which have greater density - dry air or humid air and why.

Dry  air has greater density  than humid air. Air consist of about 78 % nitrogen  gas and about 21 % oxygen gas .So dry air will have basically oxygen and nitrogen. In case of humid air some of this gas molecules will be replaced  by presence of water molecules.Water weighs about  18 g which is less than the molecular weight of oxygen which is 32 g and that of nitrogen which is 28 g.Thus humid air will have less density than dry air. Temperature and pressure have much more marked effects on the density of air.