In one word, changeable! The area in the Alps around Mont Blanc has its own micro-climate and during our trekking period, at Mont Blanc Treks, we have experienced all types of weather (sometimes in one day!). You need to be prepared as described in the equipment section above for possible poor weather. You also need to be prepared, with a high factor sunscreen, sunglasses and head shade for those scorching days with breath taking views.
Average valley temperatures for Chamonix at 1100m, allow for temperature drop for altitude gain as the cols at 2500m will be significantly cooler.