The nighttime cloud effect

About 67% of Earth’s surface is covered by clouds at any moment*. These extensive cloud layers play a major role in regulating Earth’s temperature.
Each cloud droplet forms on an aerosol particle (mostly from natural sources, such as desert dust or sea salt). Human-emitted aerosols (especially fossil-fuel combustion) create more, smaller droplets in these clouds, which can suppress rain and alter cloud amount. But until recently, it was unclear when these aerosol effects mattered most.

The climate impact of aerosols, remains highly uncertain because they interact with clouds in complex ways. Much of this uncertainty comes from how aerosols affect extensive low-level marine clouds called stratocumulus. These clouds behave very differently during the day and night, yet most observations to date have focused only on their daytime conditions.

A new analysis by Geoffrey Pugsley, Edward Gryspeerdt and Vishnu Nair shows that the decisive processes occur at night. Although stratocumulus clouds thin and break up during the day, they produce little rain then, so aerosols have limited influence on daytime behaviour. At night, the clouds thicken and drizzle more. Aerosols suppress this nighttime drizzle, helping the clouds rebuild. This stronger nighttime recovery sets the cloud’s state at sunrise and determines how much sunlight it reflects during the following day.

These insights reveal that aerosol–cloud interactions cannot be understood by studying daytime clouds alone. They highlight the need for nighttime observations and improved representation of the full daily cloud cycle in climate models, especially when assessing climate forcing and evaluating ideas such as marine cloud brightening.

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