Water vapor in the lower atmosphere can vary in amount from a mere trace in arid regions to about 4% by volume in moist areas.
大气低层的水汽量在干燥地区（量）甚微，而在潮湿地区按体积占4%左右。
2
Water vapor accounts for about 4% by volume in moist areas.
在气候潮湿地区，水汽按体积占(空气体积)4%左右。
3
The precipitation area has a good relationships with low level positive vorticity, water vapor, pressure energy, moist enthalpy, potential vorticity etc.