Precipitation refers to any form of water that falls from the atmosphere to the Earth’s surface. It plays a fundamental role in Earth’s weather patterns and the global water cycle.
The Formation of Precipitation
The journey of precipitation begins with the water cycle, a continuous process where water moves between the Earth’s surface and the atmosphere. Sunlight heats water from oceans, lakes, and land, causing it to evaporate and transform into water vapor. This water vapor then rises into the atmosphere.
As the water vapor ascends, it encounters cooler temperatures in the upper atmosphere, leading to condensation. During condensation, the water vapor changes back into tiny liquid water droplets or ice crystals. These microscopic droplets or crystals form around minute particles in the air, known as condensation nuclei.
Once these tiny droplets or ice crystals form, they grow within the clouds through various mechanisms. One process is collision-coalescence, where smaller water droplets collide and merge with larger ones, becoming heavier. Another process, the Bergeron process, occurs in colder clouds where ice crystals grow by attracting water vapor from surrounding supercooled water droplets. When these water particles become too heavy for air currents to support, gravity pulls them down to the Earth’s surface as precipitation.
Major Forms of Precipitation
The specific type of precipitation that reaches the ground depends largely on the temperature profile of the atmosphere, from the clouds to the surface. Slight variations in temperature layers can result in distinctly different forms of precipitation.
Rain
Rain consists of liquid water droplets that fall from the atmosphere. It forms when atmospheric temperatures from the cloud base to the ground remain above freezing, allowing water droplets to fall as a liquid. Raindrops typically have diameters greater than 0.5 millimeters. While rain often begins as liquid water, it can also originate as ice crystals or snowflakes in colder parts of a cloud that completely melt as they fall through warmer air layers before reaching the ground.
Snow
Snow is composed of ice crystals that form directly from water vapor in clouds where temperatures are at or below freezing. These individual ice crystals then stick together to create snowflakes as they fall. Snow can still reach the ground even if the surface temperature is slightly above freezing.
Sleet (Ice Pellets)
Sleet, also known as ice pellets, forms under specific atmospheric conditions involving multiple temperature layers. It begins as snow falling from a cloud, which then encounters a warm layer of air aloft and completely melts into raindrops. As these raindrops continue to fall, they pass through a deep layer of air at or below freezing temperatures near the surface, causing them to refreeze into small, translucent ice pellets before hitting the ground. Sleet is distinct from hail, which forms differently.
Freezing Rain
Freezing rain occurs when precipitation falls as liquid rain but freezes upon contact with surfaces that are at or below freezing temperatures. This happens when a deep layer of warm air is present high above the ground, causing initial frozen precipitation to melt into rain. Near the surface, there is a shallow layer of air at or below 0 degrees Celsius. The raindrops become supercooled as they pass through this shallow cold layer but do not freeze in the air. Instead, they freeze upon striking cold objects, creating a coating of ice known as glaze ice.
Hail
Hail is a form of solid precipitation consisting of irregular lumps or balls of ice, known as hailstones. Hail forms exclusively within strong thunderstorms characterized by powerful updrafts, which are currents of rising air. These updrafts carry water droplets high into extremely cold regions of the cloud, where they freeze into ice pellets. The hailstones grow as they are repeatedly carried up and down within the thunderstorm by updrafts and downdrafts, collecting additional supercooled water droplets that freeze onto their surface. Hailstones measure larger than 5 millimeters in diameter.