The barometric pressure on Mauna Kea is usually in the range 600-610 millibars (approximately 18 inches of mercury) - just under 60% of the sea-level atmospheric pressure: Mauna Kea is above 40% of the atmosphere. Water boils at a temperature of approximately 86 degrees Celsius (187 degrees Fahrenheit) at the top of Mauna Kea.
The weather at the Mauna Kea summit can be severe, especially during the winter months, when heavy storms commonly deposit several feet of snow. Minimum nighttime winter temperatures at the summit are around -4 degrees Celsius (25 degrees Fahrenheit); maximum daytime temperatures are about +4 degrees Celsius (40 degrees Fahrenheit), but wind chill and the high altitude can make it seem much colder. Between April and November the weather is milder, with daytime temperatures varying from freezing to almost 15 degrees Celsius (60 degrees Fahrenheit).
The Mauna Kea weather center at University of Hawaii is currently developing specialized weather prediction tools for Mauna Kea and its immediate surroundings. These predictions help astronomers to plan their observing to make the most efficient use of each night's atmospheric conditions.