Water boils at 10°c when it placed at 27km above ground.why?

Water boils at 10°c when it placed at 27km above ground.why?
Because the pressure is much lower at that altitude.

All liquids boiling points are pressure dependent. Manipulating gas pressures (compressing the gas) is at the core of all heat pumps (air conditioners). Gaseous refrigerant is pumped to higher pressure in order to increase it’s boiling (and condensing) point above the outside air temperature so it can condense. After that the liquid refrigerant is passed through a valve into the low pressure area inside the house. At lower pressure it boils and takes on the heat from the house (latent heat).

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