Storm Station

The Field Book/Vol. IV, The Water/FB-WAT-002

Humidity variables

Humidity is measured in several ways, each answering a different question: how much water the air holds, how close it is to saturation, how cool evaporation can make it, and how much would fall if all of it rained out. This plate sets them side by side and shows which ones a rising parcel of air carries unchanged.

Plate FB-WAT-002Phase changeRevision 1, 2026-09-25Status draftAlso called humidity, dew point, frost point, relative humidity, mixing ratio, specific humidity, absolute humidity, wet bulb temperature, precipitable water, vapour pressure, psychrometer

Table of nine measures of humidity: vapour pressure, dew point, frost point, relative humidity, mixing ratio, specific humidity, absolute humidity, the wet bulb and precipitable water, each with what it is, its unit, and what it does when a parcel of air is lifted without condensing; the mixing ratio and the specific humidity hatched as the two it carries unchanged.1122334455667788AABBCCDDEE© 2026 STORM STATION 247THE FIELD BOOKPLATE FB-WAT-002-ATHE MEASURES OF HUMIDITY, EACH WITH WHAT IT IS, ITS UNIT, AND WHAT IT DOES WHEN AIR IS LIFTED WITHOUT CONDENSINGFIG. A THE MEASURES OF HUMIDITYMEASUREWHAT IT ISUNITWHEN AIR IS LIFTED, NOT CONDENSINGVapour pressurethe vapour’s share of the air’s pressurembfalls, with the pressureDew pointthe temperature at which the air would be saturated°Ffalls about 1 °F per 1,000 ftFrost pointthe same, over ice, below freezing°Ffalls, as the dew point doesRelative humiditythe vapour against what the air could holdper centrises as the air coolsMixing ratiothe vapour’s weight over the dry air’sper centstays the sameSpecific humiditythe vapour’s weight over all the air’sper centstays the sameAbsolute humiditythe vapour’s weight in a volume of airgrains per cubic footfalls, as the air expandsWet bulbthe temperature evaporation can cool air to°Ffalls, more slowly than the airPrecipitable waterthe depth of water if all the column’s vapour fellinchesa whole column, not a parcelHATCHED: THE TWO THAT A LIFTED PARCEL CARRIES UNCHANGED UNTIL IT SATURATES1234TITLEHumidity variables, the measures of humidityVOL. IV THE WATER · PHASE CHANGETYPECLASSIFICATIONSCALENOT TO SCALEREVREV 1 DRAFT SHEET 1 of 3DATE2026-09-25IDFB-WAT-002-ADRAWN AS linework on paperSOURCES AMS, WMO
Fig. A The measures of humidity. The measures of humidity, each with what it is, its unit, and what it does when air is lifted without condensing NOT TO SCALEMaximizeThe sheet, SVG, 11 by 17
  1. Dew point, falling about 1 °F per 1,000 ft
  2. Relative humidity, rising as the air cools
  3. Mixing ratio, unchanged
  4. The wet bulb

What it is

Humidity has no single measure. Each one answers its own question, and Fig. A sets nine of them side by side. Three do most of the work. The dew point says how much water the air holds. The relative humidity says how close the air is to saturation. The mixing ratio is the vapour's weight against the dry air's, and it is the one a moving parcel carries unchanged. The physics behind all of them is on Water vapour as a gas FB-ENG-011.

What it is not

The relative humidity is not the amount of water. It rises overnight as the air cools and falls in the afternoon as it warms, with the same water in the air all day.

The wet bulb is not the dew point. It lies between the dew point and the air temperature, and equals both only in saturated air.

Lookalikes

Relative humidity as the amount of water
It is a ratio: it rises overnight as the air cools and falls in the afternoon while the water in the air stays the same.
The wet bulb and the dew point
The wet bulb lies between the dew point and the air temperature; it is the dew point only in saturated air.

The machine

The wet bulb

Wrap a thermometer in wet cloth and blow air over it, and evaporation cools it to the wet bulb temperature. Fig. B works the wet bulb for every temperature and humidity by the psychrometer formula the WMO sets for such instruments. Air at 86 °F (30 °C) and 55 per cent has a wet bulb of 73.4 °F (23.0 °C). The drier the air, the further the wet bulb falls below it. The wet bulb is also the limit to how far sweat can cool a body, Extreme heat, wet bulb and WBGT FB-EVT-131.

Lifted air

Fig. C lifts air from the ground at 86 °F with a dew point of 59 °F (15 °C). As it rises it cools about 5.3 °F every 1,000 ft, and its dew point falls about 1 °F. Its relative humidity climbs from 40 to 97 per cent by 6,000 ft, while its mixing ratio stays at 1.06 per cent. At about 6,210 ft (1.89 km) the two temperatures meet and the air is saturated: that is where the cloud base forms, Lapse rates and stability FB-ENG-005.

Chart of the wet bulb temperature against the air temperature from 32 to 110 degrees Fahrenheit, one line for each relative humidity from 20 to 100 per cent, worked by the WMO psychrometer formula at 1,013 millibars: at 100 per cent the wet bulb equals the air temperature, and the drier the air the further below it the wet bulb lies; air at 86 degrees Fahrenheit and 55 per cent has a wet bulb of 73.4 degrees.1122334455667788AABBCCDDEE© 2026 STORM STATION 247THE FIELD BOOKPLATE FB-WAT-002-BTHE WET BULB TEMPERATURE AGAINST THE AIR TEMPERATURE FOR RELATIVE HUMIDITIES OF 20 TO 100 PER CENT, BY THE WMO PSYCHROMETER FORMULA AT 1,013 MBFIG. B THE WET BULB32 °F(0 °C)32 °F50 °F(10 °C)50 °F68 °F(20 °C)68 °F86 °F(30 °C)86 °F104 °F(40 °C)104 °FAIR TEMPERATUREWET BULB20%40%60%80%100%86 °F AT 55%: WET BULB 73.4 °FWMO PSYCHROMETER FORMULA AT 1,013 MB; THE LINES MEET AT 100%, WHERE NOTHING EVAPORATES123TITLEHumidity variables, the wet bulbVOL. IV THE WATER · PHASE CHANGETYPECLASSIFICATIONSCALETO SCALEREVREV 1 DRAFT SHEET 2 of 3DATE2026-09-25IDFB-WAT-002-BDRAWN AS linework on paperSOURCES AMS, WMO
Fig. B The wet bulb. The wet bulb temperature against the air temperature for relative humidities of 20 to 100 per cent, by the WMO psychrometer formula at 1,013 mb TO SCALEMaximizeThe sheet, SVG, 11 by 17
  1. Saturated, 100 per cent, the wet bulb equal to the air
  2. Dry air, 20 per cent, the wet bulb far below
  3. 86 °F at 55 per cent, a wet bulb of 73.4 °F
Chart of height against temperature for air lifted from the surface at 86 degrees Fahrenheit with a dew point of 59 degrees: its temperature falling about 5.3 degrees every 1,000 feet, its dew point about 1 degree, the two meeting at about 6,210 feet (1.89 kilometres) near 53 degrees, where it saturates and cloud begins; a table at 0, 2,000, 4,000 and 6,000 feet showing the relative humidity rising from 40 to 97 per cent while the mixing ratio stays at 1.06 per cent.1122334455667788AABBCCDDEE© 2026 STORM STATION 247THE FIELD BOOKPLATE FB-WAT-002-CAIR LIFTED FROM THE SURFACE AT 86 °F WITH A DEW POINT OF 59 °F, ITS TEMPERATURE, DEW POINT AND RELATIVE HUMIDITY WORKED UNTIL IT SATURATESFIG. C LIFTED UNTIL IT SATURATES40 °F50 °F60 °F70 °F80 °F90 °F01,0002,0003,0004,0005,0006,0007,000FTTEMPERATUREDEW POINTSATURATED AT 6,210 FT (1.89 KM), 53.0 °FFTAIRDEW PTRHMIX086.0°59.0°40%1.06%2,00075.4°57.1°53%1.06%4,00064.8°55.1°71%1.06%6,00054.1°53.2°97%1.06%MIX: THE MIXING RATIO, UNCHANGED1234TITLEHumidity variables, lifted until it saturatesVOL. IV THE WATER · PHASE CHANGETYPESOUNDINGSCALETO SCALEREVREV 1 DRAFT SHEET 3 of 3DATE2026-09-25IDFB-WAT-002-CDRAWN AS linework on paperSOURCES AMS, WMO
Fig. C Lifted until it saturates. Air lifted from the surface at 86 °F with a dew point of 59 °F, its temperature, dew point and relative humidity worked until it saturates TO SCALEMaximizeThe sheet, SVG, 11 by 17
  1. The temperature
  2. The dew point
  3. Where they meet, the cloud base
  4. The relative humidity climbing, the mixing ratio unchanged

Ingredients

Scales

time
a reading at a moment; the dew point changes over hours, the relative humidity with every change of temperature
horizontal
a point, a parcel of air, or, for precipitable water, a whole column over one place
vertical
from the surface to the top of the moist layer
orlanski
micro

Signatures

sounding
the dew point line; the gap between it and the temperature; the dewpoint depression; closing where cloud is
surface
the dew point and relative humidity each minute at an airport; the wet bulb; worked from them; for heat stress
satellite
precipitable water; worked from the microwave and infrared channels
radar

The numbers

QuantityValue, and the kind of number it is
Mixing ratioThe weight of vapour over the weight of dry air it is mixed with; unchanged when air rises or sinks without condensingStandard, Glossary of Meteorology
Specific humidityThe weight of vapour over the weight of all the air, vapour includedStandard, Glossary of Meteorology
Absolute humidityThe weight of vapour in a unit volume of airStandard, Glossary of Meteorology
Wet bulb temperatureThe temperature air reaches when cooled to saturation by evaporating water into itStandard, Glossary of Meteorology
Psychrometer formulae = e_w(T_w) − 6.53 × 10⁻⁴ (1 + 0.000944 T_w) p (T − T_w), for an aspirated psychrometer, in hPa and °CStandard, Guide to Instruments and Methods of Observation (WMO-No. 8)
Precipitable waterThe depth of liquid water if all the vapour in a column of air condensed and fellStandard, Glossary of Meteorology

How the station sees it

An airport station measures the temperature and the dew point each minute, and every other measure on this plate is worked from those two and the pressure.

How it is warned

Humidity is not warned alone. It feeds the heat index and wet bulb heat warnings, frost and fog forecasts, and the cloud base every thunderstorm forecast begins with.

See also

Sources

  1. American Meteorological Society. Glossary of Meteorology.
  2. World Meteorological Organization. Guide to Instruments and Methods of Observation (WMO-No. 8).

Definition after the Glossary of Meteorology. Plate FB-WAT-002, revision 1, 2026-09-25. The number is permanent; cite it.