June to August
Most of Jotunheimen lies above the treeline at 800 to 1,000 m.a.s.l. and reaches up to 2,500 m.a.s.l. In Norway, that means that summer starts quite late. It is not uncommon to be able to ski in Jotunheimen in June, but by the end of the month, you should be able to get out for some hiking. July and August are the most popular months and when you can expect to see the most people traveling around in this area. On very nice days you can get lucky with temperatures above 20°C, but it is more common to experience temperatures in the range of 10-15°C and even days that are much colder. Be sure to bring warm clothes even when you are traveling in mid-July.
September to December
Summer is short and by September you can expect much colder weather and even snow. The leaves turn a brilliant red and yellow during September making the landscape look amazing in and around the tree line. You will have less and less daylight from about 15 hours in September to about 5 hours in December. If you want to hike around in this area from mid-September you need to be prepared for the weather by bringing warm clothes and other necessary equipment.
January to May
The winter in Jotunheimen can be intense with large amounts of snow and average temperatures of -14°C, even dropping down to as low as -30°C. In most years you will be able to ski from January (or possibly earlier), but the most popular months for skiing in Jotunheimen are April and May due to more daylight, friendlier temperatures, and better skiing conditions.