If you are studying for your BAK or PPL exams and need some help, please post your question here. Someone on the forum is bound to help you as soon as they can.
Prerequisites for the formation of a thunderstorm (TS) [PPL 2014, Ex GM5, q.4]
plpl747 created the topic: Prerequisites for the formation of a thunderstorm (TS) [PPL 2014, Ex GM5, q.4]
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.
Stuart Tait replied the topic: Prerequisites for the formation of a thunderstorm (TS) [PPL 2014, Ex GM5, q.4]
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.
bobtait replied the topic: Prerequisites for the formation of a thunderstorm (TS) [PPL 2014, Ex GM5, q.4]
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.