Between mid/late June through September is the best time to visit the mountains in Europe as the weather is milder. Average mountain temperatures range from 65°F to 80°F but can feel like mid-90s °F when exposed to the sun at altitude.
The early months benefit the best display of wild alpine flowers, by contrast, there can be more winter snow remaining in the early weeks of summer, particularly after a harsh winter, though this varies enormously from year to year.
Be prepared. European Alpine weather can change fairly rapidly from a hot sunny day to a drenching thunderstorm. It is not unusual to have a light snowfall in the mountains during summer. The snow doesn’t last long but it will reduce the temperature significantly for a few days.
Later months are calmer. There are fewer families about from the last weeks of August as schools start in most European countries early September. The weather later summer is noticeably cooler but tends to be more stable. It’s a great time for peaceful long distance hikes.