What causes condensation to form on a cold glass?

When a cold glass is placed in warm, humid air, the air immediately surrounding the glass gets cooled below its dew point. At that temperature, the air can no longer hold as much water vapor, so the excess moisture condenses into liquid water droplets on the surface of the glass. The dew point is the critical temperature at which air becomes saturated with water vapor, and once the glass cools the surrounding air to that point, condensation forms. The colder the glass and the more humid the air, the more condensation appears. This is the same process that causes morning dew to form on grass when overnight temperatures drop low enough.