Final answer:
Air temperatures at night rarely fall below the dew point because as the air reaches this temperature, water vapor condenses, releasing heat and preventing further temperature drops. In dry climates with low humidity, the dew-point temperature is lower, allowing for greater temperature fluctuations.
Step-by-step explanation:
When still air cools by radiating at night, it is unlikely for temperatures to fall below the dew point because the dew point is the temperature at which water vapor in the air condenses into liquid water. This condensation releases a significant amount of latent heat of vaporization, which warms the air and stabilizes the temperature. As the air temperature reaches the dew point, heat is released into the environment due to the phase change of water vapor to liquid, which prevents the temperature from dropping further. At temperatures below 0°C, known as the frost point, a similar process can lead to the formation of frost rather than dew if condensation occurs, and this process likewise releases heat.
In areas with low humidity, such as deserts, the dew-point temperature is low, making condensation less likely. Thus, without the release of heat from condensation, air temperatures can drop more quickly at night. Conversely, high humidity levels lead to higher dew points and thus a smaller range of temperature change due to the frequent heat exchange associated with condensation and evaporation processes.