San José State University

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Thayer Watkins
Silicon Valley
& Tornado Alley
USA

The Dominance of the Cloud Blanket Effect
of Clouds Compared to the Greenhouse Effect
in the Atmosphere

The difference in temperature for a cloudless winter night compared with one with cloud cover can be as much as 20°F. The problem investigated here is what can account for this extreme temperature difference. The argument being presented is that the effect of greenhouse gases alone cannot account for that temperature difference. It is the reflection of the thermal radiation from the Earth's surface back to the Earth.

The greenhouse effect is where molecules in the atmosphere absorb infrared radiation and radiate it in all directions. This means that that about one half is radiated downward toward Earth's surface. The term cloud blanket effect is used to denote phenomenon in which the underside of a cloud reflects back down the infrared radiation that the Earth's surface is radiating upward.

The greenhouse effect can result in at most 50 percent of the thermal radiation from the surface being returned to the surface. The amount returning due to reflection from the underside of clouds can be higher. The albedo of cumulus clouds for the visible light range can be as high as 90 percent. The fact that clouds are white means that clouds reflect equally the the different wavelengths of light in the visible range. The colors of the clouds at sunrise and sunset indicates that clouds reflect light in the red range quite effectively. When driving across the plains one can spot the location of cities at the horizon by their light on the clouds. It is quite likely that clouds reflect infrared radiation as well as they do visible light.

There is also a greenhouse effect from the water droplets and water vapor in the clouds. The task here is to establish the importance of the reflection of infrared radiation by the undersides of clouds.

On November 22, 2008 John Bryant of the WMCTV Weather Team in Memphis, Tennessee noted that the temperature at the Memphis airport under cloud cover was about 42°F and had been so for 12 hours whereas at Dyersburg, a small city near Memphis, the sky was clear and the temperature at midnight was almost twenty degrees colder.

Comparing night time temperatures is not a matter of comparing equilibria. After the sun goes down the temperature is in disequilibrium and it decreases approximately like a negative exponential function.

To establish the importantance of the reflectivity of the undersides of clouds one need only note that the temperature under cloud cover had been maintained for about 12 hours. This means that no heat energy from the surface was escaping into space. Whatever infrared radiation that was not captured by the greenhouse gases in the air below the clouds was returned by the clouds either through their reflection of the thermal radiation or the greenhouse effect of the clouds themselves. That is effectively a 100 percent reflection. The greenhouse effect can only return to the surface 50 percent whereas reflection can return about 90 percent. If the greenhouse substances of the clouds were capturing 70 percent of the thermal radiation that reached the clouds then 35 percent would be returned and the reflectivity could at most return the 30 percent not captured by the greenhouse subtances in the cloud. This would mean a return of at most 65 percent of the radiation not captured by the greenhouse gases in the air below the clouds. This is no where near the effect 100 percent return indicated by the constant temperature at the Earth's surface under the clouds. The proportions would have to be more like 90 percent of the radiation reaching the clouds is reflected and the greenhouse substances of cloud capture less than 10 percent and return less than 5 percent. This does not give 100 percent return of radiation but is in the right neighborhood.

Conclusion

Most of the effect of clouds in moderating the night temperatures is from their reflectivity of the infrared radiation from the Earth's surface.

(To be continued.)


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