Final answer:
The coldest air-mass temperatures occur in conditions of low humidity because the lack of moisture in the air allows temperatures to drop more drastically without the release of latent heat from condensation.
Step-by-step explanation:
The conditions under which air-mass temperatures are coldest are associated with low humidity environments. When the humidity is low, the dew point of the air is also low, which means that the air temperature can drop more significantly without water vapor condensing into liquid water. The absence of this condensation means the latent heat of condensation is not released into the atmosphere, allowing the air temperature to decrease more. This principle explains why deserts, which have low humidity, can experience extreme temperature drops at night. The dew point is a critical factor because it is the temperature to which air must be cooled for saturation to occur, leading to condensation if the air is sufficiently humid.