dry air

Dry air (i.e., low humidity) is of interest regarding astronomy, being an advantage for many types of observation. Water in the air is a source of opacity: some wavelength regimes include many water lines, affecting infrared and microwave observation (including millimeter and submillimeter waves). There is less water in Earth atmosphere at high altitudes, and on some mountains and plateaus, less water vapor makes atmospheric windows larger and more transparent, making observation possible that would be impossible at sea level. Some high-altitude locations of particular value for astronomical observation due to their dry air:

• Chile's Atacama Desert, many observatories at sites such as Chajnantor Plateau.
• Hawaii's Mauna Kea, Mauna Loa (High Altitude Observatory), and Haleakala (Haleakala Observatory).
• Canary Islands' La Palma island (Roque de los Muchachos Observatory), and Mount Teide (Teide Observatory).
• South Pole Station (South Pole Telescope); cold temperatures also keep air dryer.

Very-high-altitude sites can feature weather conditions that make the air especially dry, i.e., an inversion layer of the atmosphere that can dip low enough to trap the atmosphere's moisture below the observatory's altitude.

Low humidity is also an advantage of note to amateur astronomy, largely because high humidity at "ordinary" sites is associated with turbulence, affecting seeing.