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Temperature is one of the several factors that are necessary for the terraforming of a world. In order for plants to survive, the temperature must be between 200,000 and 374,000, whereas animals and Humans need a temperature between 237,000 and 337,000 to survive. The units of temperature are in Microkelvins, or 1/1000ths of a degree Kelvin. Note that degrees Kelvin=degrees Celsius+273.15 Water is also dependant from temperature. In areas at or below 263000mK (0 Celsius) water freezes and becomes solid ice, while at or above 373000mK (100 Celsius), it's so hot that water boils, evaporates and turns into its gaseous state of water vapor, which is held by the atmosphere. Increasing temperature above the freezing point will unfreeze the water and will turn it into liquid state, and decreasing it below the boiling point will condense the vapor and make it fall on the planet as it becomes colder and colder.
There are several different ways to control Temperature. in the Research menu, you can unlock the Cooling Plant, Aerostat Platform, Solar Shade, Heating Cluster, Borehole, and Orbital Mirror in the Temperature section. However, temperature can also be increased by the Hydrogen Processor (Pressure section), as well as the Ecosimulator (Biomass section, if Biospheres are turned on), and decreased by the Comet Sling (Water section). In addition, if Biospheres are on, a gene is available that raises the temperature of the planet by 1 mK per 100% of the species. This effect does stack multiple times, if multiple copies of the gene are present in the same species.
It is also important to note that the atmosphere retains heat as well. At optimal atmospheric pressure, 4,600 mK are retained.
Earth starts the game with a constant 'climate change', which increases the temperature of the earth by 10 mK per minute.
Starting temperatures for planets:
Mercury starts at 720,000 mK
Venus starts at 736,800 mK (414,000 of which are from atmospheric pressure)
Earth starts at 287,000 degrees mK (4,600 from atmospheric pressure)
The Moon starts at 274,500 degrees mK
Mars starts at 218,027 degrees mK (28 from atmospheric pressure)