Switzerland has a mild climate
Even though Switzerland is known for its mountains and snow, the climate is mild rather than cold. The climate is strongly influenced by the relatively nearby Atlantic ocean. Winds from the Atlantic transport moist and mild maritime air into Switzerland. The lower valleys in particular have quite a mild climate.
A varying climate
The mountains add an extra dimension to the mild Swiss climate, as the elevation is an important factor for the temperature at a specific location. Also, mountains can stop clouds, resulting in a moist cool climate on one side of a mountain, and a dry sunny climate on the other side.
And then there's the so-called "Föhn": a high speed cross-alpine wind, similar to the Chinook in the Rocky Mountains. If the wind blows from the south, it results in rain in the southern Alps, and dry warm fall winds in the northern Alps. If the Föhn occurs in winter or spring, it causes the snow to melt quicker. If the wind blows from the north (which is more rare), it results in rain in the northern Alps, and dry, relatively cool high speed fall winds in the southern Alps. The Föhn can occur all year long but is most common in winter.
Because of all this, the weather for any specific place in Switzerland, especially in the Alps, is quite hard to predict. Temperatures up to 35 °C (95 F) are no exception in July and August. Temperatures of -15°C (5 F) are no exception in winter. You'd best be prepared for anything when packing your cloths.
Unfortunately, global warming causes significant changes to the climate in Switzerland.
Snow in Switzerland
Switzerland is not covered in snow all year long, not even in winter. The higher areas, from about 1,500 m, are mostly covered in snow in winter (late December to March). Many mountain tops from 3,000 m are always covered in snow.
January and February are the best winter months to find snow even down in the valleys. There's normally lots of snow in the mountains around that time. March and April are suitable as well if you don't mind going up into the mountains to enjoy the snow. December can be good too in case of early snowfall, but it's different each year.
In warm summer months like July and August you'll only find snow on the highest peaks. Chances of snow in summer are diminishing because of global warming. Weather in the mountains is highly variable though, and changes quickly. A rare summer snow storm is still possible even in the valleys, but the snow will melt quickly.
For snow activities and winter sports, please go here.