Unlike the summer months, winter brings a broader range of precipitation types due to sub-freezing temperatures. Since the atmosphere is variable and seldom uniformly warm or cold, precipitation comes in several forms of the frozen variety.
Simply put, temperatures above the surface of the earth are important to determine the type of precipitation. How much (if any) of the mid-levels of the atmosphere is above freezing goes a long way to determining if what falls will be frozen, semi-frozen, or liquid.
Snow: It's All Frozen
Snow is the simplest form of frozen precipitation and requires no special conditions in the upper atmosphere to form. It is also the most common. The only requirement for snow to form is that temperatures are at or below freezing at all levels of the atmosphere.
Depending on the temperatures, snow can take on various types. Among these types are stars, dendrites, columns, plates, and needles. Each has a set of conditions necessary for formation.
For example, the two most stereotypical snowflakes require it to be quite cold in the section of the atmosphere where clouds form. Stars need a temperature of -15C (+5F). They are typically flat. However dendrites are more three dimensional and need temperatures of -20 to -25C (-4 to -13F), and also a high level of moisture content in the air.
Finally, there is also another type of snow commonly referred to as "wet snow." This is snow which contains a great deal of water content, and usually occurs when air temperatures are near freezing.
Sleet: Partially Melted Snow
Sleet is another common form of frozen precipitation, and needs some special conditions in order to form. At the surface, temperatures must be below freezing. As one moves up through the atmosphere, temperatures rise until for a short period they are above freezing. Nearer to the cloud layer, temperatures fall back to sub-freezing levels.
As precipitation falls from the clouds, it is snow. When it passes through the shallow layer of above-freezing temperatures it will melt, but not completely. This allows the partially melted snow to refreeze, and become pellet-like in shape.
The size of sleet pellets will vary, and at one time was mistakenly thought to be hail (and vice versa). However, unlike hail, the size of pellets are typically only millimeters in size. Even though they are small, these sleet pellets can make roads and sidewalks slippery due to their round shape.
Freezing Rain: It's Exactly What It Says
Freezing rain is probably the most dangerous of the winter precipitation types, although it is also the least common as it takes special circumstances for it to occur. Whereas there is only a shallow layer of warm air with sleet, in freezing rain events that layer is much larger.
There is only a very shallow layer of sub-freezing air at the surface when freezing rain forms. The rest of the atmosphere, sometimes even including the cloud deck itself, could be above freezing. Since this cold air layer is so small, it only freezes when it comes in contact with the surface.
Freezing rain can accumulate up to several inches. The weight of the ice can cause power lines to snap, tree limbs to break, and in some extreme circumstances even cause property damage.
With several different types of winter precipitation, it can be confusing to differentiate them when the weatherman talks about them on the local news. However, understanding why it happens will make it easier to deal with when it actually happens.
Source: Winter Precipitation Types and their Environments, NOAA
