Precipitable Water Column

Integrate supplied specific humidity over increasing pressure in measured column with gravity/liquid density conversion; partial observed column remains partial and condensate or unsampled atmosphere is not inferred.

Description

Precipitable water is the liquid depth corresponding to vapor mass in an atmospheric column.

Precipitable water is the liquid depth corresponding to vapor mass in an atmospheric column. A pressure-coordinate integral requires specific humidity as water-vapor mass fraction, rather than mixing ratio per dry-air mass.^[1]

When to use Precipitable Water Column

Use this tool to integrate supplied specific humidity over increasing pressure in measured column with gravity/liquid density conversion; partial observed column remains partial and condensate or unsampled atmosphere is not inferred.

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

How to use Precipitable Water Column 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 pressureLevelsPa, specificWaterVaporMassFractions, gravityMPerSecondSquared, liquidWaterDensityKgPerM3 using the field table and calculation rules, then run the tool.
Read observedColumnWaterVaporKgPerM2, observedColumnPrecipitableWaterMm. 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
pressureLevelsPa	Pressure levels Pa	list of number	Required
specificWaterVaporMassFractions	Specific water vapor mass fractions	list of number	Required
gravityMPerSecondSquared	Gravity m per second squared	number	Required
liquidWaterDensityKgPerM3	Liquid water density kg per m3	number	Required

Use positive strictly increasing pressure levels in Pa, paired vapor fractions in [0,1], and positive gravity and liquid density. Apply trapezoidal integration over the observed pressure span and convert kg/m² to millimeters.

vapor mass per area = ∫q dp/g; water depth mm = 1000 × mass per area/liquid density

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

Output field	Meaning	Value type
observedColumnWaterVaporKgPerM2	Observed column water vapor kg per m2	number
observedColumnPrecipitableWaterMm	Observed column precipitable water mm	number

json

{
  "input": {
    "pressureLevelsPa": [
      50000,
      100000
    ],
    "specificWaterVaporMassFractions": [
      0.01,
      0.01
    ],
    "gravityMPerSecondSquared": 10,
    "liquidWaterDensityKgPerM3": 1000
  },
  "output": {
    "observedColumnWaterVaporKgPerM2": 50,
    "observedColumnPrecipitableWaterMm": 50
  }
}

Limitations and assumptions

A partial column remains partial. Unobserved layers and condensate are not reconstructed.

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 Virtual Temperature Correction: Compute exact ideal moist-air virtual temperature with vapor and suspended-liquid specific mass fractions relative to total mixture; differs from vapor mixing-ratio approximations and excludes ice unless caller independently models it. For a complementary task, use Water Balance Surplus Deficit Classifier: Conserve sequential single-bucket soil water with precipitation, PET, actual evaporation, overflow surplus and unmet-demand deficit; no groundwater recharge/irrigation guarantee and all bucket processes are declared simplifications.

References

Precipitable Water Column — tool.io

Similar or alternative tools

Virtual Temperature Correction
Compute exact ideal moist-air virtual temperature with vapor and suspended-liquid specific mass fractions relative to total mixture; differs from vapor mixing-ratio approximations and excludes ice unless caller independently models it.
Water Balance Surplus Deficit Classifier
Conserve sequential single-bucket soil water with precipitation, PET, actual evaporation, overflow surplus and unmet-demand deficit; no groundwater recharge/irrigation guarantee and all bucket processes are declared simplifications.