Rain

Rain occurs when water droplets in the atmosphere become large and heavy enough to overcome air resistance and fall from clouds to the ground under the force of gravity.

How is rain formed?

Rain formation begins with evaporation, where water from oceans, lakes, rivers, and even soil transforms into invisible water vapor. This warm, moist air rises into the atmosphere. As it ascends, the surrounding air becomes cooler, causing the water vapor to condense into tiny liquid droplets. These droplets cluster together to form clouds. When they grow large enough through further condensation and collision, they fall as rain.

What qualifies as rain?

Not all liquid precipitation qualifies as rain. Several criteria must be met:

• Liquid state: Rain is composed of liquid water droplets. Other forms like snow, sleet, graupel, or slush involve frozen or partially frozen water and are classified separately.
  
• Droplet size: Rain droplets are typically larger than 0.5 mm in diameter. Smaller droplets (less than 0.5 mm) are usually classified as drizzle.
  
• Cloud origin: Rain originates from the condensation of water vapor within clouds. Moisture that condenses on surfaces (like dew) or drips from fog-covered objects is not considered rain, as it doesn’t fall directly from clouds.
  

How big are raindrops? 

Raindrops have sizes ranging from 0.5 to 9 mm (0.0039 to 0.3543 in) mean diameter but develop a tendency to break up at larger sizes. Smaller drops are called cloud droplets, and their shape is spherical. As a raindrop increases in size, its shape becomes more oblate, with its largest cross-section facing the oncoming airflow.

How is rain classified?

Rain is often classified based on intensity and duration:

By intensity (measured in mm or inches per hour):

• Light rain: Minimal precipitation with low accumulation.
• Moderate rain: Steady rainfall with noticeable accumulation.
• Heavy rain: Intense precipitation with rapid accumulation, which may cause flooding.

By duration:

• Showers: Brief and often sporadic rainfall, starting and stopping suddenly.
• Continuous rain: Persistent rainfall lasting for an extended period.