Final answer:
When the wet-bulb depression is 0°, the air is saturated (100% relative humidity), and the wet-bulb and dry-bulb temperatures are equal, indicating the dew point where condensation may occur.
Step-by-step explanation:
When the depression of the wet-bulb thermometer is 0°, it indicates that the air is saturated with water vapor. In this state, the wet-bulb and dry-bulb temperatures are identical because the evaporation of water from the wet-bulb has ceased due to the high moisture content in the air, meaning that the relative humidity is 100%. As a result, no additional moisture can be absorbed, and this is typically referred to as the dew point, where condensation can occur, forming dew or fog if the air temperature drops further. The dew point is the temperature at which the air is fully saturated and is an important concept in understanding weather patterns and the potential for condensation or freezing, as water vapor condensing out of the air releases heat, which prevents the temperature from falling further. In conditions where the dew point is below 0°C, the possibility of freezing increases, an essential consideration in agriculture and other outdoor activities.