August is the best month to visit Yellowstone National Park. You can expect just 8 days of rain during the month, taking an umbrella is not necessary. There are some people, who may feel a slight chill at 21.9 °C (71.4 °F), but others will feel perfectly fine. The night temperature of 2.4 °C (36.3 °F) is not inviting for evening and night activities except for visiting a bar and having beer or two.