On airless bodies, the lack of any significant greenhouse effect allows equilibrium temperatures to approach mean surface temperatures, as on Mars, where the equilibrium temperature is 210 K (−63 °C; −82 °F) and the mean surface temperature of emission is 215 K (−58 °C; −73 °F). There are large variations in surface temperature over space and time on airless or near-airless bodies like Mars, which has daily surface temperature variations of 50–60 K. Because of a relative lack of air to transport or retain heat, significant variations in temperature develop. Assuming the planet radiates as a blackbody (i.e. according to the Stefan-Boltzmann law), temperature variations propagate into emission variations, this time to the power of 4. This is significant because our understanding of planetary temperatures comes not from direct measurement of the temperatures, but from measurements of the fluxes. Consequently, in order to derive a meaningful mean surface temperature on an airless body (to compare with an equilibrium temperature), a global average surface emission flux is considered, and then an ' effective temperature of emission' that would produce such a flux is calculated. The same process would be necessary when considering the surface temperature of the Moon, which has an equilibrium temperature of 271 K (−2 °C; 28 °F), but can have temperatures of 373 K (100 °C; 212 °F) in the daytime and 100 K (−173 °C; −280 °F) at night. Again, these temperature variations result from poor heat transport and retention in the absence of an atmosphere.