Planning for the weather is a good way to make your trip to Idrija relaxing and enjoyable, especially when you’re splurging. The hottest months are usually July and August with an average temp of 64°F, while the coldest months are January and February with an average of 36°F. The rainiest months in Idrija are September, November, May, and July, with each month seeing an average of 8 inches of rainfall.