Sleet vs. Freezing Rain

A common wonder to many people is the difference between sleet and freezing rain. The two are formed very similarly, however I hope to clear up any overlap of the two you might have now. Thanks to my Aunt Kim for submitting this great question! (You may also submit a question of your own simply by scrolling to the bottom of the page and clicking on 'Leave a Response'.)

Sleet
Sleet has many different meanings around the world. For example, in Australia and the UK they refer to sleet as partially melted snow that falls from the sky. Their name for the precipitation we in the United States call sleet is put in a much simpler version, ice pellets. So how does sleet form? Well way up in the atmosphere a snowflake begins to fall from a cloud. On it's way down it may hit a layer of air that happens to be above freezing, causing the snowflake to become a partially melted snowflake or a cold raindrop. As this partially melted snowflake or cold raindrop keeps falling, it may encounter a deep layer of air next to the ground that happens to be below freezing (32 degrees Fahrenheit, 0 degrees Celsius). This causes it to turn back into ice in the shape of a tiny pellet. Hence, sleet is formed. Sleet is typically transparent and only has a diameter of about 1/5 of an inch or less.


Sleet.
Photo Courtesy: http://upload.wikimedia.org/wikipedia/commons/9/9f/Sleet_on_the_ground.jpg

A common characteristic that most people notice is its tendency to bounce when it strikes the ground. Or perhaps your more familiar with the tapping sound it makes when it comes in contact with glass or metal. If it helps you remember better, think of sleet as "frozen" rain.

Freezing Rain
The same process occurs during the formation of freezing rain as that of sleet; however, the layer of freezing air closest to the ground may be too shallow, prohibiting the raindrops to refreeze before they hit the ground. So freezing rain hits the ground as a supercooled water droplet. Supercooled essentially just means that a water droplet exists at below freezing temperatures. These supercooled liquid droplets will spread out and freeze almost immediately when it hits a cold object. This forms a thin layer of ice, which can be dangerous on roadways. Ice storms that occur are the result of freezing rain accumulating. Below are a couple of pictures I managed to snap of a tree branch and a lock during the January 2007 ice storm that struck the Houston area.




When freezing rain hits a ground that is below zero, it will freeze immediately forming something we call rime. Rime traps air between the droplets once on the ground, which helps explain why rime is white in appearance.


Rime.
Photo Courtesy: http://upload.wikimedia.org/wikipedia/commons/8/8b/Rime_ice.jpg

Airplanes can accumulate rime ice on the wing as it flies through a cloud with tiny supercooled liquid droplets, causing a redistribution of air flow. Luckily, rime is lighter in weight when compared to clear ice, making it easier to remove with de-icers. Icing is heaviest and most severe in temperatures between 14 - 32 degrees Fahrenheit, mostly because the concentration of liquid water is higher in warmer air. Airports generally prepare planes if this kind of weather is expected during the flight by spraying the planes down with antifreeze before take-off. On a side note, a substantial buildup of ice on the masts of sailing boats at sea can cause the vessel to capsize.


Sources:
Ahrens, C. Donald. "Precipitation." Meteorology Today: an Introduction to Weather, Climate, and the Environment. Belmont, CA: Brooks/Cole, CengageLearning, 2009. 180-183. Print.
Burroughs, William James., and Richard Whitaker. Weather. San Francisco, CA: Fog City, 2007. 222-223. Print.