365 Days of Climate Awareness 186 – Cloud Cover

Clouds are one of the hardest components of the climate system to characterize with respect to global warming. Their effects can vary, as can the conditions which generate them. The basic principle is always the same: excess humidity in the air causes condensation into droplets or solid deposition as ice. But warm vs cold conditions have widely differing effects on how easily it happens. Likewise, clouds’ climatic effects vary from day to night.

Air at higher temperatures can hold far more vapor than cooler air. Further, that storage capacity increases far more rapidly at high temperatures than low. In the tropics air temperatures are generally 18°C/64.4°F or higher. Deep convective clouds are common. In the polar regions, generally near or well below freezing, such clouds are much less so, due to the lack of convective heat energy. But clouds do occur, even if of the high, wispy cirrus variety. In these colder regions, due to air’s relative incapacity to store moisture, even a small increase in temperature will generate more clouds, than in the tropics.

Correlation (linear regression slope) between cloud cover and 1983-2009 temperature anomalies. Note the extremely strong correlation over the ENSO region, and the varying positive and negative correlations worldwide. 

There are two main effects of clouds on climate, and both have to do with their reflectivity. First is albedo. Clouds, especially the thick, convective type, reflect large amounts of incoming solar short-wave (ultraviolet) radiation. However, clouds also reflect long-wave (infrared) radiation back to earth. These effects are opposed.

Temperature and cloud trends, 1983-2009: (a) Temperature anomaly (red = day, purple = night) versus mean; (b) Annual mean daytime (red) and nighttime (purple) global temperatures; ( c ) Cloudiness trend, 1983-2009; (d) Daytime-nighttime temp. differences plotted against cloud cover change. 

Comprehensive data on cloudiness is a recent dataset, largely from the 80’s on, so long-term trends are tentative. But a complicated picture emerges, where cloud cover has increased in some parts of the world and diminished in others. There seems to be, in tropical and temperate regions, a real correspondence between decreased cloud cover and daytime maximum temperature, where the clouds’ albedo reflecting incoming UV rays dominates. Likewise there is a correlation between increased cloud cover and maximum nighttime temperature, where the clouds’ reflection of outgoing IR radiation dominates.

Tomorrow: changing snowfall patterns.

Be brave, and be well.