The summer months of June through September are very hot, though moderated by extremely low humidity. July is the hottest month, with an average daytime high of 104. 2 °F (40. 1 °C). On average, 134 days per year reach or exceed 90 °F (32 °C), of which 74 days reach 100 °F (38 °C) and 7 days reach 110 °F (43 °C). During the peak intensity of summer, overnight lows frequently remain above 80 °F (27 °C), and occasionally above 85 °F (29 °C). While most summer days are consistently hot, dry, and cloudless, the North American Monsoon sporadically interrupts this pattern and brings more cloud cover, thunderstorms, lightning, increased humidity, and brief spells of heavy rain. The window of opportunity for the monsoon to affect Las Vegas usually falls between July and August, although this is inconsistent and varies considerably in its impact from year to year. Summer in Las Vegas is marked by a significant diurnal variation; while less extreme than other parts of the state, nighttime lows in Las Vegas are often 30 °F (16. 7 °C) or more lower than daytime highs.