S Alexander, of Waitaki Boys High School, asks :-

What kind of weather does a towering cumulonimbus cloud bring?

Tony Trewinnard, a meteorologist with Blue Skies Weather in Christchurch, responded.

In a word, the answer is a thunderstorm.

Cumulonimbus is one of ten basic cloud classifications used by meteorologists. The main types of low cloud are called `stratus', which are the type of amorphous grey layer clouds often seen on the coast in the early mornings, and `cumulus', which are the puffy white clouds often seen inland on a fine afternoon. Another category covers the clouds that are a hybrid of these two - `stratocumulus' is the name given to a layer cloud that has some vertical development within it. If rain is falling from the clouds we add the word `nimbus' to the terms to give us `nimbostratus' which is rainy layer cloud, and `cumulonimbus' which is a shower cloud.

These five cloud terms are used for low level clouds, generally with bases below about 2000m. Above that level we identify two types of middle level cloud, `altostratus' (a layer cloud, like the nor'west arch we often see in Canterbury) and `altocumulus' (middle level puffy clouds). We also have three terms for ice crystal clouds very high in the atmosphere - `cirrus' is the familiar `mares tails' cloud often seen in New Zealand skies, `cirrostratus' is a layer cloud, and `cirrocumulus' is a puffy cloud that is probably the rarest of the ten main types.

As well as these categories, there are many other descriptive terms meteorologists use to describe certain features of individual clouds. The word `towering' is used to describe cumulus and cumulonimbus clouds that are taller than they are wide. When cumulonimbus clouds reach this stage they are usually producing thunder and lightening, hence the common definition of a thunderstorm cloud - a towering cumulonimbus.