WEATHER Wise

Drizzle, rain, sleet, hail, or perhaps even snow, the Met Office menu this week offers us a choice of precipitation. here is a guide to let you know what's soaking you.

Rain, in its various forms, happens when moist, warm air rises and is cooled beyond its condensation point. At that moment, tiny water droplets form to make clouds. These cloud droplets are only about a thousandth of a millimetre in diameter and are easily held aloft by the rising air.

They do not threaten to fall until they have grown into droplets about a tenth of a millimetre in diameter - which is the size of a drop of drizzle. A shower of drizzle may begin to fall within an hour or two of a cloud forming. Raindrops - which are a whole millimetre across - are far more interesting.

As cloud rises and cools below 0C, the water droplets within it do not freeze immediately. (It's to do with the problems the surrounding air has in absorbing the latent heat released when water turns to ice.) So, as the cloud gets colder, we get a mixture of super-cooled water droplets and ice crystals. Now it's all a question of what happens when the ice crystals fall.

Snow is falling ice crystals. Sleet is a mixture of ice crystals and rain (or drizzle). Rain is ice crystals that melted on the way down. Hail is raindrops refrozen as ice.