The moisture found on grass in the morning, often sparkling with countless tiny droplets, is predominantly dew. This moisture is a direct result of atmospheric water vapor changing phase onto the cool surface of the grass. The process is a common, localized event in the water cycle, driven by the relationship between air temperature, humidity, and surface cooling.
Defining Dew
Dew is water in the form of droplets that appears on exposed surfaces, like blades of grass, due to condensation. It forms when moisture already present in the air settles directly onto a solid object. This water vapor makes up the humidity in the atmosphere, and dew formation is a visible indicator of high moisture content in the air close to the ground.
The Mechanism of Condensation
The formation of dew is a phase change known as condensation, where water transitions from vapor to a liquid state. Air’s capacity to hold water vapor is directly related to its temperature; warmer air holds more moisture than cooler air. When air holds the maximum possible amount of water vapor at a specific temperature, it is considered saturated.
The temperature at which air becomes saturated and condensation begins is called the dew point. When the grass surface cools to this temperature, the air immediately in contact with the blades can no longer hold its water vapor. This excess moisture changes into liquid water, depositing as tiny droplets onto the cooled surface.
The Role of Nighttime Cooling
The process of dew formation primarily happens at night due to a phenomenon called radiative cooling. After sunset, the Earth’s surface and objects on it, like grass blades, rapidly lose heat by radiating thermal energy into the atmosphere. Since grass is a thin, exposed object with poor thermal conductivity, it cools down more quickly and efficiently than the air a few feet above it.
This heat loss is most effective on clear nights because clouds act like a blanket, reflecting radiated heat back toward the ground, which prevents the grass from cooling sufficiently. Calm, still air is also an important factor, as wind would mix the cooler air layer near the ground with warmer air from above, hindering the temperature drop. When these conditions align, the grass blades can cool several degrees below the surrounding air temperature, dropping to or below the dew point and initiating condensation.
When Wet Grass Is Not Dew
While dew is the most common cause of wet morning grass, another process, called guttation, can also be responsible for moisture droplets. Guttation is water expelled directly from the plant itself due to root pressure, not condensed atmospheric moisture. This biological process typically occurs when the soil is saturated with water, and the plant cannot release enough moisture through transpiration at night because the pores on its leaves are closed.
Distinguishing Guttation from Dew
To manage this internal pressure, the plant secretes excess water, which contains various organic compounds and sugars, through specialized pores. This results in distinct, larger droplets concentrated along the margins of the leaves. Unlike dew, which coats the entire surface more uniformly, guttation is a sign of a well-watered plant and is often distinguishable by its location and sometimes by a slightly sticky feel caused by the expelled plant sugars.