Dry air mass contains no water vapor. Also air with low relative humidity and low dew point is theoretically dry. For example; when air with low dew point is collected from outside and is heated inside the relative humidity of the air will decrease. Air could be regarded as dry even when the outside relative humidity is high enough but dew point is low. Why do we say this? this is because, even if the air has high relative humidity outside and is brought inside a laboratory and heated the relative humidity will decrease appreciably.