De Martonne Aridity Index

Compute annual De Martonne P/(T+10) from twelve monthly normals, with explicitly positive shifted temperature; not a calibrated drought category or validity at cold singular temperatures.

Description

The De Martonne index combines annual rainfall with a temperature-dependent denominator.

The De Martonne index combines annual rainfall with a temperature-dependent denominator. Its shifted temperature convention requires careful handling of cold climates near the denominator's zero.^[1]

Compute annual De Martonne P/(T+10) from twelve monthly normals, with explicitly positive shifted temperature; not a calibrated drought category or validity at cold singular temperatures.

When to use De Martonne Aridity Index

Use this tool to compute annual De Martonne P/(T+10) from twelve monthly normals, with explicitly positive shifted temperature; not a calibrated drought category or validity at cold singular temperatures.

Prepare the climate observations and independently chosen model parameters. Confirm units, daily coverage, chronological order and the intended seasonal window.

How to use De Martonne Aridity Index and understand its results

Prepare the climate observations and independently chosen model parameters. Confirm units, daily coverage, chronological order and the intended seasonal window.
Enter monthlyPrecipitationMm, monthlyMeanTemperaturesC using the field table and calculation rules, then run the tool.
Read annualDeMartonneIndex. 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
monthlyPrecipitationMm	Monthly precipitation mm	list of number	Required
monthlyMeanTemperaturesC	Monthly mean temperatures c	list of number	Required

Pair exactly twelve nonnegative monthly precipitation totals in mm with monthly mean temperatures in °C. The tool uses the unweighted mean of monthly temperatures and requires that mean plus ten be strictly positive. It returns the displayed annual index.

Paired lists must be nonempty and have equal lengths.

index = annual precipitation/[mean monthly temperature + 10]

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

Output field	Meaning	Value type
annualDeMartonneIndex	Annual de martonne index	number

json

{
  "input": {
    "monthlyPrecipitationMm": [
      50,
      50,
      50,
      50,
      50,
      50,
      50,
      50,
      50,
      50,
      50,
      50
    ],
    "monthlyMeanTemperaturesC": [
      10,
      10,
      10,
      10,
      10,
      10,
      10,
      10,
      10,
      10,
      10,
      10
    ]
  },
  "output": {
    "annualDeMartonneIndex": 30
  }
}

Limitations and assumptions

A mean temperature at or below −10 °C is rejected. The result is not automatically mapped to a drought category, and the equation does not represent soil water storage or seasonal water stress.

Numeric inputs must be finite. Follow the calculation-specific rules for overflow, underflow and exceptional cases. Numeric lists are limited to 100,000 entries; the rules above may require smaller limits. Missing required fields, extra fields, wrong value types and violations of the calculation rules are input errors.

Alternative or Complementary methods

Use UNEP Aridity Index P over PET: Compute annual UNEP P/PET from matched monthly normals and show seasonal nonnegative climate deficits; no fixed aridity classification or inference of actual crop water stress. For a complementary task, use Continentality Gorczynski Index: Evaluate named Gorczynski annual thermal-range continentality, retaining signed index and rejecting equatorial singularity; no geography or universal climate category.

References

De Martonne Aridity Index — tool.io

Similar or alternative tools

UNEP Aridity Index P over PET
Compute annual UNEP P/PET from matched monthly normals and show seasonal nonnegative climate deficits; no fixed aridity classification or inference of actual crop water stress.
Continentality Gorczynski Index
Evaluate named Gorczynski annual thermal-range continentality, retaining signed index and rejecting equatorial singularity; no geography or universal climate category.