Question

Sounding plotted on skew-T diagram

In this figure, and those that follow, the blue line between the temperature and dewpoint profiles represents the wet-bulb temperature profile.

Examine this skew-T diagram to answer the following.

How would you characterize the temperature lapse rate from the surface to 500 hPa?
a) Stable
b) Unstable
c) Neutral

The correct answer is (a) Stable. The lapse rate is less than moist adiabatic.

Are there any layers from 1000-500 hPa that exhibit POTENTIAL instability?
a) Yes, 700-500 hPa
b) Yes, 750-600 hPa
c) Yes, 1000-750 hPa
d) No, all layers are potentially stable.

The correct answer is c) Yes, the 1000-750 hPa layer. While the temperature profile exhibits a stable lapse rate, the dewpoint profile dries out rapidly with height. Consequently, the lapse rate of wet-bulb temperature exceeds that of the moist adiabat, rendering the layer potentially unstable. The lesson here is that soundings with stable temperature profiles, but which dry with height, need to be carefully examined to determine if potential or convective instability exists.
Your answer (700-500 hPa) is incorrect. Examine the lapse rate of the wet-bulb temperature and try again.
Your answer (750-600 hPa) is incorrect. Examine the lapse rate of the wet-bulb temperature and try again.
Your answer (all layers are potentially stable) is incorrect. Examine the lapse rate of the wet-bulb temperature and try again. Note the clear kink at 750 hPa separating potentially stable and potentially unstable regions.
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