Discussion
An unsaturated moist air parcel undergoes adiabatic ascent in 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?
Answer: A
The correct plot is P.
As the parcel rises its total pressure falls, and since no water is added or removed its mixing ratio stays fixed, so its vapour pressure falls in proportion to the total pressure: slowly, by roughly 10% per kilometre. The saturation vapour pressure depends only on temperature, and the parcel cools at the dry adiabatic rate of about 9.8 °C per km, so it falls much faster. With no nuclei for condensation, the two curves simply cross and the parcel becomes supersaturated above the crossing point. Plot P shows exactly this: a gently decreasing solid line crossed by a steeply decreasing dashed line.
In Q the vapour pressure falls much faster than the saturation value, in R the saturation vapour pressure increases with height, and in S the vapour pressure stays constant although the pressure decreases.
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