PCA has a great book called "Design and Control of Cement Mixtures". It is easy reading and has pictures!
It helps us mechanical types enter the civil engineering world.
Now to answer your question. From what I know it will depend. Adding water to your mixture will weaken it. If the concrete freezes while it is going through its setting phase, the water could expand and break setting concrete. As the chemical reaction of the concrete is exothermic, a small amount of moisture and cold will not hurt. You will have to be more specific with your application, the temperature, etc. to give you a better answer. Also, I am a mech engineer that occasionally dables with concrete so I am probably not the best person to answer you question.
If you are pouring concrete when snow could fall, i.e. air temperature is below freezing, you should at a minimum be covering your concrete with insulating blankets. This will help keep the concrete warm, i.e. speed set, help protect the concrete from freezing, and keep the snow off the concrete.
dwedel, was right, if you want more/better information, post more detail.
If you are placing concrete and weather reports indicate snow is coming or if the temperatures at at or below 32F, you need to protect the concrete from freezing. The water in the concrete mix will freeze and thaw and expand and cause cracking and disruption of the cement aggregate paste. Not a good idea.
Snow on a fresh, plastic slab may cause a dilution of the water/cement ratio. This will lower compressive strength and make the concrete less dense and more permeable. Strong durable concrete need to be cured and protected from freezing.
Fred J. Croen, RSM
Engius, LLC
Boston, MA
www: engius.com
