The temperature difference experienced on a cloudy summer day compared to a clear one is explained by the physics of atmospheric energy transfer. This phenomenon relates to how solar energy interacts with the Earth’s surface and the atmosphere. Understanding the cooling effect of clouds involves recognizing two distinct energy forms: incoming radiation from the sun and outgoing heat from the Earth. The balance between these two energy flows—the Earth’s radiation budget—determines the temperature we feel on the surface.
The Mechanism of Clear-Sky Summer Heating
High summer temperatures under clear skies are established through a specific energy transfer process. The sun sends energy to Earth primarily as shortwave radiation, which includes visible and ultraviolet light. This shortwave energy passes easily through a clear atmosphere and is absorbed by the planet’s surface, such as land and oceans. Darker surfaces, like asphalt, absorb a high amount of this incoming solar energy.
Once the ground absorbs the shortwave radiation, its temperature increases significantly. The warmed surface releases this absorbed energy as lower-energy longwave radiation, which is commonly known as infrared radiation or heat. This terrestrial longwave radiation warms the air layer immediately above the ground, creating the high daytime air temperatures characteristic of summer.
How Clouds Reduce Incoming Solar Energy
Clouds are composed of billions of tiny water droplets or ice crystals that act as highly effective reflectors of sunlight. When intense shortwave radiation encounters a thick cloud layer, a significant portion is scattered and reflected back into space. This property is quantified by the cloud’s albedo, which measures its reflectivity.
Thick, low-level clouds, such as stratus, have a high albedo and can reflect up to 90% of the incoming solar radiation. Even thin clouds can reflect between 30% and 50% of this energy, preventing it from reaching the surface. Because less shortwave radiation is absorbed by the ground, the surface temperature does not increase as much as it would on a clear day.
The ground then re-radiates less longwave heat energy back into the atmosphere. This reduction in the primary heat source for the air near the surface is the direct cause of the cooler temperatures experienced on a cloudy summer day.
The Dual Role of Clouds: Blocking Versus Trapping
Clouds have two opposing effects on the Earth’s energy budget: blocking incoming solar energy and trapping outgoing heat. The daytime cooling effect results from the clouds’ high albedo, where the reflection of shortwave radiation dominates the thermal balance. This blocking prevents the surface from absorbing the energy needed to reach high summer temperatures.
Conversely, clouds absorb the longwave radiation emitted by the Earth’s surface, similar to greenhouse gases. The clouds then re-radiate some of this infrared energy back toward the ground, acting as a thermal blanket and slowing the rate of heat loss. This is the trapping effect, which is a warming influence.
During the day, the magnitude of incoming shortwave energy is so great that the cooling effect from reflection far outweighs the warming effect from trapping. This is especially true for low, thick clouds, which efficiently block sunlight. At night, the blocking effect disappears because there is no incoming shortwave radiation. The trapping mechanism remains, however, as the cloud layer continues to absorb and radiate longwave energy back toward the surface. This is why cloudy nights are often warmer than clear nights.