Condensation and how air cools

Detail

What condensation is

• Condensation is the change of water vapour into liquid water. Sometimes vapour turns straight into ice without becoming liquid first.
• Cause: the air loses heat.
• Moist air cools down to its dew point. The dew point is the temperature at which the air becomes saturated, which means it is holding all the vapour it can.
• Below the dew point the air can hold no more vapour.
• The extra vapour condenses into droplets or ice crystals.

• Condensation gives out latent heat. This is the heat that was stored in the vapour when the water first evaporated, and it now warms the surrounding air.

• What decides how much condensation happens:

• how much the air cools;

• the relative humidity of the air. This is the amount of vapour in the air compared with the most it could hold at that temperature, given as a percentage.

• Condensation also depends on four properties of the air: its volume, temperature, pressure and humidity.

Conditions for condensation (NCERT list)

No.	Condition	What happens
(i)	Temperature falls to the dew point while volume stays the same	The air reaches saturation without expanding. Examples are dew, frost and fog on calm nights.
(ii)	Both volume and temperature fall	Cooling and shrinking together bring the air to saturation.
(iii)	(Not given in the textbook)	—
(iv)	Moisture is added by evaporation	Extra vapour raises the humidity until the air is saturated. An example is steam fog over warm water.

• Exam trap: the most favourable condition for condensation is a fall in air temperature. Adding moisture is not the main condition.
• Contact with a colder object also causes condensation. Air touching a cold surface cools to its dew point and deposits:
• dew, when the dew point is above 0 °C;
• frost, when the dew point is below 0 °C (vapour turns straight into ice).

Cloud condensation nuclei (CCN)

• Cloud condensation nuclei are tiny particles in the air that water vapour condenses on. They are hygroscopic, which means they attract and soak up water.
• Good nuclei include dust, smoke and sea salt.
• The WMO defines a condensation nucleus as a nucleus on which water vapour can condense [6].
• Why nuclei are needed:
• Without nuclei, droplets form only at very high supersaturation. Supersaturation means the air holds more vapour than it normally can at that temperature.
• Real air rarely gets that far above saturation.

• So almost every cloud droplet and fog droplet in nature forms on a nucleus.

• Link to pollution:

• Cities and farm fires add smoke and other particles (aerosols: tiny solid or liquid particles floating in the air).

• More nuclei → fog forms more easily and is thicker → smog (smoke + fog). An example is the winter smog of the Indo-Gangetic Plain and Delhi.

• Link to cloud seeding:

• Hygroscopic seeding adds salt-type particles at the base of warm clouds. Larger droplets form and rain falls sooner.
• Glaciogenic seeding uses silver iodide in cold clouds to form ice particles [2].
• CAIPEEX (Cloud Aerosol Interaction and Precipitation Enhancement Experiment) was run by IITM Pune under the Ministry of Earth Sciences [2].
  • It held randomised seeding trials in 2018 and 2019, following WMO recommendations [2].
  • The trials were over the rain-shadow region of Solapur, Maharashtra [2].
  • Rainfall rose by about 18 ± 2.6% over 100 sq km downwind of the seeding point [2].
  • At some places the rise was up to about 46 ± 13% [2].

Adiabatic process (beyond NCERT)

• Adiabatic process: a parcel of air changes temperature without taking in or giving out heat to the air around it. The change comes only from expanding or being squeezed.
• Rising air:
• Air pressure falls with height.

• A rising parcel expands and uses up its own energy, so it cools.

• Sinking air:

• Pressure rises as the parcel sinks.
• The parcel is compressed and warms.
• Its relative humidity falls, so sinking air is dry.

Rate	Value	When it applies	Why
Dry adiabatic lapse rate (DALR)	~9.8 °C per km, about 10 °C per 1,000 m [3]	Unsaturated air, before it reaches the dew point	Cooling comes only from expansion
Saturated (moist) adiabatic lapse rate (SALR)	~4–7 °C per km	Saturated air, above the cloud base	Condensing vapour releases latent heat, which warms the parcel, so it cools more slowly [4]
Environmental lapse rate (ELR)	~6.5 °C per km on average	The actual still air around the parcel	Set by heating from the ground (see insolation and heat budget)

Lifting condensation level (LCL)

• Lifting condensation level is the height at which rising air cools to its dew point and becomes saturated.
• Step by step:
• Air rises and cools at the DALR.
• It reaches the dew point at the LCL.
• Condensation starts and a cloud forms.

• Above the LCL the air cools at the slower SALR.

• The flat base of cumulus clouds shows where the LCL is.

• The WMO calls this the condensation level: the level at which lifted air becomes saturated [6].
• NOAA studies treat the LCL as a reasonable estimate of cloud-base height when air is forced upward [5].
• Together these explain cloud formation: rising air cools adiabatically → it reaches the LCL → vapour condenses on CCN → visible cloud droplets [3][6].

Where sinking (descending) air keeps places dry

• Subtropical high-pressure belts lie near 30° N and 30° S.
• Air sinks here and is warmed by compression.

• This gives clear skies and hot deserts such as the Sahara, the Arabian desert and the Thar (Rajasthan).

• Foehn is a warm, dry wind blowing down the northern (lee) slopes of the Alps in Europe.

• Chinook is a warm, dry wind on the eastern slopes of the Rocky Mountains (USA and Canada). It is called the "snow eater" because it melts snow quickly.
• How these winds get so warm and dry:
• The wind goes up the windward slope. It cools at the slower SALR after clouds form and drops rain there.
• It comes down the lee slope as dry air. It warms at the faster DALR.
• So it reaches the foot of the lee side warmer and drier than when it started.

Four ways air cools

Route	How it works	Where it is seen
1. Lifting (adiabatic cooling)	See the four kinds of lifting below	See below
2. Contact with a cold surface	Air touches cold ground or water and cools to its dew point	Dew and frost on winter nights in Punjab and Haryana; fog where warm air moves over a cold ocean current (Grand Banks, Newfoundland)
3. Radiation at night	The ground loses heat on clear, calm, long nights, and the air just above it cools	Radiation fog over the Indo-Gangetic Plain in December–January, which delays trains and flights
4. Mixing of air masses	Warm, moist air mixes with cold air, and the mixture passes its dew point	Steam fog; "breath" fog on cold mornings

Four kinds of lifting:

• Convection: the ground heats the air, and the warm air rises. This gives afternoon cumulonimbus thunderstorms. Examples are equatorial rain and pre-monsoon Kalbaisakhi (Nor'westers) in West Bengal and Odisha.
• Orographic lifting: wind is pushed up a mountain (orography means mountain relief). It rains on the windward side.
• Western Ghats: Mumbai and Mahabaleshwar get heavy rain. Pune and Solapur, on the lee (rain-shadow) side, stay dry.

• Meghalaya: Mawsynram and Cherrapunji.

• Frontal lifting: warm air rises over cold air along a front, the boundary between two air masses. Examples are temperate cyclones and the western disturbances that bring winter rain to north-west India.

• Convergence: winds flow together and the air is forced up. Examples are the ITCZ (Inter Tropical Convergence Zone: the low-pressure belt near the equator where the trade winds of the two hemispheres meet) and the monsoon trough over north India.

Stability: will the rising air keep rising?

• Compare the ELR of the surrounding air with the parcel's own cooling rate.

Situation	Result	Weather
ELR greater than DALR (air around cools faster with height)	The parcel stays warmer than its surroundings and keeps rising, so the air is unstable	Towering cumulonimbus, thunderstorms, hail
ELR between SALR and DALR	Dry air is stable, but saturated air is unstable. This is called conditional instability	Fair at first; storms once the air reaches the LCL
ELR less than SALR, or a temperature inversion (temperature rises with height)	The parcel becomes colder and heavier than its surroundings and sinks back, so the air is stable	Flat stratus clouds, fog, trapped smog (Delhi in winter)

Prelims Hooks

• The most favourable condition for condensation is a fall in air temperature, not added moisture.
• Air cooled to its dew point with a dew point below 0 °C gives frost. With a dew point above 0 °C it gives dew.
• Dust, smoke and sea salt act as hygroscopic cloud condensation nuclei. Without them, droplets would need very high supersaturation.
• Dry adiabatic lapse rate ≈ 9.8 °C/km. Saturated rate ≈ 4–7 °C/km. Average environmental lapse rate = 6.5 °C/km.
• The saturated rate is lower than the dry rate because condensation releases latent heat [4].
• The flat base of cumulus clouds marks the lifting condensation level [5].
• Sinking air warms by compression. This explains the dry subtropical highs (~30° N/S) and the foehn (Alps) and chinook (Rockies) winds.
• CAIPEEX (IITM, MoES) seeding trials in 2018–19 over the rain-shadow region of Solapur, Maharashtra raised rainfall by ~18% over 100 sq km [2].
• Seeding pairs: hygroscopic seeding → warm clouds; silver iodide (glaciogenic) seeding → cold clouds [2].
• Stability trap: a temperature inversion gives stable air (fog and smog). An ELR above the DALR gives unstable air (thunderstorms).

Mains Points

• How a cloud forms: air is lifted (by convection, mountains, fronts or converging winds) → it cools adiabatically at the DALR → it reaches its dew point at the LCL → vapour condenses on CCN → latent heat slows further cooling to the SALR → the parcel becomes unstable → cumulonimbus clouds and rain. This chain explains rain on the windward Western Ghats and Meghalaya and the rain shadow over the Deccan interior (GS-I).
• Aerosols and pollution (GS-III, environment): more nuclei from farm-fire smoke and vehicle exhaust, combined with radiation cooling and winter inversions, cause long spells of fog and smog over the Indo-Gangetic Plain. The results include health risks and disrupted flights and trains, which calls for clean-air measures and fog forecasting.
• Cloud seeding as drought help (GS-III): CAIPEEX showed that hygroscopic seeding can add rain to suitable clouds in rain-shadow areas such as Solapur. It works only if suitable clouds are present, so it cannot replace watershed management or rainwater harvesting [2].
• Descending air and dry regions (GS-I): compression warming under the subtropical highs explains why hot deserts sit near 30° N/S on western continental margins, including the Thar. Foehn and chinook winds explain early snowmelt, avalanche risk and fire risk in mountain valleys.