National Weather Service United States Department of Commerce

Weather Glossary: K's

Thunderstorm potential
K-index value Thunderstorm Probability
Above 35 Numerous thunderstorms
31 to 35 Scattered thunderstorms
26 to 30 Widely scattered thunderstorms
20 to 25 Isolated thunderstorms
< 20 None

It is a measure of the thunderstorm potential based on vertical temperature lapse rate, moisture content of the lower atmosphere, and the vertical extent of the moist layer. The temperature difference between 850 mb and 500 mb is used to parameterize the vertical temperature lapse rate.

The 850 mb dew point provides information on the moisture content of the lower atmosphere. The vertical extent of the moist layer is represented by the difference of the 700 mb temperature and 700 mb dewpoint. This is called the 700 mb temperature-dew point depression.

The index is derived arithmetically and does not require a plotted sounding. K-index = [(850 mb temperature - 500 mb temperature) + 850 mb dew point - 700 dew point depression]. The K-index favors non-severe convection, especially heavy rain producing convection. Threshold values vary with season, location, and synoptic situation.

K-indices are also used to determine the potential of flooding. When your K-index is high (above 35), it means that you will likely see numerous thunderstorms develop. If these thunderstorms track across the same area, you may have a various serious flooding situation on your hands.

Keetch-Byram Drought Index (KBDI)
A soil/duff drought index that ranges from 0 (no drought) to 800 (extreme drought) and is based on a soil capacity of 8" of water. Factors in the index are maximum daily temperature, daily precipitation, antecedent precipitation, and annual precipitation.
Keetch-Byram Drought Index values
KBDI What does it mean?
600 - 800 Often associated with more severe drought with increased wildfire occurrence. Intense, deep burning fires with significant downwind spotting can be expected. Live fuels can also be expected to burn actively at these levels.
400 - 600 Typical of late summer, early fall. Lower litter and duff layers actively contribute to fire intensity and will burn actively.
200 - 400 Typical of late spring, early growing season. Lower litter and duff layers are drying and beginning to contribute to fire intensity.
0 - 200 Soil moisture and large class fuel moisture are high and do not contribute much to fire intensity. Typical of spring dormant season following winter precipitation.
Kelvin (Temperature Scale)
A temperature scale in which 0°K is the point at which all molecular motion ceases, called absolute zero (approx. -473°F/-280°C).
The fear of thunder and lightning. See Astraphobia, Astrapophobia, Brontophobia, Ceraunophobia, and Tonitrophobia.
An electron tube used as a low-power oscillator or a high-power amplifier at ultrahigh frequencies. Noted for exceptional stability over long periods of transmission.
Unit of speed used in aviation and marine activities which is equal to 1 nautical mile per hour or about 1.15 statute miles an hour (1.85 km/h).

Slang for lumpy protrusions on the edges, and sometimes the underside, of a thunderstorm anvil. They usually appear on the upwind side of a back-sheared anvil, and indicate rapid expansion of the anvil due to the presence of a very strong updraft. They are not Mammatus clouds.

See also cumuliform anvil and anvil rollover.

The fear of waves.