METEOROLOGIST JEFF HABY
Convective instability occurs when dry mid-level air advects over very warm and moist air in the lower troposphere.
Convective instability is released when
dynamic lifting from the surface to the mid-levels produces a moist adiabatic
lapse rate of air lifted from the lower troposphere
and a dry adiabatic lapse rate from air lifted in the middle troposphere. Over time, this increases the
in the troposphere and can cause a troposphere with little or no
Surface Based CAPE to
change to one with large SBCAPE (relative to a parcel of air lifted from the surface). Dry air cools
more quickly when lifted compared to moist saturated air.
Convective instability exists when the mid-levels of the troposphere are fairly dry and high
dewpoints (and near
saturated conditions) exist in
the PBL. Water vapor imagery detects moisture in the 600 to 300 millibar
range in the troposphere. A dark color on
water vapor imagery implies a lack of moisture in the middle and upper levels of the troposphere. The surface, 850 mb,
and 700 mb charts can be used to assess the low level moisture profile. The best way to analyze convective instability
is by the use of a Skew-T diagram. A
hydrolapse (rapid decrease of dewpoint with height) will exist at the boundary
between the near saturated lower troposphere and dry mid-levels.
Below is a sounding displaying convective instability. The morning sounding shows no
significant CAPE. However, a forecaster
would expect daytime heating to increase SBCAPE. If lift also occurs in this sounding environment (from
dynamic lifting mechanisms such as
low level convergence, upper level divergence
PDVA)) then CAPE will
increase even further because the lifting will cool the mid-levels at a rate greater than the low levels.