Question

An unsaturated moist air parcel undergoes adiabatic ascent in the atmosphere without mixing with surrounding air. Air is so clean that there is no possibility for heterogeneous nucleation. Which one of the following plots depicts the vertical variation of water vapor pressure (shown as continuous line) and saturation water vapor pressure (shown as dotted/dashed line) of the parcel?

• A. P
• B. Q
• C. R
• D. S

Hint:

In adiabatic ascent, water vapor pressure decreases with altitude, but the saturation water vapor pressure decreases more rapidly.

Answer 1

The Correct Option is A

In the adiabatic ascent of an unsaturated moist air parcel, the water vapor pressure will decrease with height as the air parcel expands and cools. The saturation water vapor pressure (represented by the dashed line) will decrease similarly, but at a different rate compared to the water vapor pressure. The plot P best represents this scenario where the water vapor pressure decreases but the saturation pressure decreases more rapidly, which is characteristic of an unsaturated air parcel undergoing adiabatic ascent.
Thus, the correct answer is:
\[ \boxed{\text{(A) P}}. \]