READ: Changes of State

This beautiful beaded net is something very common in nature: a spider web. The “beads” on the spider web are actually drops of water. The drops of water are not raindrops, and they weren’t there on the previous day. Where did they come from?

From Gas to Liquid

The drops of water on the spider web are dewdrops. They formed overnight when warm moist air came into contact with the cooler spider web. Contact with the cooler web cooled the air. When air cools, it can hold less water vapor, so some of the water vapor in the air changed to liquid water. The process in which water vapor—or another gas—changes to a liquid is called condensation. Another common example of condensation is pictured in the Figure below. 

Dew Point

When air is very humid, it doesn’t have to cool very much for water vapor in the air to start condensing. The temperature at which condensation occurs is called the dew point. The dew point varies depending on air temperature and moisture content. It is always less than or equal to the actual air temperature, but warmer air and moister air have dew points closer to the actual air temperature. That’s why glasses of cold drinks “sweat” more on a hot, humid day than they do on a cool, dry day. 

Q: What happens when air temperature reaches the dew point?

A: When air temperature reaches the dew point, water vapor starts condensing. It may form dew (as on the spider web above), clouds, or fog (Shown in the Figure below). Dew forms on solid objects on the ground. Clouds form on tiny particles in the air high above the ground. Fog is a cloud that forms on tiny particles in the air close to the ground.