In spite of Spitsbergens northern location, the archipelago has a relatively mild climate compared to other areas on the same latitude.
In winter the average temperature in Longyearbyen is around -14° Celsius, though it’s not unusual for there to be long periods with temperatures between -20° and -30°. In combination with wind chill, the effective temperature can be extremely low. The weather on Svalbard changes quickly, and local variations can be huge, so it is better to have a few too many clothes with you just in case.
July is the warmest month of the year, with average temperature of a balmy +6° Celsius. During the summer months, temperatures stay above zero around the fjords and near the coastline. Days above +10 degrees are somewhat rare, as are days below +2 degrees. Days suitable for wearing T-shirts and shorts in Spitsbergen are about as rare as snowfall in Dubai. Therefore, you should bring with you a variety of warm clothes and leave the lightest summer layers back home. Indoor temperatures in Longyearbyen, on the other hand, are pretty high in all buildings, so you might find good use for a T-shirt or light long-sleeved top while in town.