When liquid rain transitions into solid snow, the change is governed by a complex set of conditions throughout the entire vertical structure of the atmosphere, not just ground temperature. Understanding the difference between rain and snow requires examining the temperature profile from the cloud base down to the surface. The conditions that must change involve eliminating warm air layers and establishing a deep, consistent cold environment. This article explores the atmospheric adjustments necessary for rain to give way to falling snow.
The Critical Atmospheric Temperature Profile
Rain typically falls because the atmosphere contains a “melting layer,” a region elevated above the surface where temperatures rise above 0°C (32°F). This warm stratum intercepts snowflakes formed higher in the cloud, causing them to melt into raindrops before they reach the ground. The change from rain to snow depends on the complete removal or cooling of this melting layer.
For snow to persist all the way to the surface, the atmospheric temperature profile must establish a continuous “cold column.” This column requires that the air temperature remains at or below 0°C (32°F) from the generating cloud base down to the earth. The temperature gradient should be uniformly cold, preventing mid-level warming that would initiate melting.
The vertical extent of this cold air mass is a major factor in determining the precipitation type. If the layer of air above freezing is substantial in both temperature and depth, precipitation will fall as rain. Conversely, transformation to snow requires a sustained invasion of cold air that cools the entire column below the freezing point.
Even a relatively thin layer of air slightly above 0°C can completely melt snowflakes, causing them to fall as rain. Therefore, the necessary change is the replacement of elevated warm air with a deep, pervasive mass of air that fully supports ice crystals from their inception to their landing. The duration for which this cold air mass settles dictates how long the snowfall can continue.
The Role of Evaporative Cooling
The temperature profile for snowfall can sometimes succeed even when the air temperature measured at the surface is marginally above freezing, perhaps around 1°C to 2°C (34°F to 36°F). This apparent contradiction is resolved by evaporative cooling. This mechanism is relevant when precipitation falls through a layer of relatively dry air near the ground.
When liquid rain descends through unsaturated air, the raindrops begin to evaporate. Evaporation requires a substantial amount of energy, known as the latent heat of vaporization, which is pulled from the surrounding air mass.
The removal of heat causes the air temperature to decrease rapidly. The air continues to cool until it reaches its saturation point. This dynamically cooled temperature is the real determinant of the precipitation type, often dropping the atmospheric temperature to the freezing point.
Therefore, a necessary condition for rain to become snow in a marginal temperature scenario is the presence of a dry air mass below the cloud deck. The evaporative process acts as a natural refrigerating agent, cooling the near-surface air enough to prevent the final melting of the snowflakes. This cooling preserves the ice crystals formed aloft, allowing them to reach the ground intact as snow.
Distinguishing Snow from Sleet and Freezing Rain
The transformation of rain to snow is distinguished from other winter precipitation types by the depth and consistency of the cold column. Both sleet and freezing rain occur because atmospheric cooling is only partially achieved, allowing a melting layer to persist aloft. The difference between these two types depends on the vertical extent of the sub-freezing air near the ground.
Sleet, which falls as small ice pellets, requires a specific layered profile. Snowflakes melt in a warm layer high up, turning into raindrops. These raindrops then fall through a deep layer of sub-freezing air near the surface. This deep cold layer provides sufficient time for the liquid drops to completely refreeze into solid ice pellets before impact.
The conditions that yield freezing rain, conversely, involve a warm layer that is often deeper and more extensive than the one associated with sleet. The distinction is that the subsequent sub-freezing layer near the surface is very shallow. The liquid drops do not have adequate time or depth to fully freeze before reaching the ground.
As a result, the supercooled liquid precipitation remains liquid until it contacts an object at or below freezing. This immediate solidification upon impact defines freezing rain. Therefore, the condition that must change for rain to become snow is the replacement of any elevated warm layer with a uniformly deep column of air entirely below the freezing point.