The fluffy, white clouds you see on fine, summer days are called ‘cumulus’ clouds. When these rise up to form dark ‘cumulo-nimbus’ thunderclouds, there is a good chance of a heavy shower or a thunderstorm.
‘Cirrus’ clouds are the thin, wispy clouds you sometimes see very high in the sky on dry days. They usually mean a front is arriving, so you can expect the weather to change. As a front moves nearer, the cloud gets lower, turning into ‘alto-stratus’ or small ‘alto-cumulus’. Tall heaps of alto-cumulus means there may be a storm. Lower down, grey ‘nimbo-stratus’ clouds often cause continuous rain. ‘Stratus’ clouds form as a low sheet and often cause fog.