Wet Spell Length Analyzer

Analyze full consecutive wet spell distribution at a caller inclusive precipitation threshold; supplied sequence must have no missing days and zero means no observed qualifying spell.

Description

Wet spell length measures how precipitation persists across consecutive days.

Wet spell length measures how precipitation persists across consecutive days. The wet-day threshold separates meaningful rainfall from dry days or trace amounts, and a spell ends as soon as a day fails that threshold.^[1]

Analyze full consecutive wet spell distribution at a caller inclusive precipitation threshold; supplied sequence must have no missing days and zero means no observed qualifying spell.

When to use Wet Spell Length Analyzer

Use this tool to analyze full consecutive wet spell distribution at a caller inclusive precipitation threshold; supplied sequence must have no missing days and zero means no observed qualifying spell.

Prepare the climate observations and any independently chosen thresholds or model parameters. Confirm units, daily coverage, time order and the intended seasonal window.

How to use Wet Spell Length Analyzer and understand its results

Prepare the climate observations and any independently chosen thresholds or model parameters. Confirm units, daily coverage, time order and the intended seasonal window.
Enter dailyPrecipitationMm, callerWetDayLowerThresholdMm using the field table and calculation rules, then run the tool.
Read wetSpellStartIndices, wetSpellLengthsDays, meanWetSpellLengthDays. 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
dailyPrecipitationMm	Daily precipitation mm	list of number	Required
callerWetDayLowerThresholdMm	Caller wet day lower threshold mm	number	Required

Supply a nonempty list of nonnegative daily precipitation in mm and a positive wet-day threshold. Equality counts as wet. The tool finds all maximal consecutive wet runs and returns zero-based start indices, lengths and their arithmetic mean. If there are no wet runs, both lists are empty and the mean is zero.

wet_i = (P_i ≥ wet threshold); mean length = Σ wet-run lengths / number of wet runs

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

Output field	Meaning	Value type
wetSpellStartIndices	Wet spell start indices	list of number
wetSpellLengthsDays	Wet spell lengths days	list of number
meanWetSpellLengthDays	Mean wet spell length days	number

json

{
  "input": {
    "dailyPrecipitationMm": [
      0,
      2,
      2,
      0,
      3
    ],
    "callerWetDayLowerThresholdMm": 1
  },
  "output": {
    "wetSpellStartIndices": [
      1,
      4
    ],
    "wetSpellLengthsDays": [
      2,
      1
    ],
    "meanWetSpellLengthDays": 1.5
  }
}

Limitations and assumptions

The result assumes consecutive daily observations without gaps. It does not infer missing days, distinguish storms or predict future rainfall. Runs touching the supplied period's ends can be portions of longer spells.

Numeric inputs and returned numeric values must be finite; an unrepresentable returned value produces an error. 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 Simple Daily Intensity Index SDII: Compute SDII using observed precipitation only on wet days at caller inclusive threshold; zero observed wet days is undefined rather than inventing a daily intensity. For a complementary task, use Monsoon Onset Date Threshold Lookup: Audit caller monsoon-onset wet-run and following dry-break policy with fully observed confirmation window; incomplete future windows cannot confirm onset and no regional calendar is embedded.

References

Wet Spell Length Analyzer — tool.io

Similar or alternative tools

Simple Daily Intensity Index SDII
Compute SDII using observed precipitation only on wet days at caller inclusive threshold; zero observed wet days is undefined rather than inventing a daily intensity.
Monsoon Onset Date Threshold Lookup
Audit caller monsoon-onset wet-run and following dry-break policy with fully observed confirmation window; incomplete future windows cannot confirm onset and no regional calendar is embedded.