There are good conditions for visiting in Highlands in September. There are only 10 days of rain in the whole month, so you can leave your umbrella at home. There are some people, who may feel a slight chill at 23 °C (73 °F), but others will feel perfectly fine. The temperature of 11 °C (52 °F) at night may make it better to dine inside or to put on more clothes for a dinner outside.