When reading the storm predicition center's day 1 convective outlook, what does "2500 j/kg" mean...

Question

Again, on NOAA's National Weather Service site, they provide a day 1 or 2 convective outlook and often refer to " j/kg " , "steep lapse rates" or " mlcapes" Can anyone define these terms and preferably in laymens terms. Thank you.

Answer 1

2500 j/kg is the amount of energy in joules per every kilogram of air. This energy comes from heat and moisture. Two common types of CAPE are SBCAPE and MLCAPE. SBCAPE is surface based CAPE. This means that it is the energy a parcel of air has when it originates at the surface. MLCAPE is mean layer CAPE which is the average CAPE for the lowest 100 millibars of the atmosphere. 2500j/kg is a strong CAPE and can support strong to severe thunderstorms. This is because the energy in the parcel of air results in more buoyancy and so updrafts are stronger. Strong updrafts support large hail, heavy rain, and just a generally stronger/bigger thunderstorm. Add wind shear to this situation and you will get supercellular thunderstorms which can support even larger hail and spawn tornadoes.

The lapse rate is the cooling of air with height due to decreasing pressure. A steep lapse rate means that the ambient temperature is decreasing quickly with height which supports thunderstorms. This is the called the environment lapse rate. There are also the lapse rates of the rising parcels of air. Those lapse rates are adiabatic becuase they do not exchange heat with their environment. So for a rising parcel of air, the lapse rate is either called the dry adiabatic lapse rate or the wet adiabatic lapse rate. The latter is called wet becuase it takes into account the latent heat released by moisture in the parcel condensing once it has reached its dewpoint. The lifted index (LI) is the difference between the environmental temperature and the parcel temperature at 500 millibars. A negative number for this value indicates that the parcel is warmer than the ambient temperature and can continue rising to form thunderstorms.
This process stops at the equilibrium level (EL), which is when the parcels tempurature becomes equall to the environmental temperature.

Answer 2

These are measures of how unstable an airmass is, that is, how likely it is to make thunderstorms and how strong they'll be if they form. A steep lapse rate means that the air temperature falls faster going up in elevation than you'd normally expect, so that if you lift a volume of air from the surface, it will become proportionally warmer than its surroundings as it goes up. The steeper the lapse rate the easier it is to get storms going.

"2500 j/kg" refers to another measure of how much energy is available for forming thunderstorms--the Convective Available Potential Energy (CAPE). Bigger numbers will result in stronger updrafts in any thunderstorms that form, which will cause bigger hailstones and greater chance of tornadoes. It's determined by imagining lifting a volume of air from some layer and determining how much energy it will gain by condensing water vapor out of it. You can pick different imaginary volumes to calculate it, MLCAPE uses the "mixed layer," which is about the lowest 2000 feet of air. You might also see MUCAPE, which picks the most unstable layer of air to calculate the CAPE and so should have the biggest value. 2500 is quite a large value, and could possibly result in updrafts of well over one hundred miles per hour.

A slight correction to an earlier answerer, it tells the energy per kilogram of air, not liquid.

Answer 3

Let's oversimplify: The CAPE measures the buoyancy (tendency to rise) of a typical "parcel" of air. The energy in Joules (J) per kilogram (kg) of air is due to the "parcel" of air being warmer than its surroundings and being, therefore, able to rise a certain height because of this extra energy.

What height? Well, if the CAPE is 2500 J/kg, a kg of air would consume 2500 J of energy by rising roughly 250 m (more precisely 255 m = 2500/9.81, because 9.81 m/s^2 is the gravitational field on the surface of the Earth).

That height is a nice visualization of how unstable a mass of warm air really is; it "wants to" rise several hundred meters, like a hot air balloon would...

Answer 4

Its the CAPE value - its a measure of instability (how easily/big a thunderstorm can form). It is measured as joules (a unit of energy) / kg (of liguid suspended).

Answer 5

Just wait until they start talking about LLC, CCL, LFC, Total totals, and all the other things they expect the normal "non-weather person" to know....