Rain and snow mixed

A mixture of rain and snow, also known as sleet, or a wintry mix, is a type of precipitation that happens most often in the warmer parts of winter -- that is, the beginning and end. It is common throughout the winter in the warmer parts of the temperate zone, however. This kind of precipitation can lead to extremely slippery conditions, accumulating on roads as a layer of slush, and may change over to snow or rain depending on how the overall storm system that is causing it develops.

The snow accompanying the rain is usually very wet, and often partly melted. This makes conditions at ground level even messier, especially if there is already snow on the ground to be partly thawed by the falling sleet.

Several different kinds of conditions can produce sleet. One of the most common ways sleet comes into being is when a storm represents a transition between above-freezing and below-freezing weather. Regardless of whether the warm or cold air is winning out, a mixture of rain and snow can occur at the boundary between above and below freezing temperatures. The temperature of the air may be enough to freeze some, but not all, of the precipitation falling at this line, and sleet will result.

Two other scenarios for sleet production depend on the air being layered with warmer and colder levels between the clouds and the ground. If the clouds are cold but a warmer, above-freezing layer of air exists at the ground, then the snow falling from the clouds may partially melt before reaching the surface. The warm air layer needs to be thick enough so that there is enough time for some of the snow to melt into droplets, but shallow enough so that all of the snow does not melt.

If there is a more complex layering of air temperatures, then rain and ice pellets may fall in place of rain and snow. If a layer of warm air is sandwiched between two layers of cold air, one aloft and one lying along the ground, then snow falling from the clouds melts into droplets in the warm layer, and some of it then refreezes while passing through the cold layer to the ground. The conditions are not right for the droplets to change back into snowflakes, which is a delicately-balanced process that can't occur in the mile or so of cold air needed to refreeze some of the droplets.

Of course, if the layer of cold air at the ground is too deep, then a fall of ice pellets without any liquid rain will occur. If it is too shallow, then freezing rain will probably result, rather than sleet.

All in all, it can be seen that rain mixed with snow is a product of very unstable winter weather conditions, with warm and cold air mixing together in various ways and precipitation thawing and refreezing as it falls. These unstable conditions can only exist with some above-freezing air in the region, meaning that they occur either near the subtropical latitudes, or close to the seasonal boundaries when tropical air is starting to move weakly north or has not yet been completely pushed out by the dry, cold, stable air of winter.