawn, or a spiderweb are drenched in moisture, even when the night sky was perfectly clear. This phenomenon, which occurs without any rainfall, is known as dew. Dew is not deposited from above like rain, but rather grows directly on the surfaces we observe, transforming invisible atmospheric moisture into visible water droplets. The formation of dew is a natural consequence of the air’s moisture content interacting with a drop in temperature near the ground.
Understanding the Dew Point
Air always holds invisible water vapor, a condition called humidity. Warmer air can hold significantly more moisture than cold air, as the capacity to hold vapor is related to temperature. The specific temperature at which air becomes completely saturated (100% relative humidity) is defined as the dew point.
When the air temperature cools to the dew point, it can no longer contain all the moisture as a gas. This excess water vapor transitions to visible liquid water through condensation. This phase change creates the tiny droplets of dew we see on surfaces. A higher dew point indicates more water vapor is present, requiring less cooling for condensation to begin.
Radiative Cooling and Temperature Drop
The temperature drop required is achieved overnight through radiative cooling. During the day, surfaces absorb solar energy, but at night, they emit this stored heat outward as infrared radiation. On clear nights, this heat escapes directly into the upper atmosphere without being reflected back down to the ground.
This heat loss causes surfaces to become colder than the surrounding air, which is particularly efficient on materials with high emissivity, such as grass. A calm atmosphere is favorable for dew formation because strong wind would mix the cold air layer near the ground with warmer air, preventing sufficient cooling. The cooling effect is cumulative, with the heaviest dew accumulation occurring just before sunrise.
Distinguishing Dew from Related Moisture
Other forms of moisture found in the morning can sometimes be mistaken for dew. Fog, for example, is essentially a cloud that forms at or near ground level. Unlike dew, which forms directly on a surface, fog occurs when the entire layer of air cools to its dew point, causing water vapor to condense around microscopic particles suspended in the atmosphere.
Frost
Frost forms under conditions identical to dew, except that the surface temperature has dropped below the freezing point of water, 0°C (32°F). Water vapor bypasses the liquid phase entirely, changing directly into solid ice crystals through a process called deposition. Frost is not simply frozen dew, but a separate crystalline structure.
Guttation
Not all morning droplets on plants originate from the atmosphere; some moisture is expelled from within the plant itself through a process called guttation. Guttation occurs when high root pressure forces excess water out of specialized pores called hydathodes, typically located at the tips or edges of leaves. These droplets are xylem sap, containing minerals, and result from the plant’s internal water regulation, not atmospheric condensation.