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.

The lime scale is basically calcium carbonate, so it could be compressed together to make chalk!  Calcium carbonate is also used in paint, ceramics, as the starting ingredient for builders lime, in nappies, adhesives, fillers and mixed with putty for stained glass windows.

It’s actually quite good for your health; calcium carbonate can also be used as a calcium supplement, to help build strong bones and teeth or as an antacid.

Its down to a chemical called myoglobin.  Myoglobin is a bit like haemoglobin, the red coloured stuff that ferries iron around in the bloodstream, except myoglobin is locked up in muscle (and meat is muscle). Red meat contains a lot more myoglobin than white meat, as the muscles that tend to be red are the ones most active in an animal.  The legs of a standing animal will be redder, and have more myoglobin, as the muscle has been tuned up for long term activity.

Muscles that aren’t used as often, fast-twitch muscles, tend to have low blood supply, little myoglobin, and therefore little colour (white meat).  Chicken breast and wings don’t get a huge amount of use (as chickens don’t fly), and so they are white muscle.

With fish, most of our salmon has a red colour because we tend to buy farmed salmon, and to keep the meat looking a healthy colour the fish are fed astaxanthin.  They would get this in the wild environment from yeast and from algae, it’s an antioxidant similar to the chemical which makes carrots orange.  Shrimps eat the algae, salmon eat the shrimps and the colour passes through the food chain.

Yes, but as the temperature of the atmosphere increases, it can hold more water vapour.  This means that there will be more gaseous water in the atmosphere, but not necessarily more liquid water on the ground.