Water constantly cycles through the atmosphere, moving from clouds to the Earth’s surface in various forms. This falling water is not always the simple liquid of rain or the crystalline solid of snow. The specific physical properties and formation processes in the atmosphere create a spectrum of precipitation types. These varied states, from liquid droplets to frozen pellets and simultaneous mixtures, are categorized based on their composition and interaction with air layers.
The Broad Scientific Term for Falling Water
The umbrella term for any form of water falling from the atmosphere and reaching the Earth’s surface is precipitation. This single category encompasses all liquid and frozen water particles, including rain, drizzle, snow, hail, sleet, and freezing rain. Precipitation is considered a type of hydrometeor, which is a technical term referring to any product of the condensation or sublimation of atmospheric water vapor, whether it is falling, suspended, or blown by the wind.
The distinction between the different forms depends on the physical state of the water when it lands. Rain is liquid water droplets, while snow consists of clustered ice crystals forming snowflakes. The specific type of precipitation is determined by temperatures within the atmosphere and at the ground level. This classification system helps categorize the more complex, mixed forms that combine characteristics of both rain and snow.
Specific Names for Mixed Forms
When both liquid water and frozen crystals fall simultaneously or change state just before reaching the ground, they are classified into distinct categories. The most direct answer to what rain and snow together are called is a rain/snow mix, often referred to as a wintry mix in general forecasts. This occurs when snowflakes and liquid raindrops fall at the same time, usually when the temperature near the surface is hovering just above the freezing point, causing partial melting of the snowflakes. This type of precipitation is typically translucent and slushy, marking a transitional phase from one form to the other.
Another distinct type is sleet, which in the United States is officially called ice pellets. Sleet begins as snowflakes that melt into raindrops as they fall through a layer of warm air aloft. These raindrops then fall through a deep layer of freezing air just above the surface, refreezing completely into small, hard, translucent ice pellets. This process results in precipitation that is entirely solid upon landing, unlike the slushy mixture of a rain/snow mix.
A third form is freezing rain. This occurs when liquid raindrops fall through a very shallow layer of sub-freezing air at the surface, which is not deep enough to allow them to freeze into sleet. The liquid drops become “supercooled,” remaining liquid below the freezing point until they strike a surface at or below 32 degrees Fahrenheit (0 degrees Celsius). The immediate freezing on contact creates a smooth, clear coating of ice, known as glaze, on roads, sidewalks, and power lines.
The Role of Atmospheric Temperature in Formation
The different forms of winter precipitation, particularly the mixed types, are determined entirely by the vertical temperature profile of the atmosphere. This profile describes the temperature of the air at various heights above the surface, creating a layered structure. For snow to reach the ground, the air temperature must remain at or below the freezing point from the cloud base all the way to the surface, ensuring the ice crystals never melt.
The formation of sleet and freezing rain involves a warm layer of air “sandwiched” between two layers of freezing air. For sleet, snowflakes fall into an above-freezing layer and melt into rain. They then pass through a relatively deep layer of sub-freezing air near the surface, allowing enough time for the liquid drops to refreeze into ice pellets before impact. This deep freezing layer is the defining feature of sleet’s formation process.
Freezing rain, conversely, requires a deep warm layer aloft that causes the snowflakes to melt completely into liquid rain. This is followed by only a very shallow layer of sub-freezing air at the surface. The rain does not have enough time within this shallow layer to refreeze into ice pellets. This results in supercooled liquid drops that instantly turn to ice upon encountering a frozen object on the ground.