Richard Hammond investigates the crucial role water plays. Without water there would be almost no weather: no rain, no snow, no hail, no clouds. So Richard goes in pursuit of water 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 with the aid of a cattle trough, some humidifiers and atmospheric scientist Dr Jim McQuaid. But will their cloud float in the air like a real 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. In a scientific first, and with the help of Jim Stratton and Craig Zehrung from Purdue University, Richard sets about firing ice and hail at a board to find out which does the most damage. Finally, in conjunction with the WSL Institute for Snow and Avalanche Research SLF in Davos, Richard joins Walter Steinkogler 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.