Precipitation is the process by which atmospheric water vapor condenses into sufficiently large masses and falls to the Earth’s surface under the influence of gravity. It is the primary “input” mechanism in the global hydrological cycle and the water budget of any region.
Forms of Precipitation
The form of precipitation is determined by the vertical temperature profile of the atmosphere—from the cloud level down to the ground.
| Form | Characteristics |
| Rain | Liquid water drops with a diameter greater than 0.5 mm. It is the most common form in tropical and temperate regions. |
| Drizzle | Very small, uniform liquid drops (diameter < 0.5 mm) that seem to float in air currents. Usually associated with Stratus clouds. |
| Snow | Formed when water vapor turns directly into ice crystals (sublimation) in sub-freezing temperatures. Falls as flaky hexaganol structures. |
| Sleet | Also known as frozen raindrops. It occurs when rain falls through a layer of freezing air near the surface, turning into small, clear ice pellets. |
| Hail | Hard, rounded pellets of ice (5 mm to 50 mm or more). Formed in Cumulonimbus clouds due to strong updrafts that cycle ice through supercooled water layers. |
| Glaze (Freezing Rain) | Rain that freezes instantly upon contact with cold objects or the ground, forming a coat of ice. |
Types of Rainfall (Based on Origin)
Convectional Rainfall
Common in the equatorial regions. High solar insulation causes the air to warm, expand, and rise. As it rises, it cools adiabatically, leading to condensation and heavy “4 o’clock” showers accompanied by thunder and lightning.
Orographic (Relief) Rainfall
Occurs when moisture-laden air is forced to rise over a mountain barrier.
- Windward Side: Receives heavy rainfall due to cooling and condensation.
- Leeward Side (Rain Shadow Area): Receives very little rainfall as the air descends and warms (e.g., the Deccan Plateau behind the Western Ghats).
Cyclonic (Frontal) Rainfall
Occurs when two air masses of different temperatures and humidity meet. The warmer, lighter air is forced over the colder, denser air, leading to condensation and widespread, persistent rainfall.
Measurement of Precipitation
Precipitation is measured as the vertical depth of water that would accumulate on a flat, level surface if none of it escaped.
Instrument: The Rain Gauge
The standard instrument is the Symon’s Rain Gauge. It consists of a funnel, a receiver, and a cylindrical metal container.
- Standard Unit: Measured in millimeters (mm) or centimeters (cm).
- Recording vs. Non-Recording: * Non-Recording: Provides only the total depth of rain in 24 hours (e.g., Symon’s gauge).
- Recording (Automatic): Provides the intensity of rainfall over time using mechanisms like the Tipping Bucket, Weighing Bucket, or Floating-type gauge.
Measurement of Snowfall
Snow is measured using a snow gauge or a graduated ruler. To include it in the water budget, it is converted to a Rainfall Equivalent.
- General Ratio: 10 units of snow are approximately equal to 1 unit of liquid water (10:1 ratio), though this varies with the density of the snow.
Key Meteorological Terms for UPSC
- Isohyets: Lines on a map connecting places that receive equal amounts of rainfall during a given period.
- Rainfall Intensity: The ratio of total amount of rain to the duration of the period (expressed in mm/hr). High intensity often leads to flash floods.
- Cloud Seeding (Artificial Rain): The process of spreading dry ice or silver iodide into clouds to stimulate precipitation.
- Virga: Precipitation that evaporates or sublimates before reaching the ground, often seen as streaks hanging from the bottom of a cloud.
Spatial and Temporal Variation in India
- Highest Rainfall: Mawsynram and Cherrapunji (Meghalaya) due to the funneling effect of the Khasi Hills.
- Lowest Rainfall: Jaisalmer (Rajasthan) and Leh (Ladakh).
- Variability: In India, rainfall variability is inversely proportional to the amount of rainfall. Areas with high rainfall (Western Ghats) have low variability, while arid regions (Rajasthan) have very high variability, making them more prone to unpredictable droughts.