It's well known that when the outdoor temperature is below 32 degrees Fahrenheit (0 degrees Celsius), most precipitation falls as snow rather than rain. However, in certain circumstances, rain can still fall, only freezing once it hits the ground. The reason for this is that the atmosphere is not one solid temperature; it is formed of several layers, some of which may be wildly different temperatures than each other. If precipitation initially falls as snow but passes through a layer with a temperature above 32 F, it melts into rain before freezing once again when it reaches the ground. Conversely, if the precipitation begins its "life" as rain, but passes through a sub-freezing layer, it may become sleet or even snow.
Consider what happens when it's sleeting out. When it's very cold outside, precipitation is almost always seen as snow -- single, crystallized droplets of frozen water that slowly fall to the ground and, if the ground temperature is low enough, accumulate atop one another. However, if the sky's temperature warms enough, snow can sometimes melt mid-fall, resulting in freezing rain. Alternatively, if the ground's temperature is lower than the sky's, rain can sometimes freeze too quickly, resulting in heavy pellets of ice known as sleet.
Hailstones provide yet another example that displays the effects of varying atmospheric temperatures that can cause rain when it seems to be "freezing" out. Individual hailstones begin their relatively short, happy lives as ice pellets in clouds, usually during thunderstorms. The ice pellets merge with drops of water, the latter freezing when strong winds force the pellet into colder regions of the cloud. Through this process -- up and down in colder and warmer regions of the cloud, drops of water freezing and then meeting more drops of water -- hailstones get bigger, eventually hitting the ground. The longer the poor, hapless hailstone spends getting bounced around up and down in the cloud like a lottery ping-pong ball, the bigger it gets. When they finally get to come in for a landing, they can be as big as oranges or the size of kernels of corn [source: weather.com].
Ever wondered why sometimes it rains and sometimes it snows when the temperature is below freezing? (iStockPhoto)
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