The mean global surface temperature of a planet in a circular orbit is given by (adapted from Qiu et al., 2003):
where G = greenhouse effect (W/m^{2}) or greenhouse forcing, and T_{eq} = equilibrium temperature (K). Both A and g are numbers between 0 and 1 that are necessary to understand the temperature of planets. They do not only depend on the surface and atmospheric properties of the planet but also on the surface temperature. For example, Raval & Ramanathan (1989) determined the terrestrial g for clearskies globally, but for a particular month (April 1985), as:
where T_{s} = sea surface temperature (SST), and only valid for temperatures between 275 K to 300 K. Eq. 1 can be easily extended to elliptical orbits assuming that both A and g are nearly constants as function of eccentricity (i.e. constant with orbital changes of T_{s}). Table 1 show some approximate values of A and g for Venus, Earth, and Mars. Table 1. Necessary data to calculate the surface temperature of Venus, Earth, and Mars from Eq. 1. Solar luminosity L = 1.0 and f = 1. This solution can be extended to exoplanets given appropriate estimates of A and g.
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