Glaciers are formed as a result of having more snowfall than snowmelt. In areas that receive regular snowfall, the length of the winter and summer are important factors for glacial activity for those very reasons. If you are going to have a glacier form at all, there must be residual snow and ice left from a winter season. Glaciers form as a result of layer after layer after layer of snow pushing down and compacting into ice. It would make sense that a longer cold spell, such as is produced in wintertime, would be ideal to preserve the accumulation of all this snow. On the flipside of the equation, it is equally important to have shorter summers, or, at the very least, cooler summers, so as not to melt the accumulated ice away. The Earth is currently warming slightly, so glacial ice is under attack the year round as average global temperatures continue to climb.