Stability of the Atmosphere
K Index
The K index is a measure of thunderstorm
potential based on the vertical temperature lapse rate, and the
amount and vertical extent of low-level moisture in the atmosphere.
K = T(850 mb) + Td(850 mb) -
T(500 mb) - DD(700 mb)
in degrees C, where T represents
temperature, Td represents dewpoint temperature, and DD represents
dewpoint depression at the indicated level.
|
K below 30: |
Thunderstorms with heavy rain
or severe weather possible (see note below). |
|
K over 30: |
Better potential for thunderstorms
with heavy rain. |
|
K = 40: |
Best potential for thunderstorms
with very heavy rain. |
In general, the higher the ambient or inflow K index value, the
greater the potential for heavy rain. However, beware of low (less
than 30) values of K. Since the K index includes the dewpoint
depression (i.e., difference between the temperature and dewpoint
temperature) at 700 mb, dry air at this level will cause a low
K value. However, given moisture below 700 mb, unstable air, and
a lifting mechanism, strong or severe organized thunderstorms,
and even heavy rain, can still occur. Scattered diurnal convection
occurring in an environment containing high K (and PW) values
can cause a quick burst of very heavy rain.
Source: NWS
Other UKAWC Stability Indices:
Ag Weather Center, Department of Biosystems &
Agricultural Engineering, University of Kentucky
