Cool by Day, Warm by Night

February 08, 2016

: David K. Lynch and Paul M. Adams
Summary Authors: David K. Lynch and Paul M. AdamsFebruary 2016 Viewer's Choice

We all know that shade is cooler than sunny areas. But did you know that the same shady areas are warmer at night than open areas?

Sunlight heats the Earth’s surface but in the shade it stays cooler. At night the situation is reverse. Shaded areas remain warmer than those exposed to the sky. The surface cools at night because energy from sunlight is radiated out to space in the 10-micron (wavelength) region of the spectrum. Caves cannot radiate to the sky because they're surrounded by rock. As a result, they stay warmer at night.

This situation is illustrated here. The upper photo was taken during the day and the lower one was taken that night with an infrared camera operating in the 10-micron region. Infrared images measure temperature. The south facing openings in the rock remain in shade nearly all day and so are cooler than areas directly illuminated by the Sun. At night, however, the caves retain much of their daytime heat and stay warmer than the exposed surface rock. Photos taken in December 2013.