Wednesday, January 14, 2015

Sorting Out Winter Precipitation

Now that we are the mid-point of January Mother Nature has seen fit to provide most of the country with some winter weather. When most people in the colder part of the country hear "winter precipitation" they are most likely to think of snow. One of the challenges to winter precipitation measurements is the menu of precipitation that can occur.

The precipitation can be broken into two categories, frozen, and freezing or unfrozen precipitation. In the frozen category we have snow, graupel, ice pellets (sleet), and snow pellets. In the unfrozen or freezing category we have freezing rain or drizzle, and "plain" rain or drizzle. Let's look at these one at a time.

Snow is precipitation of ice crystals, mostly branched in the form of six-pointed stars. The shape and size of snowflakes can vary, from large "fluffy" flakes to smaller ice crystals. The type of snow and the snow density is dependent on many complex factors. Among these are are the temperature and water vapor within the dendritic growth zone in the clouds (where the temperature is -12°C to -18°C), and the depth of the dendritic zone. Dendrites are beautiful, ornate snow crystals that most people are familiar with. Additional factors affecting snow density include the shape of the snow crystals, vertical motion in the clouds, the amount of liquid water in the cloud,and the thermal profile of the layers from cloud level to the surface. The large "silver dollar" snowflakes tend to occur when the air is warmer and moisture is plentiful. What appear to be huge snowflakes are at times multiple snowflakes that have collided and stuck together. Go here for more information on snow density and the snow-to-water ratio.

Snow pellets/graupel
Rime on tree branches
Snow pellets are white, opaque grains of ice, with diameters about five times the size of snow grains. The real distinction between snow grains and pellets is that snow pellets are brittle, crunchy, and bounce upon hitting a hard surface. Sometimes they will break on impact. The are usually opaque white, as opposed to the clear ice pellets.  Also, they fall in the form of showers, from mainly cumulus congestus clouds.They typically form when snow crystals come into contact with supercooled droplets, and these droplets freeze onto the surface of the crystal. The process is similar to that which you see when water droplets in fog freeze to the outer surface of objects, forming ice crystals (riming).

Credit: Wikipedia

Snow Grains
Snow grains are very small, white, and opaque grains of ice, the solid equivalent of drizzle (or as I like to call it, "snizzle"). They are flat and elongated, with very small diameters. They fall in small quantities, and never in the form of a shower. When these grains hit the surface, they neither bounce or shatter.

Ice Pellets
Commonly called sleet, ice pellets are transparent or translucent pellets of ice which are round or irregular. Typically they are hard grains of ice consisting of frozen raindrops or largely melted and refrozen snowflakes. It often looks like it's raining, but the ice pellets often bounce when they hit a flat surface and they accumulate. Driving on a a few inches of sleet is like driving in sand or on tiny ball bearings.

All of the above precipitation types fall as frozen precipitation and are reported as "new snowfall" on the CoCoRaHS Daily Precipitation report.

Freezing Rain and Freezing Drizzle
The challenge of freezing rain.
Freezing rain or freezing drizzle occur when rain becomes supercooled and freezes on impact with objects on the surface. Typically snow falls though an elevated warm layer where the snow melts and becomes rain. The rain drops then fall through shallow layer of cold air near the surface, where they become supercooled. When the supercooled but still liquid rain drops come in contact with objects below freezing (the ground, trees, power lines, etc.) they immediately freeze. The resulting glaze of ice, if heavy enough,  can cause extensive damage and make travel difficult if not impossible.

Since freezing rain falls as liquid, it is reported the same as normal rain after melting the ice on the inside of the gauge and measuring the liquid.

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