What is Precipitation?

You might think that precipitation sounds like a bit of a dull subject, but we're not just talking about rain and drizzle experienced here in this country - it also includes extreme weather such a hail storms and blizzards. In this film we're going to be looking at those examples of extreme weather and finding out how rain, hail, and snow are formed.

All precipitation develops in clouds, and clouds are formed when water vapor in the atmosphere cools and condenses. As the vapor condenses, it forms droplets, and if a cloud develops within or moves into a part of the atmosphere that is below freezing, then the droplets form ice crystals.

The cooling process which first forms clouds can happen in many different ways. Air may be forced to rise along a warm front, and as it cools, deep layers of clouds can develop - sometimes reaching many thousands of meters, all the way down to ground level. Air may also be force to rise as a cold front approaches. In this instance, a wedge of cold dense air pushes under the warm moist air ahead of it, lifting it. Weather fronts tend to bring generally cloudy skies with relatively long spells of rain and drizzle or sleet and snow at colder times of year. And finally, air may be forced to rise simply because it's heated by the Earth's surface, and it convects. This is what we often see on a summer's day, with cumulus clouds building up and sometimes developing into towering cumulonimbus clouds. Then, we can get very heavy rain and even hail.

Let's look first at rain and drizzle. The only real difference between rain and drizzle is the size of the droplets. Drizzle is composed of very small droplets, half a millimeter or less in diameter. The tiny droplets that fall from the cloud itself merge together or "coalesce" until the droplet size becomes large enough to fall from the cloud under its own weight. Even though drizzle is made up of small droplets, it can still produce some very poor weather. Heavy drizzle can seriously reduce visibility because the droplets are small and close together. In tropical climates where deep clouds develop in warm air, this coalescing of droplets can produce quite heavy rain, but in our latitudes, anything more serious than drizzle usually starts off as ice crystals high in the atmosphere.

When clouds build in air that's below freezing, we often get a mixture of ice crystals and super-cooled water droplets (that is, water that is just below freezing but hasn't yet turned to ice). It is this mix of ice crystals and super-cooled droplets that is really the key to understanding how most precipitation forms. The ice crystals are able to grow rapidly where there is super-cooled water nearby, because as soon as the super-cooled water makes contact with the ice crystal, it immediately freezes. Then, as the crystals drift through the cloud they connect with other ice crystals in a process called "aggregation," and large snowflakes are formed. This process of aggregation can take place in any fairly deep cloud - whether it's formed in layers along a weather front or by the convective "bubbling up" process.

If the air between the base of the cloud and the ground is no warmer than two degrees Celsius, then the snowflakes reach the ground in tact. In warmer conditions when the air beneath the cloud is above two degrees Celsius, then the snowflakes will melt and arrive on the ground as rain drops. This is the way in which almost all significant rain is produced in our latitudes. It starts as snow but melts before it reaches the ground.

The most energetic cloud types and the ones that can produce really heavy rain are driven by strong convection. As the air cools, the water vapor condenses into water droplets forming clouds. As the cloud grows up through the atmosphere, it will soon pass the freezing level, and water droplets within the cloud will freeze and form ice crystals. That is called "glaciation" - and anvil shape that is typical of cumulonimbus or thunder clouds. The fuzzy edges of an anvil cloud show that there are ice crystals present, and the cloud has grown well beyond the freezing level. These ice crystals grown as other water droplets nearby freeze onto their surface, creating a coating of ice, and small hailstones are formed. Inside the cloud, there will be rising currents of warm air and some quite severe downdrafts of cold air. Small hailstones may fall through the cloud, helping to create the down drafts. They can also be carried back up through the cloud by the rising currents of warm air. This can happen many times, with the hailstone melting and freezing and developing fresh coats of ice during its journey through the subzero part of the cloud.

If you're brave enough to venture into a hail storm, you can actually cut a hailstone in half and see how those layers of ice build up like an onion as the hailstone travels up and down within the cloud. In really severe storms, that process is repeated so often and the currents of air are so strong, that hailstones as big as golf balls can be supported. In really severe storms, hailstones as big as tennis balls have been recorded. The largest hailstone ever recorded in recent times came from Bangladesh in 1986 and measured a whopping 15 centimeters across. It weighed over a kilo, that's the same as a bag of sugar!

Next time you're out in the rain, have a think about what's going on above your head and the journey those rain drops may have made from water vapor into cloud droplets, snow, maybe even hail, before falling to earth as rain.

If you want to learn more about precipitation or anything to do with weather and climate, visit the Met Office website (www.metoffice.gov.uk).