The efficiency of a heat pump is decreased in colder climates. If it is more than moderately cold outside, supplemental and traditional heating systems may be needed to heat the structure in question efficiently.
Heat pumps are least effective in extreme climates. In extreme cold, it is very difficult to efficiently operate a heat pump to heat the interior of a building since the heat pumps do not generate heat, rather they transfer it from the ground or the air. In moderate climates, a heat pump has an additional advantage, it can be used to cool as well as heat a building.