What causes rain to become sleet?

Answer 1

Ice pellets (sleet) frequently occurs mixed in with freezing rain, and is made of frozen raindrops. Sleet forms in advance of a warm front in the wintertime in a narrow band, usually sandwiched between an area of snow and an area of rain or freezing rain associated with an extratropical cyclone. Rain being produced in a warm layer aloft falls into a cold air layer below. If the cold layer is deep enough, then the raindrops freeze. If the cold layer is very dry, sleet will more readily form due to evaporative cooling.

Answer 2

Freezing rain is liquid precip which hits a surface that is at or below 0 degrees C (32 degrees F). The liquid touches the colder surface and thus "freezes the rain" to become ice.

Most precip falls straight down to the ground due to gravity. In a larger storm cloud, such as a cumulonimbus, there might be updrafts or currents of air that are rising upward, perpendicular to the ground. Suspended in a cloud are supercooled water droplets which freeze together. One of these droplets gets big enough that gravity pulls it down. It then hits an updraft which is strong enough to push it back higher into the air. As it rises, it hits more supercooled water droplets, causing the precip to grow in size. Gravity starts to win again, pulling it back down hitting more droplets adding to the size. Another stronger updraft comes to push it back up, gravity down, updraft up, etc. The longer it stays suspended in air, the larger the hail grows. Eventually, gravity wins and causes softball size hail which dents your car.

Hail and sleet are very similar. Hail is larger than 3 mm in diameter, sleet is smaller than 3 mm in diameter. Most people call everything hail; if it is bigger than a pea you can call it hail, smaller than it is probably sleet.

Answer 3

Sleet doesn't actually turn into snow, despite what it might seem like on the ground. Snow crystals MUST grow within clouds under the right conditions, and only if the atmosphere from the clouds all the way down to the ground remains below freezing will you observe snow at the surface. Sleet begins as snow, but it melts when it falls into a layer of air that is above freezing. It then refreezes into sleet (pellets of ice) when it encounters a deep layer of sub-freezing air lower towards the surface. A condition which might aid in this would be a temperature inversion, where temperature increases with height.