Even a small cloud can weigh as much as four tonnes – but gravity, chemistry and temperature keep them floating in the sky.
Good for you, bad for the air?
New research is spotlighting personal care products, such as shampoos and skin lotions, as a significant source of chemicals that contribute to urban air pollution.
False-color image of ozone concentrations above Antarctica on Oct. 2, 2015.
NASA/Goddard Space Flight Center
Earth's ozone layer shields us from harmful ultraviolet radiation. Nations have been working to reduce ozone-depleting chemicals since the 1980s, but recent studies show that there is still work to do.
Flames and smoke shroud State Route 33 as a wildfire burns in Ventura, California, Dec. 5, 2017.
Daniel Dreifuss via AP
Intense wildfires in southern California are triggering air quality alerts. Health experts know surprisingly little about how inhaling smoke affects human health, especially over the long term.
Wildfire creates an orange glow in a view from a hilltop Oct. 13, 2017, in Geyserville, California.
AP Photo/Marcio Jose Sanchez
Wildfires in California have triggered a public health emergency. One threat is smoke inhalation: Some air readings have registered pollution levels comparable to bad air days in Beijing or Mumbai.