Dew is a natural phenomenon where water appears as tiny droplets on exposed surfaces. This moisture is not rain, but rather the result of atmospheric water vapor changing into a liquid form. It commonly covers grass, leaves, and other objects outdoors.
How Dew Forms
Dew forms through condensation, when water vapor in the air changes into liquid water. This happens as surfaces cool down. Surfaces lose heat to the environment, especially after sunset.
As these surfaces cool, they also cool the layer of air directly above them. Cooler air can hold less water vapor than warmer air. When the temperature of the surface drops to a specific point, known as the dew point, the air immediately surrounding that surface becomes saturated with water vapor.
At this point, the excess water vapor in the air condenses directly onto the cool surface. These particles then accumulate to form visible dew droplets. This process is similar to how water droplets form on the outside of a cold drink on a warm day.
Environmental Conditions for Dew
Certain environmental conditions favor dew formation, primarily those promoting significant surface cooling. Clear skies are important because they allow heat to radiate away from the Earth’s surface into space. This process, known as radiative cooling, enables surfaces to cool efficiently.
Calm or very light winds are also beneficial for dew to form. Strong winds can mix the cooler air near the surface with warmer, drier air from higher up, preventing the surface from reaching its dew point. Light winds, however, allow the cool, moist air to remain in contact with the cooling surfaces, facilitating condensation.
High humidity provides a greater amount of water vapor for condensation. When there is ample moisture, the dew point temperature is higher, meaning surfaces do not need to cool as much to reach it. Moist soil can also contribute to dew by releasing water vapor into the air near the ground as it cools.
The Usual Timing of Dew
Dew typically begins to form in the late evening, continues overnight, and is most noticeable in the early morning hours. This timing aligns with when temperatures are lowest. After the sun sets, surfaces no longer receive solar radiation and begin to cool rapidly through the emission of infrared radiation.
As the night progresses, the temperature of exposed objects continues to drop. This prolonged cooling allows these surfaces to eventually reach the dew point of the surrounding air. Once the surface temperature falls below this point, water vapor condenses into dew droplets.
The process often continues until sunrise, or shortly after, when increasing solar radiation causes the surfaces to warm up and the dew to evaporate. Observing dew is most common from several hours after sunset until the early morning.
Why Dew Doesn’t Always Form
Dew does not form every night, as its appearance depends on specific atmospheric conditions. One primary reason for its absence is the presence of strong winds. Vigorous air movement prevents the ground-level air from becoming sufficiently cool and saturated, as it constantly mixes with warmer, drier air.
Cloudy skies also hinder dew formation. Clouds trap the heat radiated from the Earth’s surface, preventing it from escaping into space. This reduces the amount of radiative cooling, which means surfaces may not cool enough to reach the dew point.
Furthermore, very low humidity in the air can prevent dew. If there is insufficient water vapor, the dew point temperature will be very low, making it unlikely for surfaces to cool enough to reach it. Even if surfaces cool considerably, there might not be enough moisture to condense into droplets.