Full snow cover in the region lasts from around December to April, but the effect of the Gulf Stream takes a little of the edge off the bitter winters in Murmansk and around the Kola Peninsula's coast and even down in the Solovetsky Islands. So while these places will be several degrees cooler than St Petersburg in summer they have similar temperatures in winter. Murmansk temperatures range from -10°C to -15°C in January and February, and from 8°C to 14°C in July. Arkhangelsk, with less Gulf Stream effect, experiences temperatures between -12°C and -20°C in January.
As well as higher temperatures, summer also brings many more hours of daylight - two months without sunsets in Murmansk. While there's still fun to be had in winter, most of it has to be had by electric light!