Final answer:
To determine the mass of water that must condense due to a temperature drop from 25.0°C to 15.0°C with an initial relative humidity of 90.0%, one must use the saturation density values at both temperatures.
Step-by-step explanation:
The question pertains to the calculation of the mass of water that must condense out of the air due to a drop in temperature, and how this relates to the concepts of relative humidity and dew point.
Assuming a day with a temperature of 25.0°C and a relative humidity of 90.0%, when the temperature falls to 15.0°C, the amount of water that must condense can be calculated using the saturation densities at those temperatures. The saturation density represents the maximum amount of water vapor the air can hold at a certain temperature. When the air's temperature decreases, its capacity to hold water vapor reduces, and the excess must condense out.
For instance, if the saturation density at 25°C is greater than at 15°C, the difference in the amount of water the air can hold at these temperatures will give us the mass of water that needs to condense.
Without specific saturation densities provided, we can look up these values in a reference table and proceed with the calculation. The result will be in grams of water per cubic meter of air. This phenomenon explains why a sudden drop in temperature can lead to heavy dew or fog, as the excess water vapor condenses into liquid water droplets.