Answer:
In order to determine the relative humidity from dry-bulb and wet-bulb thermometer readings, we need to use a psychrometric chart or a calculation based on the difference between the two temperatures.
In this case, the dry-bulb temperature and the wet-bulb temperature are the same (66°F). This indicates that the air is fully saturated with water vapor, meaning that the relative humidity is 100%.
When the dry-bulb and wet-bulb temperatures are the same, the air is said to be at its dew point temperature. At this temperature, the air is holding as much moisture as it can, and any additional cooling will cause condensation to occur. This is why surfaces can become covered in dew when the temperature drops at night.
In summary, when the dry-bulb temperature and the wet-bulb temperature are the same, the air is fully saturated with water vapor and the relative humidity is 100%.