Mist and Fog are both created by water droplets. The main difference is density. Density is the amount of water droplets in a given volume of air. Fog is denser than mist and tends to last longer. In terms of visibility (how far you can see through it), fog reduces it to less than 1 kilometer (0.6 miles) while mist can reduce visibility to between 1 and 2 kilometers (0.6 - 1.2 miles).
This will vary depending on the size of the system and the number of nozzles to be used.
The cooling efficiency of a misting system is depending on the temperature, relative humidity and atmospheric pressure. Here you can calculate the evaporative cooling for your area: Calculator
Yes. The evaporative cooling concept will work any time you are able to achieve evaporation(See "How does it work?"). The more complete the evaporation, the more effective the system will work. For areas with humidity above 80%, temperature reductions will be as high as 10 degrees. For humidity levels between 40% and 80%, temperature reductions will be as high as 20 degrees. Below 40% humidity, the temperature reductions may be as much as 35 degrees.
Water temperature does not have a great effect upon the cooling produced through evaporation. At 90º F (32º C) it takes 9,000 BTUs (2.268 kilocalories) to evaporate a gallon of 50° F (10º C) water and 8,700 BTU (2.192 kilocalories) to evaporate a gallon of 90º F. water. In this example, the water is 180% warmer and results in only a 3% reduction in the amount of heat absorbed.