Why does water vapor condense at the dew point?
Condensation is driven by a dynamic balance between water molecules hitting a surface and becoming stuck because they don’t have enough thermal energy to escape and water molecules already part of the liquid getting enough thermal energy to escape. At all times there are some molecules sticking and escaping. The dew point is where more get stuck than escape; below the dewpoint more are getting stuck, so the quantity of stuck water molecules increases.
