Planning for the weather is an important factor for making your trip to Untergrainau a success, especially when you’re splurging. The hottest months are usually July and August with an average temp of 58°F, while the coldest months are January and February with an average of 29°F. The rainiest months in Untergrainau are June, August, May, and July, with each month seeing an average of 9 inches of rainfall.