When the temperature of the air or Earth changes condensation or evaporation.
If the air temperature gets cool enough, the water vapor has to condense on particles in the air to form clouds; this is extremely noticeable when plants have dew on them in the morning or frost forms on your parents' car in the winter.
Click on Percipitation for the next step.
Link to Northern Michigan University
Melissa Teasley: mteasley@nmu.edu