When does the temperature on a wet-bulb thermometer equal the dry-bulb temperature?

The temperature on a wet-bulb thermometer equals the dry-bulb temperature when the air is saturated with moisture. This occurs because the wet-bulb thermometer measures the temperature of air that is influenced by the evaporative cooling effect of water. When the air is fully saturated, it cannot hold any more moisture, meaning that the liquid water will not evaporate and cool the thermometer. As a result, both thermometers will read the same temperature, which indicates 100% relative humidity.

In contrast, when there is little to no moisture in the air, the wet-bulb temperature will be lower than the dry-bulb temperature due to evaporation. Below freezing conditions can also yield readings where the wet-bulb thermometer is indicating a lower temperature than the dry-bulb, depending on moisture levels. The notion that such a condition can never occur is incorrect, as saturation is a standard condition observed in meteorological measurements.