Dave, Norfolk asked:
Is rainwater clean enough to drink? After rainfall, my car seems to be covered in a layer of dirt, so is this dirt in the rain?
It depends on where you are. Everyone assumes that clouds are sterile, but scientists have recently discovered that clouds contain a species of bacteria called Pseudomonas. These bacteria live in the air and seem to use clouds as a way of transporting themselves. Itís able to do this because it has a way of causing ice nucleation Ė Itís got certain chemicals on itís surface that makes tiny ice crystals form, and this makes the cloud form ice crystals around the bacterium. This makes it heavier, and so it flutters down to earth Ė using the clouds and winds as a transport mechanism. These bacteria donít seem to cause any harm to humans though.
It's also interesting to note that some of the dust that rain deposits on your car has come all the way from the Sahara desert. Dust from the Sahara gets blown high into the atmosphere and is distributed accross the globe.