Precipitation falls to Earth in various forms, including the familiar liquid drops of rain and the solid flakes of snow. Confusion often arises when conditions introduce sleet, a type of frozen precipitation distinct from both rain and snow. Understanding the difference between rain and sleet depends on the precise temperature structure of the atmosphere from the clouds to the ground. Meteorologists rely on vertical temperature profiles to accurately forecast which precipitation type will reach the surface.
Defining Rain and Sleet
Rain is defined as liquid water droplets that fall from the atmosphere and reach the Earth’s surface. These drops are typically greater than 0.5 millimeters in diameter. Sleet is defined as ice pellets, which are small, translucent balls of ice. These ice pellets are essentially frozen raindrops. Sleet is recognized by its solid state upon impact, often bouncing off the ground or other objects.
The Atmospheric Recipe for Rain
The formation of rain requires a specific atmospheric temperature profile. For precipitation to fall as liquid rain, the entire column of air from the cloud base down to the surface must remain above the freezing point (32 degrees Fahrenheit or 0 degrees Celsius). Any ice crystals that form high in the cloud melt completely as they descend through this warm layer. The resulting liquid water then travels unimpeded to the ground.
The Atmospheric Recipe for Sleet
Sleet requires a layered temperature structure in the atmosphere. The process begins where precipitation forms as ice crystals or snowflakes. As this frozen precipitation falls, it encounters a mid-level layer of air where the temperature rises above freezing, often between 3,000 and 5,000 feet above the ground. This warm layer causes the snowflakes to melt completely, turning them into liquid raindrops.
The third layer for sleet formation is a deep, sub-freezing layer of air near the surface, where temperatures drop back below 32 degrees Fahrenheit. As the raindrops enter this colder air, they cool down below the freezing point. If this freezing layer is sufficiently deep, the drops have enough time to completely refreeze into solid ice pellets before they hit the ground.
How to Tell Them Apart
The way to distinguish sleet from rain is by observing its physical characteristics upon impact. Rain hits the ground and surfaces with a quiet patter, instantly wetting them and pooling into puddles. Sleet, being solid ice pellets, makes a distinct clicking or rattling sound as it strikes cars, windows, and pavement. The translucent pellets will also bounce off hard surfaces rather than splashing. Sleet tends to gather as a layer of granular ice.