METEOROLOGIST JEFF HABY
A thunderstorm is defined as convection that has at least one stroke of lightning that produces audible thunder.
You may have noticed that some thunderstorms have much more lightning and thunder than others. Why is this?
There are three factors that contribute to a storm having an exceptionally large amount of lightning and
of these will be discussed individually.
HIGH INSTABILITY RELEASE: High
instability is a condition in which the ambient tropospheric temperature decreases
rapidly with height, especially in the top lower to mid-levels of the troposphere. When instability is high,
thunderstorm updrafts will be more intense. The stronger the thunderstorm updraft, the deeper the thunderstorm
column will be. As air rises in a thunderstorm it cools. When the storm height is very high, the top of the
thunderstorm will cool to very cold temperatures. This intense cooling glaciates the top of the storm and this
can be seen as the
thunderstorm anvil. The glaciation process produces a charge differential in the storm cloud.
In cases where very rapid and intense glaciation occurs (very high CAPE), lightning and thunder will be generated
to a significant degrees. All thunderstorms have ice in the upper portions of the storm. How fast the ice
develops, the depth of the icy portion of the storm and how quickly the precipitation moves within the cloud are
important to the lightning process. There is still much research that needs to
be done to fully understand this process.
HIGH MOISTURE CONTENT: Operational meteorologists determine the potential
moisture a thunderstorm will have by
examining lower tropospheric
dewpoint and the
precipitable water (PW) in the troposphere. Since thunderstorm
updrafts often begin in the lower portions of the troposphere, it is most important to examine moisture there.
Dewpoints of 60 F or greater in the lower troposphere will bring significant moisture into a storm. Dewpoints
of 70 F or higher in the lower troposphere are not uncommon in the warm season maritime tropical environment.
Low level moisture helps
in that it increases instability. This in turn helps lead to a stronger storm updraft. An increase of moisture
also means more ice can be produced when the moisture begins to glaciate in the updraft. Charge differential
builds up more significantly as the mass of ice and water in the thunderstorm cloud increase.
WIND SHEAR: Wind shear is wind speed changing significantly with height and/or wind direction changing
significantly with height. Wind shear enables a thunderstorm to last for a longer period of time since it
helps displace the updraft from the downdraft.
These thunderstorms are often in the form of multi-cell storms
or supercell storms. Wind shear also increases turbulence within the thunderstorm. This violent mixing of
precipitation in the air could help enhance charge separation in a storm.
Typical profile for highly active lightning storm:
CAPE: 3,000 J/kg or greater, low level dewpoints greater than 65 F,
PW 2.00 inches or greater, change in wind speed and direction with height.