If it snows, is it safe to drink the water when it melts? What sickness can I get from snow? I am in Europe and would like to know for here, but other places would be interesting too.
If it is clean, fresh snow, it is safe to drink. This is basically drinking rain water. It hasn't had time to pick up pollutants when it is newly fallen. I live in New England, and kids do this all the time. You get taught early to only do this with white snow.
Make sure that the snow is actually clean: the longer it sits, and the more urbanized an area is, the more likely it is to be dirty and to have picked up pollutants. If it's brown or gray snow beside a road or path, or yellow snow where an animal urinated, it's no longer drinkable.
As far as what sickness you can get, it will depend on what's in the snow. With yellow snow, I'd assume you could get the diseases you'd usually get from drinking water with animal waste. If it's beside a road, you'd be ingesting heavy metals and chemicals found in car exhaust.
Edit: In response to the comments as to the general safety of drinking rainwater,
http://www.harvesth2o.com/rainwater_safe.shtml#.Vo7e0pXluAY - This is from a person who tested their own cistern (a holder for rainwater), and includes no information on the cistern's cleanliness, or whether it is open to the air and debris. Even so, the water was purer than well water or tap water the tester checked in New Mexico. The only significant pollutant was bacteria, which tend to grow when you store water.
http://www.who.int/water_sanitation_health/gdwqrevision/rainwater.pdf - In the first sentence WHO simply assumes rainwater is drinkable straight from the sky, and the rest of the article is about storing and purifying after storage. If contamination directly from the sky was a major concern, it is reasonable to expect WHO to mention it when discussing rainwater harvesting.
And anecdotally, when I've lived places where drinking rainwater was the norm, the only concern was bacteria from the cistern, not airborn pollutants.
From a water purity point of view the same rules apply for drinking water. What is important though is the temperature. If you drink large amounts of snow at a low temperature (i.e. you don't heat it adequately and just drink it as it melts in your hand) then you may need to be careful.
Basically if the temperature is low enough and you're ingesting large quantities of really cold material, it may be enough to tip your metabolism into hypothermia. Typically you should melt the snow (using a stove or fire) before drinking.
As already explained in the other questions, the primary concern is possible contamination. For fresh snow and far from civilization this is very easy to identify: White is good: yellow, brown, ... not so :)
Close to roads/industry there might be a non-visible contamination but unlikely to be harmful for occasional consumption (at least in countries with enforces environmental standards). And usually this is not the place where you need to get water from snow.
A widespread argument is, that you should not drink demineralized water. Snow is very low in minerals, so this would apply. But is is not true. Minerals can be obtained with the food too. So unless you plan to drink only snow water for days without eating, you are fine. Often a WHO recommendation for adding minerals to demineralized drinking water is quoted as proof for this claim. Apparently this recommendation is to prohibit water contamination from corroded parts of the water conduit system, as demineralized water is quite corrosive.
Another argument is that it will cool you down. This is most certainly true for the mouth, the oesophagus and the stomach, which will be unpleasant and if this is badly ignored, can even cause frostbite in your face...
Russell Steen calculated the energy needed to heat 1 liter of water to body temperature as 67 dietary calories (@ab2 and wikipedia with some calculation suggest 80kCal for melting and kCal for heating, in total 117kCal - does not change the argument though). We ingest and "burn" about 2000 calories a day, so quite a bit more. So you needed to eat more than 1 liter of water worth of snow an our to match this. Of course this is no measure for whether it cools you or not, but it shows the proportions. So unless you are already almost in hypothermia this does not need to concern you too much (the coldness in your mouth and stomach will bother you first anyway).