Yellowstone's weather can vary quite a bit, even in a single day. In the summer, daytime highs can exceed 70°F (21°C), only to drop 20 or more degrees when a thunderstorm rolls through. It can snow during any month of the year, and winter lows frequently drop below 0°F (-18°C), especially at night. Bring a range of clothing options, including a warm jacket and rain gear, even in the summer.