Clouds occur in three basic shapes: puffy (cumulus), layered (stratus) and wispy (cirrus). They are also identified by the altitudes at which they form. Clouds below two kilometers (about one mile) are considered low. Those from two to six kilometers (about one to four miles) are considered mid-level, while those above that altitude are considered high. And finally, the prefix “nimbo-“ and postfix “-nimbus” signify clouds from which precipitation is falling.

Noctilucent clouds form when ice crystals begin to cling to dust and particles high in the atmosphere, forming electric-blue, rippled clouds that stretch across the sky at sunset. Noctilucent or “night-shining” clouds are a delight for high-latitude sky watchers, and their season is eagerly anticipated around the Arctic Circle.

We admit that this is a tricky one, but that’s part of what makes aerosols so hard to pin down in climate models. Various kinds of aerosols have various kinds of effects, both direct and indirect. As a direct effect, aerosols scatter sunlight back into space. As an indirect effect, they can either help or hinder the development of clouds, which in turn can either cool or warm the planet.

Aerosols can be found in the air over oceans, deserts, mountains, forests, ice, and every ecosystem in between. They can be found drifting in Earth’s atmosphere from the stratosphere to the troposhere to the Earth's surface.

A given volume of pollution-rich cloud tends to have more numerous and smaller droplets than in more pristine clouds. The high number of small droplets provides more surfaces to reflect light, making the cloud appear brighter than a cloud consisting of fewer and larger droplets.

Different kinds of clouds at different altitudes play different roles in regulating Earth’s temperature. Wispy clouds at high altitudes tend to trap infrared radiation emitted by the sun-warmed Earth, which prevents it from escaping into space and warms the atmosphere. On the other hand, thick, low-lying clouds tend to cool the planet by shading Earth’s surface, reflecting sunlight back into space. Scientists are trying to determine how global warming will affect the balance we’ve been enjoying up to now.

Aerosols range in size from smaller than the smallest virus to about the diameter of a human hair and can remain aloft anywhere from a few days to years.

Most occur naturally, including sea salt, desert dust, wildfire smoke and sulfates from volcanic eruptions. A significant minority is from human causes such as industrial pollution, cars and deliberate burning of trees and agricultural waste. But the artificial stuff can dominate the air downwind of urban and industrial areas.

Low, thicker clouds mostly reflect the Sun’s heat. This cools Earth’s surface. Clouds high up in the atmosphere have the opposite effect: They tend to warm Earth more than they cool. High, thin clouds trap some of the Sun’s heat. This warms Earth’s surface.