Final answer:
To increase relative humidity, you can lower the temperature of the air, which reduces its capacity to hold water vapor, or you can add more water vapor to the air. Both approaches result in a higher percentage of relative humidity.
Step-by-step explanation:
The two ways of increasing relative humidity are by lowering the temperature of the air and increasing the amount of water vapor in the air.
When the temperature is lowered, the air can hold less water vapor, so the existing moisture constitutes a higher fraction of the maximum, thus increasing relative humidity. On the other hand, by increasing the amount of water vapor directly, you also increase the humidity because you are moving closer to the saturation point where the air can hold no more water vapor and relative humidity reaches 100%.
Relative humidity is an important concept because it affects both the rate of evaporation and our perception of comfort. In the evenings, as the air temperature declines, relative humidity often rises since cool air holds less moisture than warm air before reaching saturation. This is the same reason why hot air can help dry items faster; the warm air can contain more water vapor, promoting evaporation.