K Index Severe Weather

Calculate named K index from 850/700/500-hPa temperature and measured 850/700 dewpoints; input levels must match and no severe-weather probability/recommendation follows.

Description

The K index combines lower-to-middle tropospheric temperature contrast, low-level moisture and the 700-hPa dewpoint depression.

The K index combines lower-to-middle tropospheric temperature contrast, low-level moisture and the 700-hPa dewpoint depression. Its specified pressure levels are essential to its meaning.^{[1, 2]}

Calculate named K index from 850/700/500-hPa temperature and measured 850/700 dewpoints; input levels must match and no severe-weather probability/recommendation follows.

When to use K Index Severe Weather

Use this tool to calculate named K index from 850/700/500-hPa temperature and measured 850/700 dewpoints; input levels must match and no severe-weather probability/recommendation follows.

Prepare the observed sounding, cloud records or forecast verification data. Check pressure and height coordinates, wind conventions and the stated units.

How to use K Index Severe Weather and understand its results

Prepare the observed sounding, cloud records or forecast verification data. Check pressure and height coordinates, wind conventions and the stated units.
Enter temperature850C, temperature700C, temperature500C, dewpoint850C, dewpoint700C using the field table and calculation rules, then run the tool.
Read kIndexC. The explanation below defines the units, signs, ordering and any flags or placeholder values.
If the tool reports an input issue, correct the values and run it again. Apply the stated assumptions when interpreting a successfully returned result.

Input field	Meaning	Value type; see rules below	Requirement
temperature850C	Temperature850 c	number	Required
temperature700C	Temperature700 c	number	Required
temperature500C	Temperature500 c	number	Required
dewpoint850C	Dewpoint850 c	number	Required
dewpoint700C	Dewpoint700 c	number	Required

Enter Celsius temperatures at 850, 700 and 500 hPa and dewpoints at 850 and 700 hPa. Require each dewpoint at or below its corresponding temperature and return the named Celsius index.

K = T850 − T500 + Td850 − (T700 − Td700)

Units, indexing, boundary conditions and exceptional cases are defined in the calculation rules above.

Output field	Meaning	Value type
kIndexC	K index c	number

json

{
  "input": {
    "temperature850C": 20,
    "temperature700C": 10,
    "temperature500C": -10,
    "dewpoint850C": 15,
    "dewpoint700C": 5
  },
  "output": {
    "kIndexC": 40
  }
}

Limitations and assumptions

The index does not provide severe-weather probability or account for the full sounding.

Numeric inputs and returned numeric values must be finite; an unrepresentable returned value produces an error. Missing required fields, extra fields, wrong value types and violations of the calculation rules are input errors.

Alternative or Complementary methods

Use Showalter Index: Estimate Showalter 850-hPa-origin parcel index via explicit supplied LCL and caller piecewise dry/moist lapse approximation; differs from supplied surface-parcel lifted index, not a full pseudoadiabatic solver or validated thunder forecast. For a complementary task, use Lifted Index: Difference measured environmental and caller independently lifted surface-parcel temperatures at 500 hPa; parcel lift is not computed and index alone does not classify storm occurrence.

Further background: Atmospheric stability index glossary.

References

K Index Severe Weather — tool.io
Atmospheric stability index glossary — National Weather Service

Similar or alternative tools

Showalter Index
Estimate Showalter 850-hPa-origin parcel index via explicit supplied LCL and caller piecewise dry/moist lapse approximation; differs from supplied surface-parcel lifted index, not a full pseudoadiabatic solver or validated thunder forecast.
Lifted Index
Difference measured environmental and caller independently lifted surface-parcel temperatures at 500 hPa; parcel lift is not computed and index alone does not classify storm occurrence.