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 SKEW-T: A LOOK AT KI

METEOROLOGIST JEFF HABY

1. What is KI?

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:

 K INDEX
 15-25 Small convective potential 26-39 Moderate convective potential 40+ 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.

4. Pitfalls:

a. May not pick up a capping inversion that prevents storms from developing.

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 differential advection situation in which an elevated mixed layer advects over mT air.