Richard Hammond investigates the crucial role water plays, in all its forms. He tries to weigh a cloud, finds out how rain could crush a car and gets involved in starting an avalanche. Along the way, he tries to find out why clouds float by building his own cloud. He also drops in on renowned hail scientist Charles Knight in his lab in Boulder, Colorado, to discover that there is far more to hail than meets the eye. Richard joins the WSL Institute for Snow and Avalanche Research SLF as he starts an avalanche in an attempt to find out how something as delicate and fragile as a snowflake can travel at extraordinary speeds of up to 250mph on the ground.