Met office http://www.metoffice.gov.uk/education/primary/students/index.html http://www.metoffice.gov.uk/education/secondary/students/thunderstorms.html#light http://www.metoffice.gov.uk/education/primary/students/experiments.html
BBC: Useful background on all things weather http://www.bbc.co.uk/weather/features/understanding/
BBC: Cloud types http://www.bbc.co.uk/weather/features/weatherbasics/cloud_types.shtml