Relative humidity

Also called: RH · Topic: Water in the Atmosphere: Humidity, Condensation, Clouds and Precipitation · NCERT: Class 7, Ch 2 "Understanding the Weather"; Class 11, Ch 10 "Water in the Atmosphere"

Meaning

Relative humidity (RH) is the amount of water vapour in the air, given as a percentage of the most vapour that air could hold at the same temperature. WMO defines it more exactly: the observed vapour pressure (the push of water-vapour molecules in the air) as a percentage of the saturation vapour pressure (the highest vapour pressure possible) at the same temperature and pressure. It is written as % or %rh [3].

RH is the humidity figure used in weather reports. It tells us how close the air is to saturation, so it shows when dew, fog, cloud and rain can form. It also tells us how sticky or dry the weather will feel.

Explanation

How RH works: the same vapour can give a different RH

• Water vapour is water as an invisible gas. It makes up between 0 and about 4 per cent of the atmosphere by volume.
• It enters the air through evaporation (water from oceans, lakes, rivers and wet soil turning into vapour) and transpiration (plants giving off vapour through tiny pores in their leaves).
• RH depends on two things:
• how much vapour is actually in the air

• how much vapour the air could hold, which depends on temperature

• When air warms, RH falls.

• Warm air can hold more vapour.
• The same amount of vapour now fills a smaller share of that capacity.

• So afternoons feel drier.

• When air cools, RH rises.

• Cool air can hold less vapour.
• The same vapour now fills a bigger share of the capacity.

• So mornings bring dew and mist.

• Key point: RH can change even when no vapour is added or removed. A change in temperature alone is enough.

Saturation, dew point and condensation

• Saturated air holds the most vapour it can at a given temperature. Its RH is 100 per cent.
• Dew point is the temperature to which air must be cooled to become saturated, with no change in pressure or moisture. WMO puts it simply: "the temperature at which moist air is saturated with water vapour" [3].

• If the dew point is below 0 °C, the vapour turns straight into ice. This is called the frost point [3].

• Chain of events:

• Air cools, either at night or as it rises.
• It reaches its dew point, and RH becomes 100 per cent.

• Any more cooling makes the extra vapour condense (turn into liquid droplets), forming dew, fog or cloud.

• The gap between air temperature and dew point shows how humid the air is:

• A small gap means the air is nearly saturated and feels muggy.
• A large gap means the air is dry.

RH compared with the other two humidity measures

Measure	What it means	Unit	Behaviour
Absolute humidity	Actual weight of vapour in a unit volume of air	g/m³	Changes when air expands or shrinks
Specific humidity	Mass of vapour in a unit mass of moist air [3]	g/kg	Stays the same when air rises, expands, warms or cools, as long as no vapour is added or removed
Relative humidity	Vapour present as a percentage of the most the air could hold at that temperature	%	Falls as air warms and rises as air cools

• RH tells us how close the air is to saturation. It does not tell us the real amount of moisture.
• To compare the real moisture of two places, use specific humidity, not RH.

Measuring RH: the hygrometer

• A hygrometer is any instrument that measures humidity [3].
• Wet-and-dry bulb hygrometer (psychrometer): two thermometers sit side by side. One bulb is wrapped in a muslin wick wetted with distilled water [3].
• Water evaporates from the wet muslin and cools the wet bulb.
• Drier air means faster evaporation and more cooling.
• So a large gap between the two readings means low RH, and a small gap means high RH. With no gap, the air is saturated at RH 100 per cent.
• Hygrometric tables (or a psychrometer formula) turn the two readings into RH and dew point [3].

• Readings are most reliable when air blows over the wet bulb at about 3–5 m/s [3]. The fan-driven Assmann psychrometer is the field reference used to check other instruments [3].

• Hair hygrometer: human or synthetic hair gets longer in moist air. The change in its length depends mainly on RH [3]. It still works but is not very precise, and it is being phased out [3].

• Electronic (capacitive) sensor: a thin polymer film between two electrodes takes up vapour, and its electrical capacitance changes. It is now the leading technology in weather work [3].

Where it occurs

• World pattern: RH is higher over oceans than over continents. Oceans have endless water to evaporate. Land, especially deserts, has little water to supply vapour.
• Low-RH deserts: the Thar Desert (Rajasthan, India) and the Sahara (North Africa).
• Humid places in India:
• Kochi (Kerala, on the Malabar coast, about 10° N, on the Arabian Sea): wet clothes dry slowly, sweat stays on the skin and the air feels sticky.
• Kolkata and Chennai before the monsoon: the air temperature is close to the dew point, so it feels muggy.

• The monsoon coasts of Kerala, the Konkan and the Bengal delta have the highest humidity in India.

• Dry places in India:

• Jaipur (Rajasthan, on the edge of the Thar Desert) and Delhi have dry air for much of the year, so clothes dry quickly.
• Jaisalmer (Rajasthan) on a May afternoon: there is a large gap between air temperature and dew point.

• Ladakh and the polar regions also have very low humidity.

• Typical values: dry weather has an RH of about 20–40 per cent. Humid weather has an RH of about 60–80 per cent.

• Use in India: IMD (India Meteorological Department) uses RH in its Heat Index (HI), also called apparent temperature (how hot it actually feels). The index combines air temperature and RH and is based on Steadman's formula (1979) [2].

Don't confuse with

• Absolute humidity: this is the actual weight of vapour per unit volume (g/m³), not a percentage. It is highest over warm tropical oceans and changes when air expands, while RH changes with temperature.
• Specific humidity: this is measured in g/kg and is the only measure that stays the same when air rises, expands or cools. It is the right measure for comparing the real moisture of two places. RH is not.
• Dew point: this is a temperature, not a percentage. It is the temperature at which RH reaches 100 per cent and condensation starts.
• Saturated air: this is not a separate measure. It is the state in which RH equals exactly 100 per cent.

Prelims Hooks

• Units: absolute humidity is in g/m³, specific humidity is in g/kg and RH is in %.
• Trap: RH can rise without any vapour being added, simply because the air cools. RH falls when air warms.
• RH is higher over oceans than over continents, but absolute humidity is highest over warm tropical oceans. Don't mix up the two statements.
• Saturated air = RH 100 per cent. Cooling below the dew point causes condensation into dew, fog or cloud.
• In a wet-and-dry bulb hygrometer, a larger difference between the two readings means lower RH [3]. In a hair hygrometer, hair lengthens as RH rises [3]. Capacitive polymer sensors are now the leading method [3].
• Place–state pairs: humid Kochi is in Kerala. Dry Jaipur is in Rajasthan. Typical RH is 20–40 per cent in dry weather and 60–80 per cent in humid weather.

Mains Points

• Humid heat as a hazard (GS-III, disasters):
• High RH stops sweat from evaporating, so the body cannot cool itself.
• Coastal cities such as Chennai, Mumbai and Kolkata face dangerous heat even at moderate temperatures.

• IMD's Heat Index combines temperature and RH (Steadman 1979). Wet-Bulb Globe Temperature (WBGT) mixes the wet-bulb, globe and dry-bulb temperatures, with weights of 0.7, 0.2 and 0.1 in sunshine [2]. Both indices help heat-action plans and work-hour rules.

• Spatial pattern and correct use of RH (GS-I):

• RH explains daily weather well: dry afternoons, and dew and mist in the morning.
• It also explains why the monsoon coasts feel sticky and the Thar feels dry.

• However, RH shifts with temperature. For comparing the real moisture of two regions, specific humidity is the better measure.

• Monitoring and climate change (GS-III, environment):

• Under the Clausius-Clapeyron relation, the most vapour air can hold rises by about 7 per cent for each 1 °C of warming [5].
• IPCC AR6 (2021) expects heavy rain to become stronger at about this same rate (high confidence) [4]. In India, this means more Himalayan cloudbursts and more urban floods in Mumbai and Chennai.
• RH data from psychrometers and electronic sensors at weather stations [3] feed forecasts, crop advisories, heat warnings, and the design of storage for food and heritage objects.

Related concepts

• Humidity
• Absolute humidity
• Specific humidity
• Saturated air
• Dew point
• Hygrometer

Read more

• Humidity: how much water vapour the air holds