If you’re looking forward to lots of outings with your partner, take a look at these details about year-round temperatures in Inverarnan: The hottest months are usually July and August, with an average temperature of 12°C, while the coldest months are January and February, with an average of 2°C. The rainiest months in Inverarnan are October, December, November and January, with each month seeing an average of 216 mm of rainfall.