A wet lawn does not always mean rain fell while you slept. Dew can form when a surface cools enough for water vapor in nearby air to condense into droplets. The water was already present in the air; it did not need to arrive as a shower.
The surface has its own temperature
Grass, a car roof, and a paved path do not cool in exactly the same way. On a clear, relatively calm night, exposed surfaces can lose heat and become cooler than the air measured at a weather station. That helps explain why one object is beaded with water while another nearby looks dry.
The dew point is the temperature associated with saturation of the air with respect to liquid water. When a surface becomes cool enough, condensation can occur there. Below-freezing conditions introduce frost processes, so a weather app's single temperature does not describe every surface around your home.
Look for a pattern, not a puddle-sized forecast
On your next ordinary morning walk, compare exposed grass with grass under a tree, or a car roof with a sheltered surface. Notice shade, exposure, and whether the night was clear. This is an observation exercise, not a way to predict road ice or decide that a surface is safe.
Dew also does not tell you how much rain a garden received. A film of droplets and water delivered by a shower are different events. If watering decisions matter, use appropriate soil observations and local guidance rather than treating every wet leaf as rainfall.
The small surprise of dew is that the atmosphere can make a surface wet without a drop falling from a cloud. Once you notice the role of surface temperature, a patchy wet morning becomes much less mysterious.
Sources
- National Weather Service: Dew and frost development
Surface cooling, dew point, and conditions supporting dew or frost.