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Technical guideFree to read2012

Guide to Meteorological Instruments and Methods of Observation, 2008 edition updated in 2010 (2012 publication)

World Meteorological Organization · World Meteorological Organization (WMO)

The WMO-No. 8 guide includes humidity definitions and calculation formulas. Annex 4.B gives the saturation vapour pressure and dew-point relations needed to check the consistency of temperature and humidity inputs.

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Overview

Annex 4.B gives the water-phase saturation vapour pressure as 6.112 exp(17.62 t / (243.12 + t)) with t in degrees Celsius, and defines relative humidity as actual vapour pressure divided by saturation vapour pressure. For example, saturation vapour pressure is about 23.3 hPa at 20 C and about 42.3 hPa at 30 C; these values are applications of the formula.

The same annex gives the Assmann psychrometer relation over water: vapour pressure equals the saturation value at the wet-bulb temperature minus 6.53e-4 (1 + 0.000944 tw) p (t - tw). For instance, t = 25 C, tw = 18 C and p = 1013.25 hPa give a vapour pressure of about 15.9 hPa and a relative humidity of about 50%. The inverse relation yields dew point.

The record points to the original WMO document hosted by Radiometrics. The formulas are defined for the Celsius convention and the stated range of applicability. The calculation is an approximation for checking whether dry-bulb, dew-point and relative-humidity inputs agree with one another; it is not a substitute for sensor calibration. The guide itself is a broad document on meteorological observation methods; this record uses only the humidity definitions and the Annex 4.B formulas.

Key points

  1. Saturation vapour pressure (water phase): 6.112 exp(17.62 t / (243.12 + t)), t in C.
  2. Relative humidity is actual vapour pressure divided by saturation vapour pressure.
  3. Assmann psychrometer relation: vapour pressure = saturation value at wet-bulb minus 6.53e-4 (1 + 0.000944 tw) p (t - tw).
  4. The inverse relation gives dew point.
  5. Use the Celsius convention and stated range; the calculation is not sensor calibration.

Using it in design

  • Use the stated Celsius convention and applicability range when comparing dry-bulb, dew point and relative humidity.
  • Run this calculation as a consistency check to warn the user when the input triple disagrees.
  • Label the result as an approximation and state that it does not replace sensor calibration.

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Who should read it

Software developers writing humidity and dew-point calculations and mechanical engineers validating ambient conditions.

Related standards

  • WMO-No. 8

Publication details

Authors
World Meteorological Organization (WMO)
Publisher
World Meteorological Organization
Year
2012
Document number
WMO-No. 8
Document language
English
Source type
Government / public
Access
Free to read
Licence
Copyright retained; citation only — the publisher’s terms apply to the original document

Citation

World Meteorological Organization (WMO) (2012). Guide to Meteorological Instruments and Methods of Observation, 2008 edition updated in 2010 (2012 publication). World Meteorological Organization. WMO-No. 8. https://radiometrics.com/wp-content/uploads/2021/10/wmo_8_en-2012.pdf

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