Conditional Instability Vs Convective Instability

[03jtrickey]

Hi,

I have just started the Climatology section of my Geography course.

I am struggling to understand the distinction between conditional instability and convective instability and I was wondering if anyone would be able to help me out.

As I understand it, convective instability occurs when a layer of air which is more humid at the bottom than at the top, is lifted(by something - perhaps a cold front or a mountain range?). The bottom of the layer cools more slowly (briefly at DALR, then at the slower SALR) than the top of the layer which simply cools at the DALR. The temperature gradient increases and the layer becomes unstable and may overturn.

I understand conditional instability to be when the lapse rate of an unsaturated layer of air lies between the saturated and dry adiabatic lapse rates. Therefore if a parcel of air from the layer is somehow lifted, when it reaches the lifting condensation level and becomes saturated, it will begin to cool more slowly at the SALR, and may rise above the environmental temperature (at the level of free convection) and will continue rising because it is less dense than surrounding air.

However, I am finding it hard to see the difference between the two. Both types of instability seem to require the air to be lifted in some way (it is not very clear how).

Is the difference about scale? (i.e. does convective instability occur only when a whole layer is forced up, whereas conditional instability is just a parcel being forced up?)

Or does conditional instability only occur when the environmental temperature profile is right, whereas convective instability will always occur is a layer of moist to dry air is forced up?

Another question that I had is - does convection occur with both types of instability? - it is a little confusing that one type is called convective but surely convective scenarios occur under conditional instability as well?

Sorry for all the questions... I would really appreciate it if someone could help me understand these concepts better and I hope it is possible to make sense of what I have written!

Edited February 23, 2011 by 03jtrickey

[johnholmes]

Definitions that may help

Conditional Instability

If you cause dry air to rise, its temperature will drop due to adiabatic expansion. This vertical temperature gradient is called the dry adiabatic lapse rate. When the temperature gradient becomes sharper than this (i.e. the temperature drops sharply as altitude increases), the atmosphere becomes unstable.

On the other hand, if you cause air saturated with water vapor to rise as the temperature falls the water vapor condenses and lets off heat, so the temperature gradient becomes smaller. This is called the moist adiabatic lapse rate.

In other words, saturated air becomes unstable under a smaller temperature gradient than dry air. Conditions with a vertical temperature distribution under which both dry and saturated air are stable are called absolutely stable, and conditions that cause instability for both dry and saturated air are called absolutely unstable, and conditions that cause instability in saturated air but not dry air are called conditionally unstable.

The lower layer of the atmosphere is in a state of conditional instability, so when water vapor reaches saturation point, it becomes unstable, and cumulus and cumulonimbus occur.

This link is generally quite good

http://www.theweatherprediction.com/habyhints/214/

and the N W Guides section on skew-t diagrams may also help

http://forum.netweather.tv/topic/16002-a-simple-guide-to-understanding-skew-t-diagrams/

there are still I or 2 daigrams that will not open properly. I will eventually find time to correct them. Also I intend during the summer to do an actual case of instability, each kind!

Hope the above helps.

Do pm me if you want anything explaining again.

jh

[03jtrickey]

Thanks very much for your help