Final answer:
To determine the dew point, we need to know the current air temperature and use it along with the provided relative humidity. However, the question lacks the specific temperature needed for the calculation, making it impossible to provide an answer. Typically, a reference table or a psychrometric chart is required to find the saturation vapor densities at different temperatures.
Step-by-step explanation:
The question is asking about the dew point, which is the temperature at which air reaches 100% relative humidity, given a certain temperature and relative humidity. The dew point can be determined using a psychrometric chart or equations that relate air temperature, relative humidity, and dew point.
Unfortunately, we are missing essential information in the question to accurately calculate the dew point since the current temperature value was replaced with a question mark. Normally, the calculation would involve knowing the current temperature and the relative humidity to find the dew point.
Additionally, the provided examples show how to calculate the dew point from specific scenarios by comparing vapor density to saturation vapor density. However, without the necessary data or a specific formula, we cannot perform the calculation for the given 39.0% relative humidity. To find the dew point, we would need the current air temperature and a reference such as Table 13.5 mentioned in the examples to find the saturation vapor densities.