Jump to content


This topic is now archived and is closed to further replies.


Conditional Instability Vs Convective Instability

Recommended Posts


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!

Share this post

Link to post
Share on other sites

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


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


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.


Share this post

Link to post
Share on other sites

  • Heatwave conditions possible to end May

    Some parts of the UK could qualify for heatwave conditions during the final days of May. With temperatures widely into the 20s Celsius. Read more here

    Jo Farrow
    Jo Farrow
    Latest weather updates from Netweather

    Dry, sunny and warm end to May but is there a pattern change on the horizon?

    The already very dry May will end with more dry, sunny and warm weather across all of the UK. This theme continues into the first few days of June, but there could be a change on the way later next week. Read the full update here

    Netweather forecasts
    Netweather forecasts
    Latest weather updates from Netweather
  • Recently Browsing   0 members

    No registered users viewing this page.

  • Create New...