Where do the morning drops on grass come from?

After sunset, the ground and grass give off heat to the sky. Thin blades cool quickly, and on a clear night they can become several degrees colder than the surrounding air.

Air carries invisible water vapor. Below a certain temperature, air can't hold all of it; that temperature is called the dew point. The more humid the air, the higher the dew point, so even a little cooling makes water form. Once a leaf's surface drops below the dew point, vapor touching it turns into droplets—just like water beading on a glass of ice.

Because people say dew 「falls」, it's easy to imagine water dropping from above. But dew forms on cold surfaces, so even in the same yard it gathers most on things that cool well toward the open sky, like grass and car roofs. Spots under eaves or trees stay drier.

On cloudy nights, heat from the ground is sent back down and the grass cools less; on windy nights the air mixes, so the layer by the leaves can't get cold. That's why dew is heaviest after clear, calm nights. If the leaves cool below freezing, frost forms instead of water.

If each grass blade has one round drop hanging right at its tip, it may not be dew. When the soil is moist at night, roots push water upward and it seeps out of tiny pores at the leaf tip. This is called guttation. Dew coats the whole leaf evenly, while guttation gathers at tips and edges.

There's an old saying that morning dew means a clear day. Clear, calm nights usually come with high pressure overhead, so the saying has real grounds.

A car windshield left outside overnight turning cloudy and wet is dew too. Parking under a roof or covering the windshield cuts the heat lost to the sky, so less dew forms.

The same goes for a camping tent that's damp outside every morning. Packing it right away because it didn't rain puts it in the bag wet, so it's better to take it down after the sun has dried the outside.