American Meteorological Society Certification (AMS) Practice

When water in clouds joins together and becomes heavy, it leads to the formation of rain. This process occurs as individual water droplets within the clouds collide and coalesce, gradually increasing in size. When these droplets become too heavy for the air currents to support them, they fall to the ground due to gravity, resulting in precipitation in the form of rain.

Frost forms at temperatures below freezing when water vapor turns directly into ice without becoming liquid first. Humidity refers to the amount of moisture present in the air, which is a measure rather than a form that results from the joining of water in clouds. Condensation is the process of water vapor turning into liquid form but does not specifically refer to the outcome of precipitation. Thus, the correct choice is rain, as it directly describes the phenomenon resulting from water droplets merging and increasing in weight within clouds.