Every psychometric chart includes vertical lines that represent the dry bulb temperatures, diagonals line represents wet bulb temperature and curved lines represent relative humidity.
Dry Bulb Temperature: Actual temperature of gas or mixture of gases.
Wet Bulb temperature: Temperature obtained by an accurate thermometer having a wick moistened with distilled water
Dew point temperature: Temperature at which the liquid droplets just appear when the moist air is cooled continuously.
Relative humidity along the saturation line is 100%.
Following observations can be made from the psychometric chart
If relative humidity has not to change with the temperature then its specific humidity will not remain constant.
If air is saturated then its relative humidity is 100% that means it can't absorb more moisture, therefore if it will be made to pass over the water it will not absorb the moisture consequently its temperature will not change.
If air is saturated then its dry bulb temperature, wet bulb temperature and dew point temperature all are the same.
The relative humidity of the air decreases when there is sensible heating and increase when there is sensible cooling.