Hey Guys,

Doing the CMET practice exam I came across this question "One of the prerequisites for the formation of a thunderstorm is -"

In the answer it states the following:

The conditions necessary for the formation of a thunderstorm are:

1. Conditionally stable atmosphere.
2. High humidity.
3. A trigger mechanism to begin the initial lifting of the air.

Conditional stability exists when dry air is stable but saturated air is unstable. That is, rising air that doesn't form cloud will eventually stop rising and cause no real problems for a pilot. However, once cloud begins to form, the cloud will become unstable and rapidly grow to form a large cumulonimbus cloud.
The correct answer is: a conditionally stable atmosphere

My understanding is that you need a conditionally unstable atmosphere " 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'

Can someone explain this to me?

Conditionally stable and conditionally unstable are two ways to describe the same conditions

Hi Incognico,

I'm only beginning my PPL studies, so take my answer with a pinch of salt.

I would say that it is 'instability' that is required, and moisture is important but maybe strictly speaking not high humidity. This leave lifting mechanism which is, I think almost always required.

I have attached some info from BOM.

1. An unstable atmosphere (evidenced by a strong vertical lapse rate, i.e. temperature falls rapidly with height) which, when associated with high surface temperatures, provides a favourable environment for the strong vertical atmospheric motions that produce thunderstorms.

2. A lifting mechanism (to initiate the vertical motion) such as low level convergence of airstreams, a frontal system, orographic uplift or local differences in heating.

3. Sufficient moisture (water vapour) in the low levels of the atmosphere. As air rises it cools, causing the invisible water vapour to condense into visible water droplets that form the cloud. The condensation process releases latent heat into the atmosphere, making the rising air more buoyant, fuelling further cloud growth.

Hi Nick

You are correct and the answer given is correct. Conditional stability and conditional instability are closely related - the difference is a matter of semantics. If the air parcel is saturated the atmosphere is said to be conditionally unstable; if the air parcel is unsaturated the atmosphere is said to be conditionally stable. Not worth losing sleep over.

Refering to the graphic below:

Absolute instability occurs when the ELR is greater than dry adiabatic rate. In this situation, an air parcel will be warmer and less dense than the surrounding air and will rise due to buoyant forces. This is not common and is generally short lived.

Neutral Stability is an atmospheric condition that occurs when the ELR is equal to the dry adiabatic rate.

Conditional stability occurs when the ELR is between the dry and moist adiabatic rates.

Absolute stability occurs when the ELR is less than the moist adiabatic rate.

Conditional instability is a state of instability that depends upon whether or not the rising air is saturated - semantics.

Note: Tropical airmasses are almost always conditionally stable.

Hi Nick

You are correct and the answer given is correct. Conditional stability and conditional instability are closely related - the difference is a matter of semantics. If the air parcel is saturated the atmosphere is said to be conditionally unstable; if the air parcel is unsaturated the atmosphere is said to be conditionally stable. Not worth losing sleep over.

Refering to the graphic below:

Absolute instability occurs when the ELR is greater than dry adiabatic rate. In this situation, an air parcel will be warmer and less dense than the surrounding air and will rise due to buoyant forces. This is not common and is generally short lived.

Neutral Stability is an atmospheric condition that occurs when the ELR is equal to the dry adiabatic rate.

Conditional stability occurs when the ELR is between the dry and moist adiabatic rates.

Absolute stability occurs when the ELR is less than the moist adiabatic rate.

Conditional instability is a state of instability that depends upon whether or not the rising air is saturated - semantics.

Note: Tropical airmasses are almost always conditionally stable.

The term 'conditional stability' was once used to describe a condition where the atmosphere is stable for dry air and unstable for saturated air. In recent years, the term was changed to 'conditional instability' which is virtually the same thing. The atmosphere is unstable for saturated air and stable for dry air. It's describing the same condition. I thought I had changed 'conditional stability' for 'conditional instability', but I must have missed that one.