Terra's Big Neighbor

[radtech497]

How does albedo react to percentage cloud coverage?

The albedo of a cloudless world is lower than that of one whose disk is obscured by clouds; the amount of change is related both to the reflectance of the clouds (clouds composed of nitrogen oxides reflect less incoming sunlight than do clouds made of water vapor or ammonia) and to the degree of cloud cover.

What's the impact of atmospheric pressure on greenhouse gas? If the atmosphere was thinned to between 0.9 to 0.5 atmospheres, how much does the planet cool off?

The three gases usually considered in calculations of radiative forcing, or the greenhouse effect, are carbon dioxide, water vapor, and methane (there are, of course, other gases, but their concentrations are usually to small to have much influence on the results). These calculations require three values be known about each gas: 1) the gas' fraction of the atmosphere by volume (f), the partial pressure (p) of the gas (which requires the mean surface pressure be known), and the optical thickness factor (k) of the gas (kCO2 = 0.029, kH2O = 0.087, and kCH4 = 0.609).

As one of the required factors, changes in the partial pressures (and thus the mean atmospheric pressure) change the amount of temperature caused by the greenhouse gases. If the pressure is increased, the influence of these gases increases, while for decreases in pressure the opposite is true.

As for the reduced pressures (0.9 and 0.5 atmospheres) requested, the temperature increases caused by the greenhouse effect are 40 K and 30 K, respectively, for an Earth-like atmosphere; thinning the atmosphere does result in lower mean surface temperatures.

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