1. Isentropic coordinate

2. Advection of Moisture on an Isentropic Surface
Moist air from low levels on the left (south) is transported upward and to
the right (north) along the isentropic surface. However, in pressure coordinates water vapor appears on the constant pressure surface labeled p in the absence of advection along the pressure surface --it appears to come from nowhere as it emerges from another pressure surface. (adapted from
Bluestein, vol. I, 1992, p. 23)

3. Relative
Humidity
305K surface
12 UTC
RH>80% = green
Pressure analysis
305K surface
12 UTC
Benjamin et al.

4. Relative
Humidity
at 500 hPa
RH > 70%
=green

5. Isentropes near Frontal Zones