AND THE CAP ON SKEW-T
METEOROLOGIST JEFF HABY
Most Skew-T's that you see on the web will have a list of abbreviations and numbers to the right of the Skew-T
and wind identifiers. On the actual diagram on the web, there will be three sounding lines (one for the dewpoint,
one for the temperature and one for the parcel lapse rate from the surface). The parcel line is easy to pick out,
it is a smooth curve first following a dry adiabat and then after saturation following a moist adiabat. The
temperature and dewpoint soundings are not as smooth in appearance. Since dewpoint is always equal to or less
than temperature, the dewpoint sounding will ALWAYS be to the left of the temperature sounding.
Now for interpretation of some of the abbreviations and numbers to the right of the diagram. The ones we will go
over today involve positive and negative buoyancy. One value is called
CAP. The CAP is the number of
degrees C the temperature needs to warm in the
boundary layer to remove the cap. This value
tells you the strength of the
inversion in the low levels of the troposphere.
An inversion is most commonly found at the top of the planetary boundary layer or the transition zone of
differential advection. The CAP is important since it can BOTH promote
severe weather OR prevent storms from forming. If the
CAP is too strong, parcels of air in the PBL will not be able to rise above the CAP. Since the CAP is an inversion,
a strong inversion of warm air prevents PBL air from rising above the CAP since the PBL parcels become cooler than
the environment when they reach the CAP. On the other hand, the CAP can trap moisture and heat in the PBL...
this will gradually weaken the CAP... the warmer and more humid the PBL gets, the weaker the CAP will become.
Once the CAP is broken, explosive development of thunderstorms can occur. A general rule is that if the CAP
is greater than 2.0, The CAP will not be broken within the next couple of hours. Once the CAP drops below 2.0,
convection is likely. The CAP is important to study in the Plains since this is the region most vulnerable to
differential advection and
convective instability. It is important to look at forecast soundings to determine
the approximate time of when the CAP may break. Some days the CAP will be too strong and no storms develop at
all, even in the heat of the day. These days are called
"busts" and are one reason why days with a moderate or
high chance of severe storms end up with no convective activity. Again, this is primarily a Great Plains
"tornado alley" problem. The CAP is not as important in other parts of the country, but can be in certain
weather situations (especially warm season thermodynamic thunderstorms). The CAP is only important to thermodynamic
thunderstorms as opposed to elevated convection. If the CAP is weak in the morning, thunderstorms are liable to
form earlier in the day and not be as severe.
Another term you will see under CAPE is
CINH. This stands for convective inhibition. CAPE is the "positive area"
of a sounding while CINH is the "negative area" (parcel cooler than surrounding environment). CINH is the amount
of energy needed to warm the PBL in order for surface parcels of air to reach the
level of free convective. If
the CAPE is high, and the CINH is low, thunderstorms are likely. If the CAPE is high and the CINH is high, then
more afternoon heating and warm/moist air advection will be needed before parcels from the surface will be able
to reach the level of free convection. CINH can also be overcome by fronts,
and others since the air is forced in the vertical to the level of free convection. Generally, CINH values of
50 and below are low while 200 and above are high. Thermodynamic thunderstorms are unlikely as long as the CINH
remains above 200. Once the CINH drops below 50 and adequate
moisture are in place,
thunderstorms are eminent. Remember, soundings can change rapidly throughout the day, especially the morning
sounding. This is one area where forecaster experience is critical.
Forecasting how a sounding will change throughout the day requires experience and an hour by hour analysis of how
the troposphere is changing. RULE: soundings change most dramatically in the low levels of the troposphere due to
thermal and moisture advection along with daytime heating. Forecast soundings can help answer several questions
of how a sounding may change throughout the day. Although CAPE can give you an idea of
upward vertical velocities
that will be associated with thunderstorms and the overall instability of the troposphere, the CAP and CINH are
just as important to study. If the CAP is large and/or CINH is large, no amount
of CAPE will produce thunderstorms.