Section 1.11Figure 2.20 shows the latitude-height cross section of mean specific humidity, a measure of water vapor content in the atmosphere, in the middle and lower troposphere for January and July. The lower tropospheric water vapor in the tropics has smaller seasonal variations than that in the middle and higher latitudes. The vertical decrease of moisture content in the tropics is greater than that in the other latitudes.
The specific humidity can be taken into account in terms of the virtual temperature. The vertical stratification of virtual temperature makes the tropical troposphere more conditionally unstable than the mid-latitude troposphere. A conditionally unstable tropospheric air column exists when the temperature lapse rate is less than the dry-adiabatic lapse rate but greater than the saturation-adiabatic lapse rate. Thus an air parcel will be stable if unsaturated, and unstable if saturated, with reference to a slight vertical displacement of the air parcel.
Chapter 2