Image of the Day: Lingering Night-Shining Clouds

Credit: P-M Hedén via NASA

These noctilucent, or night-shining, clouds are made of tiny ice particles. Because they're so high up — some 50 miles about Earth's surface — they're still illuminated by the Sun long after sunset.

One of the lingering uncertainties in climate science is whether the extra water vapor the atmosphere is expected to hold in a warmer world will form clouds like these, or low-altitude cumulus clouds instead. If it's the former, they'll help boost global warming by trapping extra heat beyond what's already being trapped by CO2 from fossil-fuel burning. If it's the latter, they could hold back warming by reflecting some sunlight back into space.