Hi everyone,

Hope to have your help on this. I got a question about Q.20 of exercise GM5, (2014 PPL p.141)
The q asks one of the prerequisite for the formation of a thunderstorm.
I thought it should be [d], but the ans is [a]. I don't quite understand why. Here's my thinking:

The reason I don't choose [a], is because even moisture is present thru considerable depth, it has to be cooled to dew point. Moisture along doesn't form TS.
However, the reason I choose [d], is a cold isothermal layer suggests an unstable condition.

Thanks,

Gidday

There are three necessary conditions for thunderstorms to develop. They are:

Low-level moisture - sufficient water vapour in the near-surface layers to allow significant buoyancy once air from this level has reached the Level of Free Convection (LFC). The LFC is the level at which a parcel of saturated air becomes warmer and therefore more buoyant than the surrounding air and begins to rise freely.

Instability - a deep layer of conditionally unstable air above the LFC which allows the parcel of saturated air to continue to rise to a considerable height, i.e. to the -20°C level and beyond.

A lifting mechanism - to lift a parcel air from near the surface to its LFC.

So (A) is correct answer

I'd like to point out that using a 2014 version of the PPL text you are missing the GAF/GPWT updates and the changes to the AIP please have a look at the Errata pages to see what has been added to the text, www.bobtait.com.au/updates-and-errata/25-ppl-errata

Cheers
Stuart

Also, an isothermal layer is an indication of extreme stability - not instability. The environmental lapse rate in an isothermal layer is zero. That indicates strong stability. Instability requires a high environmental lapse rate.