What is CAPE/LI?

[Nick L]

A few people in the convective thread were asking "how come there are no storms despite decent CAPE/LI values?", so I thought I'd write up some guides on the subject.

 

First of all, it's useful to look at what CAPE and LI actually are. CAPE stands for convective available potential energy, the clue is in the word "potential". It is the energy an air parcel would have if it were lifted some distance vertically in the atmosphere. Note the phrase "if it were lifted", so this implies we need a source of lift for this potential energy to become useful in terms of convective development.

 

LI is the lifted index, which is another measure of instability. In detail, it is the temperature difference between the atmosphere at a given level (I think it's typically the 500mb level) and an air parcel if it were lifted to that height. Or in more mathematical terms:

 

LI = Temperature of lifted air parcel - Temperature of environment at the new level

 

If an air parcel is warmer than its surroundings, then through the difference in density it is liable to rise. As we can see, this would also provide a negative value of LI. In this case the atmosphere is unstable, with more negative values indicating greater instability. Conversely, if an air parcel is cooler than its surroundings, it will be more dense than the environment, so it is unlikely to rise and may even sink. This results in a positive LI and denotes a stable atmosphere - which would suppress any convective development.

 

However, both of these quantities don't really mean much if there is no source of lift, or a "trigger" mechanism. We need to give an air parcel and initial "nudge" in the vertical to make use of the unstable atmosphere. Without this initial vertical motion then instability in the atmosphere may not be realised.

 

But what can cause the initial lift? Well, there are numerous sources which I will go through in my next guide

Edited May 10, 2014 by Nick L

[johnholmes]

I suspect you may have to use a skew-t diagram Nick at some point. For those not sure about these there is a Guide to these in the Net Wx Guides, not sure just where. If I find it I will post the link in here.

 

here it is

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

 

not sure if it has any glitches in it recently, pm me if it has please.

Edited May 21, 2014 by johnholmes

[Nick L]

I suspect you may have to use a skew-t diagram Nick at some point. For those not sure about these there is a Guide to these in the Net Wx Guides, not sure just where. If I find it I will post the link in here.

 

here it is

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

 

not sure if it has any glitches in it recently, pm me if it has please.

 

Yeah I was thinking about doing a Skew-T guide and then remembered you did one! The images are working in your initial post, but any images in subsequent posts are no longer there.

Edited May 22, 2014 by Nick L

[johnholmes]

Oh will take a look

yep seem okay in my original

anyone want any help drop me pm and I will do my best to answer for you

Edited May 22, 2014 by johnholmes