Precipitation refers to any product of the condensation of atmospheric water vapor that falls under gravity, whether in liquid or solid form. Generally, precipitation falls as a single type, such as rain or snow, but sometimes atmospheric conditions allow multiple forms to reach the ground at the same time. This simultaneous occurrence is known as mixed precipitation, where different solid and liquid hydrometeors combine in one event.
Defining Mixed Precipitation
Mixed precipitation involves the combination of two or more distinct winter weather types observed at the surface, either simultaneously or in rapid succession. Common combinations include snow falling alongside sleet, or a mixture of snow and freezing rain. Rain itself may also be a component of the mix, often appearing with wet snow.
Sleet, which the United States National Weather Service defines as ice pellets, is a major component. These small, translucent ice pellets bounce upon striking the ground, distinguishing them from the stickiness of wet snow or the liquid state of rain. Freezing rain is also included, as it involves supercooled liquid droplets that freeze upon contact with surfaces below freezing.
Atmospheric Layers and Temperature Profiles
The formation of mixed precipitation is governed by the vertical temperature structure of the lower atmosphere. Nearly all winter precipitation begins its descent from the cloud tops as snow or ice crystals. The ultimate form reaching the ground depends on a layered temperature profile, often described as a “warm nose” of air aloft.
This warm nose is an elevated layer where the air temperature is above the freezing point of 0 degrees Celsius, sandwiched between layers of sub-freezing air. As snow falls into this warm layer, it begins to melt; the depth and warmth of this layer determine the degree of melting.
If the warm layer is relatively deep, the snow fully melts into raindrops. These raindrops then fall into a shallow, sub-freezing layer near the surface, becoming supercooled and freezing instantly upon impact with the ground as freezing rain.
If the warm layer is shallower or cooler, the snow only partially melts before entering the cold air near the surface. The partially melted snowflakes or raindrops then refreeze into ice pellets, or sleet, before reaching the ground. Minor changes in the altitude, temperature, or thickness of these atmospheric layers can rapidly change the precipitation type.
Challenges in Weather Forecasting
Predicting mixed precipitation events presents a significant challenge for meteorologists because the outcome is sensitive to minute atmospheric details. The difference between heavy snow, a glaze of freezing rain, or merely cold rain often hinges on a temperature variation of just one or two degrees Celsius. This narrow temperature window must be accurately forecast across the entire depth of the atmosphere.
Weather prediction models frequently encounter difficulty in precisely depicting the vertical thermal structure. A common issue is a warm bias, where models may predict a surface temperature that is too high, leading to an incorrect forecast of rain instead of a hazardous mixed type.
Complex terrain, such as valleys and mountains, can locally enhance these forecasting difficulties by creating small-scale variations in the cold air depth. Accurately forecasting the timing and the specific amount of each precipitation type within the mix remains a difficult task.