Question

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?

Answer

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. It's able to do this because it has a way of causing ice nucleation - It's got certain chemicals on it's 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 - using the clouds and winds as a transport mechanism. These bacteria don't seem to cause any harm to humans though.

What can harm you are the other chemicals that are dissolved in the water as the rain falls down to earth. If you're isolated from pollution sources, the rain is coning from a pristine ocean and will be pretty clean. If you're in a built up area, or downwind of heavy industry, power stations etc, these things can be pumping out all sorts of chemicals - particulates, carcinogens, dioxins and even heavy metals. These particles get into clouds and encourage the clouds to form water droplets, falling as rain.

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.

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