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What's falling from the sky?

Thomas Hough, Sunday May 17, 2020 - 12:19 EST

There are many forms of precipitation, some of which can easily be confused with others.

Whilst everyone knows the difference between rain and snow, some may not be familiar with differences between these and other types of precipitation such as drizzle, sleet, freezing rain, snow grains, snow pellets and hail. Which of these we experience on the ground is determined by how raindrops and snowflakes are altered by atmospheric conditions as they fall.

Most people think of rain as any liquid drop falling from the sky, however there is a distinct difference between rain and drizzle, and that is in their size. To be classed as rain, a drop must have a diameter of 0.5mm or greater, with anything under this considered drizzle. The last of the purely liquid precipitation is freezing rain. If the cold layer under a cloud is too shallow for the droplet to completely freeze before it hits the ground, it instead hits the ground as a supercooler raindrop. When this drop hits a cold surface, it almost immediately freezes. For wondering what it's called if the raindrop is smaller than 0.5mm - freezing drizzle.

Snow is first in the list of frozen precipitation and the one most people can readily identify. Snow is white or translucent ice crystals, with complex hexagonal shapes. These often join together forming snowflakes.

Less obvious to identify are snow grains and snow pellets. Both are white or opaque, however fall from different types of clouds and are different sizes. Snow grains are less than 1mm in diameter whilst snow pellets are less than 5mm in diameter.

Another form of precipitation that is probably more widely known than snow grains and snow pellets is sleet (or ice pellets). Sleet forms when snowflakes, or frozen raindrops, partially melt as they fall, before refreezing before they hit the ground.

Rounding out the last of the list is the all too familiar hail. These ice particles can come ball shaped or in lumps of varying irregular shapes and can be transparent or opaque. Hail, unlike other types of precipitation can vary in size significantly, from that of a pea up to baseball. Unlike previous types of precipitation, hail is almost always associated with a thunderstorm.

So next you feel something falling from the sky, see if you can work out exactly what it is.

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