Todd Jopp, of Waitaki Boys High School, asks :-

How are thunderstorms formed?

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

Thunderstorms are formed in cumulonimbus clouds. For a cloud to produce a thunderstorm it needs to generate electrically charged particles. If enough of these charged particles form in the same area of the cloud, the insulating capacity of the air is broken down, electricity is discharged and causes what we call lightening. The rapid expansion of the air the lightening passes through due to heating, generates a sound wave we call thunder. Sometimes different parts of the cloud generate oppositely charged particles, and the electricity goes from one side of the cloud to another. This is called cloud to cloud lightening. Sometimes the electricity goes to the ground, and is called cloud to ground lightening.

A number of different mechanisms are responsible for generating charged particles within a cloud. In New Zealand most of our thunderstorms have ice crystals in them. This is because thunderstorm clouds rise to 5 to 10 km above the earth's surface, and temperatures at these heights are typically below -20 degrees, sometimes much colder. The contact between ice crystals, liquid water, and super-cooler water (which is liquid but with its temperature below 0 degrees) can generate static electrical charges as turbulent winds blow the water droplets around. This process also forms hail, and explains why our thunderstorms often bring hail as well as lightening.