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CAPE - Where the heck does it go?

Just seems like watching the models that Wisconsin/Minn/Iowa/Etc will have a high value of CAPE at a certain time. With prevailing winds you would think that it would drift towards Michigan or at least go somewhere but the high values just seem to dissapear - - - what am I missing?
Is this a day/night thing with models?


CAPE over 5000 in Wisconsin.

12hours later where did it go?
Simple answer? It didn't go anywhere. You are looking at a 00Z forecast (evening when surface temperatures are warmer) versus the 12Z forecast which is the early morning where surface temperatures are at their coolest.

The better answer to this is to truly understand what CAPE really is.


To me, your question sounds like you consider CAPE to be a physical object like a balloon (representing a large volume/parcel of air) or something like the boundary of a differential airmass such as a warm front that can be shifted as a whole by prevailing winds. I think you're close though because you can shift the atmospheric conditions that affect CAPE values.....not CAPE itself.

CAPE is all dependent on a specific location where you would take a parcel of air (think of the hot air ballon analogy) and lift it vertically through the atmosphere above that same location. That very parcel's physical characteristics such as dewpoint and temperature are very very important.

As you lift that parcel, it automatically cools due to decreasing air pressure. It will cool at a specific rate until it gets cool enough to become saturated...a cloud or fog in simpler terms. At this point, the air parcel continues rising, but at cools MUCH slower now that it is saturated. So, at what level in the atmosphere it becomes saturated depends on how much moisture it starts off with.

Since CAPE values are derived by calculating how much warmer the parcel is in relation to it's surrounding air temperature at a given level, the warmer it starts off from the lower levels will affect the CAPE values. So, that's why you will see CAPE values highest during the peak heating hours of the day and fall off at night.

What you are seeing on the models are likely where the surface airmass is more saturated (higher dewpoints) and warmer than elsewhere. Also, keep in mind what I mentioned above about the air temperature surrounding the air parcel. If colder air moves in aloft and the surface airmass has the same dewpoint and air temperature, then the CAPE will also increase.

Going back to the hot air balloon analogy seems the best way to learn about atmospheric thermodynamics. I know it helped me alot when I was first learning. What does a balloon pilot do to increase his altitude? He hits the burners and increases the air temperature inside the balloon relative to the outside air. Then, up he goes! :) Also, the greater the difference in air temperature inside and outside of the balloon, the faster the rate of ascent.

All of this is why I just love cooling mid level temperatures and increasing dewpoints at the surface. :)

I hope this helps at least a little bit. I'm not a meteorologist (as my sig line indicates), so others that are can offer more where needed.
All of this is why I just love cooling mid level temperatures and increasing dewpoints at the surface. :)

Ah yes but ya gotta keep in mind that CCA is a subsidence mechanism which could hinder convective development :wink:

But your explanation on where the CAPE 'went' was great. Not bad for a Holiday Inn Express Patron :lol:
As you lift that parcel, it automatically cools due to decreasing air pressure.

While it is true the temperature of air typically decreases with height (in the troposphere), and that temp. will decrease as pressure drops, this is only for a system with constant volume/density. In reality, the temperature deceases with height in the atmosphere due to radiational heating which occurs primarily at the surface of the earth.

Thanks for reply - I understand the day night thing.

I am an Mech engineer so I look for some type of conservation of E or at least it (CAPE) moving. Also since CAPE is related (among other things) to dewpoint (moisture) I feel it must have some physical "Property".

Looking at the following CAPE plot which goes with previous 2 plots - there is a loss of some 2000J/kg for the next day across Wis and Mi. I don't want to post temp plots for these same periods but I see no big differences at least on surface temp plots or 850mb. Also look at the big area of high Cape values over central Canada which shows up at 84hrs out. Please explain? Or look at the following wind plots - does the CAPE from Wis blow over to Canada?

Cape plot

Wind plot - does Cape blow up to Canada?

Or look at the following wind plots - does the CAPE from Wis blow over to Canada?

You can't "blow" CAPE around. CAPE is a function of thermodynamic properties (primarily the mid level lapse rates, cap, surface temperature, and boundary layer moisture). You can cause CAPE to increase in Canada due to advection of moisture into Canada (looks like this is the case in the forecast times you are mentioning).

Another thing that would cape CAPE to suddenly increase is the advection of cooler air aloft given that the surface properties are the same.


Did you visit the site that I posted?


Being a mechanical engineer, this will quickly clarify exactly what CAPE is. Heck, CAPE is a value of potential energy. :) I strongly urge you to study that link if you haven't done so.

As Aaron pointed out, you simply can't "move" or "blow" CAPE around. As I said, you can however shift the parameters that influence CAPE. I think what you are focusing on as far as "blowing" CAPE around is in fact moisture being shifted around due to differential airmasses such as ahead of a cool front. Again, CAPE values are calculated starting with a parcel's temperature and dewpoint (and this parcel can be from the surface or aloft...each will result in different levels of CAPE typically). The calculation becomes much clearer when looking at it on a "skew-t" diagram...which is shown quite well in the link above.

Here is an outstanding site to play around with a sounding and CAPE:


I started off with the FWD sounding (Fort Worth). It's a java applet. Do a right-click with your mouse at a particular altitude (I suggest as low as possible) and you can plug in the values for an air parcel including the dewpoint, temperature and even the altitude of the parcel you want to lift. I use this site alot to manually forecast a sounding to explore potential setups...realizing of course that the sounding profile will change throughout the day. :)

But, to keep moving forward, one thing of interest concerning moisture advection is what is called moisture convergence. I've seen many instances where moisture becomes concentrated and increases significantly on a local scale (mesoscale) along a boundary such as a cool front. This is called moisture convergence and is simply where the moisture starts piling up and becomes more concentrated and deeper. This is why you'll often see higher CAPE values in the vicinity of boundaries like a cool front where there is persistent convergence.

Look at your second set of maps and carefully look at the surface flow chart in northern Iowa and southern Minnesota. What do you see? I see good convergence sith SW/S winds meeting SE/E winds. That is where the CAPE values are highest very likely due to moisture convergence or "pooling" as it's often referred to.

As far as "evapotranspiration", it's a serious variable in the moisture equation. Moisture from evaporation, irrigation, vegetation, etc add more moisture content to the air. The greener the vegetation, the more moisture added to the air. If you look at the corn belt regions, especially Iowa over the past few weeks, you'll see outrageous dewpoint readings hovering around 80...due in large part to evapotranspiration. Compare this to Texas right now where even with persitent flow off of the GOM, we barely manage the 70's with lots of 60's because of the drought conditions we are in.
Ah yes but ya gotta keep in mind that CCA is a subsidence mechanism which could hinder convective development :wink:

But your explanation on where the CAPE 'went' was great. Not bad for a Holiday Inn Express Patron :lol:

Ah yes....it's all in the timing. :) I'm glad I didn't botch up the CAPE explanation too bad....or I'd have to change my sig line from Holiday Inn Express to Motel 6....without the light on. LOL!! :)
Tempting to think that CAPE advects

Looking at the charts above, it would appear that CAPE itself does move. It doesn't.

CAPE is a function of temperature, dewpoint and "height" (more accurately, pressure). As a rule, as dT/dz falls faster (that is, as air temperature falls more rapidly with height), CAPE increases. As surface dewpoint rises, CAPE increases. So if south winds at low levels waft the warmer and moister air over Iowa and Minnesota northward, one can expect the CAPE to increase over Manitoba.

It probably does not help comprehension that "Convective Available Potential Energy" is not itself energy (measured in joules) but a sort of energy per unit-mass (measured in J/kg, which is more evocative than denoting it as m^2/s^-2. Since it does not describe energy, it need not remain constant.