(Ski/snowboard jargon) A contraction of the words snow-rain used to describe the precipitational form of slush. Snain is partially (though not fully) melted snow that will most frequently fall when the air temperature is between about 0.5 and 3.5 degrees Celsius (33-38 degrees Fahrenheit).
In many ways, snain can be said to embody the percieved negative aspects of both snow and rain. As is the case with rain, it is very easy to get wet from being outside in the snain (it is not dry like full-blown snow), but like snow (or rather, more like slush), snain can stick to the ground and make conditions messier, provided it is cold enough to be sustained. As a result, snain is often seen as a bane by those who would prefer snow, and a pain to those who would rather see plain old rain.
While snain is a very rare occurance in most interior climates (those with less precipitation, and with temperature ranges wide enough to put what there is cleanly in the domain of either snow or rain), it can be commonplace in substantially northern (or southern) coastal regions. Hotbeds of this cold and clammy phenomenon include: British Columbia (along the coast), the rim of the Gulf of Alaska, parts of New England and Nova Scotia, Norway, and probably some parts of Patagonia and Tierra del Fuego.
To conclude, it was about three years ago that I was up at Eaglecrest Ski Area during one of its relatively common snain-spells. While I was sitting around in the lodge drying myself off, I noticed the following line scribbled on a whiteboard: The rain in snain falls mostly on our brains.