Precipitation is any form of atmospheric water that falls to the planet’s surface. This process begins with water evaporating from land and bodies of water, rising into the atmosphere, and forming clouds. When water droplets or ice crystals within these clouds grow sufficiently large and heavy, gravity pulls them down as precipitation.
Understanding the Primary Forms
Rain consists of liquid water droplets. These droplets vary in size, with smaller ones often referred to as drizzle, while larger ones can reach diameters up to 6 millimeters.
Snow is composed of individual ice crystals that form and grow while suspended in the atmosphere. Snowflakes exhibit a wide array of shapes, from simple hexagonal crystals to intricate dendritic structures, influenced by temperature and humidity during their formation.
Sleet, also known as ice pellets, refers to small, translucent ice particles typically less than 5 millimeters in diameter. These pellets form when precipitation freezes before reaching the ground.
Hail consists of solid, irregular lumps or balls of ice, known as hailstones, which typically measure 5 millimeters or more in diameter. Unlike sleet, hailstones are often layered and can grow significantly larger, sometimes reaching several centimeters across.
The Atmospheric Conditions for Each Type
The type of precipitation that reaches the ground is determined by the temperature profile of the atmosphere from the cloud down to the surface. Rain occurs when temperatures remain above freezing throughout the entire column of air from where the droplets form in the cloud to the ground. This allows condensed water vapor to fall as liquid without solidifying.
Snow forms when the atmospheric temperature from the cloud to the ground is consistently at or below the freezing point of 0°C (32°F). Ice crystals nucleate around particles in the atmosphere and grow, accumulating into snowflakes that fall to the Earth without melting.
Sleet develops under specific conditions where a layer of warm air is sandwiched between two layers of freezing air. Snowflakes originating in the upper cold layer fall through the warm layer, melting into raindrops. These liquid drops then encounter a deeper, freezing layer of air closer to the surface, causing them to refreeze into ice pellets before impact.
Hail formation requires strong thunderstorm clouds, known as cumulonimbus clouds, characterized by intense updrafts. Within these clouds, water droplets are carried upward into extremely cold regions, freezing into ice pellets. These ice pellets grow as they collide with supercooled water droplets and other ice particles, accumulating layers of ice while being repeatedly lifted and dropped by the strong updrafts and downdrafts until they become too heavy to be supported and fall to the ground.