Cumulonimbus clouds are our cloud giants on earth. They are the tallest and most hazardous in the cloud family. They are towering, dense columns of moisture, that are often associated with thunderstorms, heavy rain, hail, lightning, squalls, and sometimes tornadoes.

Thunderstorms can be tricky. They need just the right ingredients, a good push to get going, and when they finally show up, they can hit one spot while an area nearby remains untouched. You might be sitting in sunshine while just a few kilometres away, your Aunt Carol is getting hammered by hail from a booming thunderstorm cloud.

So how do they form?

Most of New Zealand’s severe weather comes from large systems like lows and fronts, which can affect wide areas for long periods. But some of our most intense weather, like very heavy rain, large hail, or even damaging winds, can come from something much smaller and shorter-lived: a severe thunderstorm.