Discussion
If the surface temperature of the air parcel is 30 degree Celsius and the moist adiabatic lapse rate 7 degree Celsius/km, what will be the temperature of the air parcel at 4 km altitude for a lifting condensation level of 2 km ?
Answer: B
The parcel is at -4 degree Celsius at 4 km.
Below the lifting condensation level the parcel is unsaturated and cools at the dry adiabatic lapse rate, about 10 degrees Celsius per km. Above the LCL it is saturated and cools at the given moist rate of 7 degrees per km.
\[ \begin{aligned} T_{2\,km} &= 30 - 10 \times 2 = 10\ ^{\circ}C \\ T_{4\,km} &= 10 - 7 \times 2 = -4\ ^{\circ}C \end{aligned} \]
Using the moist rate all the way up gives \(30 - 28 = 2\) degrees, which is not offered; using the dry rate all the way gives -10. None of -6, 0 or 6 degrees follows from applying the two rates to their own layers, and the warmer answers ignore the faster dry cooling below the cloud base.
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