July 2000
in the world. In the United States most of
thethunderstormactivitytakesplacein
the spring and summer months. Although
possible in the early morning hours, most
of these summer bashers take place in the
late afternoon and earlyevening.
Ingredients for thunderstorms are un-
stable air, an initial updraft, and high mois-
ture content. Yet,withthesesamecondi-
tions present, there can also be fairweather.
Oftenballoonpilotshavetakenoff
for an afternoon flight only to be surprised
by a rapidly developing cell. It’s atthose
timestheywishthattheywereonthe
ground. One sourceto help you determine
thelikelihoodofthunderstormsisthe
stability chart.
The stabilitychart is compiled from
observations taken by radiosondeballoons
atreportingstationsaroundthecountry.
Raw data is gathered by means of a radio-
sonde instrument carried aloft by balloon.
Pressure, temperature, anddew point are
evaluatedfromthe radiosonde transmis-
sions.This information aboutthephysi-
calpropertiesoftheatmosphereovera
particulargeographiclocationatapar-
ticular time is then transcribed to an atmo-
sphericsoundingdiagram.Itcreatesa
displaywhichincludesstability,cloud
layer, freezinglevel, potential icinglev-
els, andthe temperature at any altitude.
The stability chart is composed ofthe
LiftedandKindexes.Datagatheredis
usedtoplottheLiftedIndex(seebox
nearby for these computations). Air with
aLiftedIndexabove+4isconsidered
stable; values less than 0 indicate unstable
air; and values between 0 and +4 indicate
The K Indexmeasures themoisture
contentoftheair.Theindexwillrange
from less than -10 to more than +40. High
readings indicate wet conditions andlow
readings indicate dryconditions.
The stability indexlookslike a frac-
tion—the Lifted Index is on top and the K
ingweatheryouwantahighnumerator
andlow denominator.
The ingredients necessary for a thun-
derstormare moisture, lifting, and insta-
bility. Thestability chart allows you to see
if these conditions are present. Look at the
figures showing the stability chart and the
K Index
The Lifted Indexis compu ted by
lifting an imaginary parcel of air from
the surface up to the 500 millibar level
(about 18,000 feet). As the air is lifted,
itcoolsbyexp ansion.Thetempera-
turethattheparcelwouldbeatthat
altitude issubtractedfromthe actual
temp erature.Th ed ifferenceisthe
LiftedIndex.
Temperatures are expressed in de-
greesCelsiusandtheresultcanbe
positive, zero, or negative. Should the
liftedair be cooler thanthe measured
air,theresultwillbe positive. Thatis
the lifted air would sink, and therefore
stable. If the lifted airparcel is warmer,
theresult would benegative. Thelifted
air would rise, and the air is considered
unstable.Ifthe resultiszero,itindi-
cates that the air lifted to 500 millibars
would be the same temperature as the
existing 500 millibar temperature. This
is neutrally stable air.
The K Index indicates the mois-
ture content. It combines moisture and
stability, but does not depend on lift-
ing. K Index is computed using the
following formula:
K = [Temperature (C) at 850 mil-
libar level minus Temperature (C) at
500 millibar level] + [Dew point at 850
millibar level minus Temperature Dew
point spread at 700 millibar level]
The Lifted Index deals with the
lapse rate and is proportional to the
mean lapse rate. A large temperature
difference shows a steep or unstable
lapse rate. Put another way, the greater
the difference, the more unstable the
air. The KIndex equation involves two
elements. The first is a measure of low-
level moisture, the second a measure of
saturation. The greater the spread, the
drier the air.
