Convective Available Potential
Energy
(CAPE)
One term you will hear when listening to NOAA Weather Radio, storm chasers, and
weather experts during storm season is CAPE. Here is a little about what CAPE is
and how it pertains to severe weather.
The convective available potential energy or
CAPE, is used by weather officials and storm chasers to understand what
the potential might be for severe weather, and how powerful those
storms might become if they do materialize. The potential energy available for
convection is expressed mathematically using a standard measurement of energy
represented as Joules Per Kilogram. A high CAPE value might
also be expressed by storm chasers and weather experts by using the
term "high instability". When chasers talk about a highly unstable
atmosphere, CAPE values are usually in excess of 2500 J/kg's which
would supply ample energy for strong updrafts and violent storms should
they develop.
More specifically, CAPE represents the amount of
buoyant energy available to speed up a parcel vertically, or the amount
of work a parcel does on the environment. Storms require high
CAPE values; the higher the CAPE value, the more energy available to
promote storm growth. CAPE is especially important when air parcels are
able to reach the (LFC) or Layer of Free Convection. To find CAPE from
a skew-T thermodynamic diagram like the one below, simply locate the
area on the diagram where the parcel sounding (Thick yellow vertical
line) is farther to the right (warmer) than the atmosphere sounding
(red line). The white shaded region on the sounding below is the area
of CAPE.
The white region is called the "positive energy" region. The area of
this region is CAPE and is expressed in Joules/kg values. The larger
this area, the more CAPE, and the more energy that is available to
rising parcels. A CAPE value that is more concentrated in the lower
half would produce a stronger updraft than an equal CAPE value that is
stretched higher and narrower. Also, CAPE is directly related to
the maximum potential vertical speed within an updraft. Faster updrafts
means more powerful updrafts. Powerful updrafts aid in the creation of
large hail, supercells and violent tornadoes.
The chart below can be used to gain a greater
understanding of what numerical values of CAPE indicate as it relates
to convection of air parcels. Remember, CAPE is only the potential
energy. High CAPE values cannot be used as a predictor about whether or
not severe weather is going to take place at a given time or that
severe weather is looming.
| CAPE value |
Convective
potential |
| 0 |
Stable |
| 0-1000 |
Marginally Unstable |
| 1000-2500 |
Moderately Unstable |
| 2500-3500 |
Very Unstable |
| 3500 + |
Extremely Unstable
|
It should be remembered that CAPE represents potential energy,
and will only be used should a parcel be lifted to the level of free
convection. When values are above 3500 j/kg and storms do develop, they
may build rapidly and quickly become severe. Often these storms are
referred to as "explosive storms" by chasers and professionals. In a
high CAPE environment storms that develop can usually be seen by the
human eye as rising rapidly. Chasers discussing these kind of storms
have been overheard saying these storms resemble a nuclear explosion
as in this photo, because they rise up so quickly and powerfully.
Joules Defined: Energy here is expressed in JOULEs (J). An
energy of 1 joule is expended in moving an object through 1 meter when
it is opposed by a force of 1 NEWTON. 1 Joule is equal to 1
Newton-meter. Energy is stored work, Work being the application of a
force through a distance. Power is the rate of flow of energy, or the
rate at which work is done. Stored energy becomes working energy when
it is used.
CAPE is then expressed as: Joules
J (Energy) /kg Per
Kilogram (Mass)
NOTE: To get the full picture of severe
weather potential CAPE should be used in conjunction with a full
sounding. Also remember that CAPE when used to calculate (Wmax),
is often over stated by as much as a factor of 2x.
For more details take a class on Buoyancy and CAPE
by clicking here. This class room study
requires flash player to be installed on your web browser. A teacher will
explain step by step buoyancy and CAPE. The class is free online right now.
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