The KI (K INDEX) is an index used to assess convective potential.
2. How is KI determined?
The KI is a combination of the Vertical Totals (VT) and lower tropospheric moisture characteristics. The VT
is the temperature difference between 850 and 500 mb while the moisture parameters are the 850 mb dewpoint and
700 mb dewpoint depression.
KI = (T850 - T500) + (Td850 - Tdd700)
The equation above for the KI is stating (850 mb temperature minus 500 mb temperature) plus (850 mb dewpoint
minus 700 mb dewpoint depression)
The Skew-T below gives a KI value of 30 for that sounding. Here is how that number was determined:
850 mb Temperature = 20 C
500 mb Temperature = -7 C
850 mb Dewpoint = 9 C
700 mb Dewpoint Depression = 6 units of C
KI = (20 - (-7)) + (9 - 6) = 27 + 3 = 30
3. Operational significance of KI:
Small convective potential
Moderate convective potential
High convective potential
A high 850 mb dewpoint and a low 700 mb
dewpoint depression at the same time indicates there is a deep layer
of warm and moist air in the lower to middle troposphere. This is very beneficial to producing
instability, especially when the VT is high.
b. Index should not be used to determine severity of storms.
c. VT may be very high and contributes to causing a high KI even when
moisture is lacking. Index will be
unrealistically too unstable in these situations.
d. Works best for flat areas in low to moderate elevations. Does not work for high elevations.
e. Index value interpretation varies with season and location.
f. 700 mb dewpoint depression may be very large and thus will lead to a stable KI. Very dry air at 700 mb
will not degrade the convective potential as long as there is moisture below this layer. KI is better to
use when forecasting within a deep layer of
mT (maritime tropical) air as opposed to a
situation in which an
elevated mixed layer advects over mT air.