Humidity: how much water vapour the air holds

Detail

1. What humidity is and where it comes from

• Water vapour is water in the form of an invisible gas. It makes up between 0 and about 4 per cent of the atmosphere by volume.

• It is near 0 per cent over cold deserts and polar ice. It is near 4 per cent over warm, wet tropical areas.

• Water in the atmosphere is found in three states:

• gas, as water vapour
• liquid, as cloud droplets and rain

• solid, as ice crystals, snow and hail

• Humidity means the water vapour present in the air.

• It comes from two sources:
• Evaporation: liquid water from oceans, lakes, rivers and wet soil turns into vapour.

• Transpiration: plants release water vapour through tiny pores in their leaves.

• Hydrological cycle (water cycle): water keeps moving between the air, the oceans and the land.

• It evaporates, condenses into clouds, falls as rain or snow, and flows back to the sea.

• Mixing ratio is a related measure. It is the mass of water vapour compared with the mass of dry air. Weather services use it alongside specific humidity [3].

2. Why humidity falls quickly with height

• The source is at the surface.
• Vapour enters the air from seas, soil and plants near the ground.

• Little of it gets carried far upward.

• Cooler air holds less vapour.

• Temperature drops with height in the troposphere.

• Cold air can hold less vapour before it becomes saturated.

• Result: most water vapour stays in the lowest few kilometres of the atmosphere. Clouds, rain and storms therefore mostly form in the troposphere.

3. Three ways to measure humidity

Measure	What it means	Unit	Behaviour	Where it is high or low
Absolute humidity	The actual weight of water vapour in a unit volume of air	grams per cubic metre (g/m³)	Changes when air expands or shrinks, because the volume changes	Highest over warm tropical oceans; low over hot deserts and the poles
Specific humidity	Mass of water vapour in a unit mass of moist air [3]	grams per kilogram (g/kg)	Stays the same when air rises, expands, warms or cools, as long as no vapour is added or removed	Best measure for comparing the real moisture of two places
Relative humidity (RH)	Vapour present, as a percentage of the most the air could hold at that temperature	per cent (%)	Falls when air warms and rises when air cools, even if no vapour is added	Higher over oceans than over continents

• Absolute humidity is different from place to place.

• It is highest over warm tropical oceans, where there is plenty of water and heat drives fast evaporation.

• Specific humidity is better than absolute humidity for comparing places.

• A rising parcel of air expands, so its volume grows. Its g/m³ value then falls even though no water has left.

• A kilogram of air stays a kilogram, so g/kg does not change.

• Relative humidity is the measure used in weather reports.

• WMO definition: the observed vapour pressure (the push of water-vapour molecules in the air) as a percentage of the saturation vapour pressure (the most vapour pressure possible) at the same temperature and pressure. It is written as % or %rh [3].
• Why RH changes with temperature:
  • Warm air can hold more vapour, so the same amount of vapour gives a lower RH. That is why afternoons feel drier.
  • At night the air cools and can hold less, so RH rises. That is why mornings bring dew and mist.
• Why RH is higher over oceans: there is endless water to evaporate. Over land, especially deserts such as the Thar in Rajasthan and the Sahara, there is little water to supply vapour.

4. Humidity in daily life (Indian examples)

• Dry weather: RH of about 20–40 per cent.
• Humid weather: RH of about 60–80 per cent.
• Kochi (Kerala, Malabar coast, about 10° N, on the Arabian Sea) has humid air.
• Wet clothes dry slowly.

• Sweat does not evaporate fast, so it stays on the skin and the place feels sticky.

• Jaipur (Rajasthan, on the edge of the Thar Desert) and Delhi have dry air for much of the year.

• Clothes dry quickly because water evaporates fast into dry air.

• Process: dry air → room for more vapour → fast evaporation → quick drying and quick cooling of skin.

5. Saturated air and dew point

• Saturated air holds the most water vapour it can at a given temperature. Its RH is 100 per cent.

• If saturated air cools any further, it cannot hold all its vapour. The extra vapour condenses into droplets, forming dew, fog or cloud.

• Dew point is the temperature to which air must be cooled, with no change in pressure or moisture, to become saturated.

• 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].

• How close the air is to its dew point shows how humid it is.

• A small gap between air temperature and dew point means the air is nearly saturated. It feels muggy, as in Kolkata or Chennai before the monsoon.

• A large gap means dry air, as in Jaisalmer on a May afternoon.

• Chain of events:

• Air cools at night or as it rises.
• It reaches its dew point, RH 100 per cent.
• Vapour condenses into dew, fog or cloud.

6. Warmer air holds more water (beyond NCERT)

• Clausius-Clapeyron relation: the most vapour air can hold rises by about 7 per cent for each 1 °C of warming.

• So the capacity roughly doubles for every 10 °C of warming.

• Global evidence (IPCC AR6, 2021):

• Total water vapour in the atmosphere rises by about 7 per cent per 1 °C of global surface warming [5].
• Heavy rain at the global scale will get stronger at about this same rate, around 7 per cent per 1 °C (high confidence) [4].

• Climate models show heavy-rain extremes rising by 4–8 per cent per 1 °C of surface warming [4].

• Chain of events:

• The world warms.
• Air holds more vapour.
• Each storm has more water to drop.

• Downpours become heavier and short, intense floods more common, such as cloudbursts in the Himalaya and urban floods in Mumbai and Chennai.

• Link: climate change → extreme rainfall → disaster management.

7. Measuring humidity: the hygrometer

• Hygrometer: any instrument that measures humidity [3].

Type	How it works	Status (WMO, 2018)
Wet-and-dry bulb (psychrometer)	Two thermometers side by side. One bulb is wrapped in a muslin wick wetted with distilled water [3]	Still used, but slowly being replaced by electronic sensors [3]
Hair hygrometer / hair hygrograph	Human or synthetic hair gets longer in moist air. Its change in length depends mainly on RH [3]	Still used, but being phased out; works, but is not very precise [3]
Electronic (capacitive) sensor	A thin polymer film between two electrodes takes up vapour, and its electrical capacitance changes [3]	Now the leading technology in weather work; easy to make, maintain and calibrate [3]

• How the wet-and-dry bulb type works:
• Water evaporates from the wet muslin, and this cools the wet bulb.
• The drier the air, the faster the evaporation and the bigger the cooling.
• So a large gap between the two readings means low RH. 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 used as a field reference to check other instruments [3].

• Uses of humidity data:

• weather forecasting and climate studies
• agriculture, hydrology and aviation [3]
• food processing and storage

• museums, to protect paintings and manuscripts

• Heat-stress warnings:

• IMD uses the Heat Index (HI), also called apparent temperature (how hot it actually feels). It combines air temperature and RH, based on Steadman's formula (1979) [2].
• Wet-Bulb Globe Temperature (WBGT) is used for heat stress at work. It mixes the wet-bulb, globe and dry-bulb temperatures. In sunshine the weights are 0.7, 0.2 and 0.1 [2].

Prelims Hooks

• Water vapour in the atmosphere ranges from 0 to about 4 per cent by volume.
• Absolute humidity is measured in g/m³. Specific humidity is measured in g/kg. RH is measured in %. Only specific humidity stays the same when air expands or cools.
• Trap: RH can rise without any vapour being added, simply when air cools. RH falls when air warms.
• RH is higher over oceans than over continents. Absolute humidity is highest over warm tropical oceans.
• Saturated air = RH 100 per cent. Cooling below the dew point causes condensation.
• Humidity falls rapidly with height, because its source is at the surface and cooler air holds less vapour.
• In a wet-and-dry bulb hygrometer, a larger difference between the readings means lower RH.
• Hair hygrometer: hair lengthens as RH rises. Capacitive polymer sensors are now the leading method [3].
• Clausius-Clapeyron relation: about 7 per cent more vapour per 1 °C of warming, so capacity roughly doubles per 10 °C [5].
• Typical RH: dry weather 20–40 per cent, humid weather 60–80 per cent. Humid Kochi is in Kerala; dry Jaipur is in Rajasthan.

Mains Points

• Spatial pattern (GS-I): humidity is highest over warm tropical oceans and the monsoon coasts of India, such as Kerala, Konkan and the Bengal delta. It is lowest over the Thar, Ladakh and polar regions. Specific humidity, not RH, should be used to compare real moisture between places.
• 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, which combines temperature and RH (Steadman 1979), and WBGT help heat-action plans and work-hour rules [2].
• Climate change link (GS-III, environment): warmer air holds about 7 per cent more vapour per 1 °C. IPCC AR6 (2021) expects heavy rain to intensify at about this rate (high confidence) [4][5]. For India this means more cloudbursts, more urban floods and pressure on drainage and dam operation.
• Monitoring and adaptation: humidity 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.