• The hydrological cycle encompasses significant transformations in the state of water, involving evaporation, condensation, and precipitation.
• Condensation plays a crucial role in the creation of dew, frost, fog, clouds, and eventually, rainfall.

Introduction to Humidity:

• The water vapor existing in the air is termed humidity, with levels ranging from 0 to 4%. This moisture concentration is most prominent within the lowest 10 km of the atmosphere. Hygrometers measure humidity.
• Humidity capacity denotes the ability of a given volume of air at a specific temperature to hold the maximum amount of moisture content. When the moisture content matches its humidity capacity, the air is referred to as saturated air.
• The humidity capacity of air is directly linked to the air's temperature; higher temperatures result in increased humidity capacity. The temperature at which the air reaches saturation is known as the dew point.
• The presence of water in the atmosphere originates from water bodies through evaporation and from plants through transpiration.
•  A continuous exchange of water occurs among the atmosphere, oceans, and continents through processes such as evaporation, transpiration, condensation, and precipitation. Notably, the freezing point registers at a higher temperature compared to the dew point. The temperature at which a liquid transitions into a solid state is termed the freezing point.

Forms of Humidity:

Humidity can be categorized into three types:

1. Absolute Humidity:

• Absolute humidity is the weight of water vapor per unit volume of air, expressed as grams per cubic meter of air.
• It fluctuates with the expansion and contraction of air, even when the quantity of water vapor remains constant.
• Varies across different locations, and the air's ability to hold water vapor is temperature-dependent.

2. Specific Humidity:

• It represents the ratio of the weight of water vapor (grams) to the weight of the air (kilograms), expressed as grams per kilogram of air.
• Directly proportional to moisture pressure, increasing with rising moisture pressure and decreasing with its fall.
• Inversely related to air pressure.

3. Relative Humidity:

• Relative humidity is the percentage of moisture in the atmosphere compared to its full capacity at a given temperature.
• Changes with air temperature, affecting the air's capacity to retain moisture.
• Higher over oceans and lower over continents.

Distribution of Relative Humidity:

Zonal:

• Highest at the Equator and decreases towards the poles.
• Minimum in sub-tropical anti-cyclones.
• Increases from 30°N and 30°S but decreases towards the poles with decreasing temperature.
• Latitudinal shifting due to the apparent movement of the Sun.

Seasonal:

• Varies with latitudes.
• Between 30°N and 30°S, higher in summer than winter.
• In higher latitudes, higher in winter due to colder land temperatures.
• Temperature influences specific humidity, affecting relative humidity.

Phases of Water:

• The majority of the world's water is in a liquid state, convertible to the gaseous form (water vapor) through evaporation or the solid form (ice) via freezing.

Evaporation:

• Transformation of water into water vapor.
• Heat is the primary cause.
• Latent heat of vaporization is the temperature at which water starts evaporating.
• Increased temperature enhances water absorption and retention capacity, leading to greater evaporation.
• Maximum evaporation occurs in continents between latitudes 10°N and 10°S. In oceans, the peak evaporation takes place between 10 to 20° latitudes in both hemispheres.

Factors Controlling Evaporation:

• Humidity: Higher aridity increases the air's potential to absorb and retain moisture. Higher humidity reduces this potential, leading to slower and smaller evaporation.
• Temperature: Warmer water experiences faster evaporation than colder water. Heating the lower layer of cold air from below induces instability, turbulence, and promotes evaporation.
• Movement of Air: The air's movement plays a crucial role in evaporation. Air movement replaces the moisture-filled layer with a drier layer, enhancing moisture absorption capacity.

Condensation:

• Condensation is the transformation of water vapor into water, occurring due to heat loss. When moist air cools, its capacity to hold water vapor diminishes, leading to the condensation of excess water vapor into liquid form.
• Conditions for Condensation:
  • Presence of nuclei: Micro-particles like salt and smoke serve as nuclei.
  • Temperature below dew point: Air temperature must fall to or below the dew point.
  • Sufficient water vapor: Adequate water vapor molecules in the air.
  • Condensation also depends on temperature deficiency and relative humidity. If it directly condenses into a solid form, it is called sublimation.

Forms of Condensation:

1. Dew:

• Deposited in water droplets on cooler surfaces.
• Ideal conditions: clear sky, calm air, high relative humidity, and cold, long nights.
• Dew forms when the dew point is above freezing.

2. Frost:

• Forms on cold surfaces below freezing point.
• Ice crystals form when the dew point is at or below freezing
• Damages crops, but can be avoided with irrigation.

3. Fog:

• Formed through radiation, conduction, and mixing of warm and cold air masses near the Earth's surface.
• Occurs in winters in sub-tropical regions and all seasons beyond 35° latitudes.
• Consists of microscopic water droplets suspended near the ground surface.
• When air temperature drops below the dew point due to contact with a cold surface and the air is still, containing hygroscopic nuclei, fog forms.
• Fogs are common where warm air currents meet cold currents, creating miniature clouds through condensation around nuclei provided by dust, smoke, and salt particles.
• Various types of fog include:
  • Radiation Fog: Formed under cloud cover after rain, or when cooled air collects in depressions or valleys.
  • Advection Fog: Occurs when warm, moist air moves over cold surfaces, common along seacoasts and shores of large bodies of water.
  • Hill Fog: Results from warm moist winds rising along mountain slopes, causing condensation, often observed in temperate tropical latitudes.
  • Frontal Fog: Forms along fronts separating cold and warm air masses, prevalent in cool temperate latitudes with frequent fronts.
  • Mist: Contains more moisture than fog, with thicker layers of moisture around each nucleus. Common over mountains as warm air meets a cold surface.
  • Smog: A fog-smoke mixture, prevalent in industrial areas. Smog forms earlier and lasts longer than regular fog, contributing to air pollution and posing risks to vision and respiratory health.

Clouds:

• Clouds are defined as collections of tiny water droplets, ice particles, or a mixture of both in the air, usually high above the ground. 
• They form through the condensation of water vapor around hygroscopic nuclei, induced by air lifting and adiabatic cooling. As clouds form at some height above the Earth's surface, they exhibit various shapes.

• Clouds are generally formed when air is forced to rise at a front, over mountains or because of convection. The first classification of clouds was given by Luke Howard in 1802.
• Clouds are generally formed in the troposphere. That's why almost all meteorological phenomena take place in this layer only.

Types of Clouds

• Clouds can be classified into four types as follows.
• High Clouds (Height 6000-12000m)
• Cirrus The high altitude detached clouds having fibrous chain like or silky appearance. They are composed of tiny ice crystals and do not cause rainfall. It is often called Mares' tails and indicates fair weather.
• Cirrocumulus These are white-coloured clouds having patches of small white flakes or small globules which are arranged in distinct groups or wave- like form, so called and appear as sand.

• Clouds take on hues and extend across the sky in milky, thin sheets, resembling transparent formations that create halos around the Sun and the Moon.

Middle Clouds (Height 2000-6000m):

• Altostratus: Thin grey or blue clouds with a fibrous or uniform appearance. Thicker sheets obscure the Sun and Moon, making them appear as bright spots behind the clouds.
• Altocumulus: White and grey wavy layers or globular forms, often forming regular patterns like lines, groups, or waves. High globular groups are sometimes called sheep clouds or wool pack clouds.

Low Clouds (Height up to 2000m):

• Stratus: Dense, low-lying fog-like clouds in dark grey, seldom close to the ground. Composed of uniform layers, capable of producing light drizzle.
• Stratocumulus: Composed of globular masses or rolls arranged in lines, waves, or groups, more pronounced than altocumulus. Covers the entire sky in the winter, associated with fair weather.
• Nimbostratus: Dark, low clouds close to the ground, creating darkness due to their high thickness. Increase humidity and lead to precipitation, also known as rain clouds.

Vertical Clouds:

• Cumulus: Dense, widespread, dome-shaped clouds with flat horizontal bases and rounded tops, occurring in humid tropical regions as a fair weather cloud.
• Cumulonimbus: Dark, heavy, and dense clouds with vertical development, resembling cauliflower or an anvil. Frequently seen in tropical afternoons, especially in summer.

Noctilucent Clouds:

• These night-shining clouds resemble wispy cirrus clouds with a bluish or silvery color. Also known as Polar Mesospheric Clouds (PMCs) when viewed from space, detectable near the summer polar mesopause.

Precipitation:

• Continuous condensation in free air leads to the growth of condensed particles. When air resistance can no longer hold them against gravity, they fall to Earth, a process known as precipitation. Precipitation can occur in liquid or solid forms, including rain, hail, frost, fog, and sleet of snow.

Forms of Precipitation:

• Snowfall: Occurs when the temperature is below 0°C, resulting in fine flakes of snow in hexagonal crystal form.
• Hail: Limited and sporadic, hail forms when raindrops leave warmer air and encounter colder air below, solidifying into small pellets of ice.
• Sometimes, raindrops, upon release from the clouds, undergo solidification into small, rounded ice pieces that reach the Earth's surface, referred to as hailstones. These hailstones form as rainwater passes through colder layers, resulting in several concentric layers of ice.

Sleet:

• Sleet consists of frozen raindrops and refrozen melted snowwater. When a layer of air with a temperature above freezing overlies a sub-freezing layer near the ground, precipitation occurs in the form of sleet.
• Raindrops leaving warmer air encounter colder air below, solidifying and reaching the ground as small ice pellets, not larger than the raindrops from which they formed.

Lightning and Thunder:

• Lightning manifests as flashes of light caused by massive electrical discharges of 10 to 100 million volts, briefly superheating the air to temperatures ranging from 15,000°C to 30,000°C.
• The violent expansion of this abruptly heated air sends shock waves through the atmosphere, resulting in sonic bangs known as thunder. The distance a lightning stroke travels determines the duration of the thunder echoes.

Cloudburst:

• A cloudburst signifies an extreme amount of rain occurring in a short period, sometimes accompanied by air and thunder.
• It involves unexpected precipitation exceeding 100 mm (or 10 cm)/hour over a geographical region of approximately 20 to 30 km². 
• Cloudbursts are more likely in mountainous zones, where saturated clouds ready to condense into rain face obstacles due to the upward movement of very warm air currents.

Distribution of Precipitation:

• The global pattern of precipitation distribution depends on factors such as latitude, temperature, moisture, atmospheric disturbances, landform barriers, frontal activity, air mass movement, and differential heating. The average annual precipitation worldwide is around 80 cm (30 inches). 
• Equatorial regions and monsoon areas of Southeast Asia experience the highest rainfall, while temperate regions receive a moderate amount. Subtropical high-pressure belt and polar regions, along with the eastern margins of continents, receive less precipitation. Coastal areas experience decreased precipitation from coast to continental due to moisture dissipation.

Equatorial Regions:

• The Equatorial zone receives abundant precipitation (over 200 cm annually) due to high temperature, high humidity, unstable air, convergence of trade winds from both hemispheres, and a general upward motion of air. 
• Precipitation mainly occurs as heavy intermittent convective showers accompanied by thunder and lightning. 
• Tropical storms in this region intensify local ascending air currents, resulting in significant precipitation in the Caribbean, Southwest Pacific Ocean, Indian Ocean, and China Sea.

Sub-Tropical High-Pressure Latitudes:

• The upwelling of cold water along the west coasts of continents cools the air from below, increasing stability on the eastern sides of these anticyclones.
• Deserts like the Sahara (North Africa), Atacama (South America), and the Great Desert (Australia) form on the western sides of these subtropical highs, while eastern regions receive abundant precipitation.
• Subtropical Anticyclones: The impact of subtropical anticyclones and associated subsidence is less pronounced on the western side, while convergence with accompanying ascending air is more prominent.

Temperate Mid-Latitudes:

• In the middle latitudes, weather and precipitation are predominantly influenced by traveling extratropical cyclones and fronts.
• These systems bring abundant rainfall to most areas, except for the far interiors of Asian and North American continents. The polar front in this region witnesses convergence between cold polar air and warm, moist Westerlies, often leading to the formation of temperate cyclones.
• The seasonal migration of wind belts results in a narrow belt (between 30° and 40° latitudes) experiencing significant seasonal variability in precipitation. Winter sees the movement of depressions along the polar front, producing frontal precipitation, while summer is marked by subsiding air linked to subtropical anticyclones, resulting in reduced precipitation.

Polar High Latitudes:

• At high latitudes, especially in polar regions, low precipitation is attributed to air subsidence in high-pressure belts and the low temperatures, limiting air moisture. Throughout the year, these regions experience minimal precipitation.
• Even in summer, snow-bound areas are dominated by anticyclonic conditions, hindering the movement of poleward-traveling depressions.

Rainfall:

• Rainfall occurs when water droplets grow large enough through coalescence, making the air incapable of holding them. Warm, moist, and unstable air, along with a sufficient number of hygroscopic nuclei, are essential conditions for rainfall.

Types of Rainfall:

Convectional Rainfall:

• Convectional rainfall happens when warm and moist air rises due to convection. Heating of the land surface induces the ascent of air, creating convection currents. Upon reaching a certain height, the warm and moist air becomes saturated, forming dark cumulonimbus clouds that result in substantial rainfall.

• Such rain is common in the summer or the hotter part of the day.
• It is very common in the equatorial regions and interior parts of the continents, particularly in the Northern hemisphere.

Orographic Rainfall

• When warm and moist air is obstructed by any hill or plateau, it starts ascending along the slope of the hill or plateau and gets cooled.
• As a result, it gets saturated and the process of condensation starts. The rainfall caused in this process is called orographic rainfall. 
• This type of rainfall occurs more frequently where hills or mountains are situated near and parallel to the coast.
• The slope which faces the warm and moist wind and receives heavy rainfall, is called windward slope.
• But, as the wind starts descending along the slope of the opposite side, it becomes warm and dry and due to decreased humidity, a little rainfall occurs.
• This region is called the rain shadow region or leeward slope. This is also known as the relief rain.

Cyclonic or Frontal Rainfall

• It is caused due to the ascending of moist air and adiabatic cooling caused by convergence of two extensive air masses of entirely different physical properties.
• In the temperate region, where Westerlies and polar winds meet, warm air is lifted upward along the front. Whereas, cold air being heavier settles down.
• The warm air lying over cold air is cooled and gets saturated and condensation begins, resulting ultimately into rainfall.
• In tropical regions, this type of rainfall is called cyclonic rainfall and in temperate regions,this rainfall is called depression rain or frontal rain.
• Depressional rains are usually lighter than convectional rains and is as much greater duration, cyclonic rains occur throughout the doldrum where trade winds meet

Artificial Rainfall:

• In this method, artificial clouds are created to induce rain. Meteorologists suggest that this technique can generate rainfall from dense, rain-bearing clouds located at an altitude of 3 km. 
• The selected clouds for artificial rainfall should be at least 1500 m thick and possess a relative humidity of 50 to 60%. Another approach involves charging clouds with electrical or shock waves, particularly effective with cumulonimbus clouds that yield substantial rainfall after thunderstorms. Successful artificial rainfall initiatives have been undertaken in Israel, China, and Russia. In India, Project Raindrop was launched to address drought issues in the Saurashtra region of Gujarat.

Factors Influencing Rainfall Distribution:

• Mountain Slopes: Mountain ranges receive significant rainfall on their windward slopes, while the leeward slopes experience minimal rainfall. The moisture-laden Westerlies from the ocean hit the windward slopes, ascend due to mountain barriers, and release their moisture, resulting in heavy rainfall on the western sides of continents. Conversely, the eastern areas remain comparatively dry.
• Supply of Humidity: The moisture content in the atmosphere is a crucial factor in determining rainfall patterns. Tropical regions, with high evaporation rates, receive more rainfall, while polar regions, with limited evaporation, experience lower rainfall.
• Direction of Winds: Trade winds and Westerlies play a pivotal role in precipitation. When these winds move from the sea to continents, they bring about rainfall. However, when they move from land to sea, they create dry winds.
• Cold Air: Regions influenced by cold air currents tend to have lower humidity levels, resulting in reduced rainfall. Conversely, regions affected by warm air currents experience higher specific humidity and, consequently, heavier rainfall.
• Ocean Currents: Ocean currents, categorized as cold and warm, influence rainfall distribution. Areas with cold currents exhibit lower humidity and less rainfall, while those with warm currents experience higher specific humidity, leading to increased rainfall.

Prelims Facts

• Humidity is measured by which instrument?

- Hygrometer [APPSC (Pre) 2013]

• Humid heat is experienced in summer when the weather is

- Maggy UPPSC (Pre) 2022]

• On what factor the amount of rain depends?

- Hamidity in abmosphere [UPPSC (Pre) 2016]

•  Which basin experiences the convectional rainfall?

- Amazon [APSC (Pre) 2016]

• The amount of moisture in the atmosphere is associated

- Latitude [LAS (Pre) 2005]

• The ability to keep the moisture in the form of water vapour is related to

- Temperabure [LAS (Pre) 2006]

•  The process of conversion of water vapour into ice is

- Condensation [MPSC (Pre) 2017

• The rain fed and rain shadow areas are the characteristics of

- Orographic rainfall IMPSC (Pre) 2017)

• The doud formation is the result of

- Condensation [UKPSC (Pre) 2016]

• Which type of clouds is often accompanied by heavy rains?

- Carmalonimbus [UPPSC (Pre) 2012

• What is meant by the term nimbus regarding clouds?

- Roin imeminert (IPSC Pre) 2013]

•  Sun's halo is produced by the refraction of light in

- Ice orystals in cirras clods (UPPSC (Pre) 2002)

•  What does the term Fog means?

- A Camalonimbus [IPSC (Pre) 2013]

•  Which cloud is responsible for highly intense rain?

- Nimbostretus (IPSC (Pre) 2005]

• The line joining points with equal cloud cover is called

- Isoneph (Odisha PSC (Pre) 2018]

•  The clouds flood in the atmosphere because of their low

- Density TEAS (Pre) 1995]

• The cloudy nights are warmer compared to clear cloudless nights because

- it reflect back the heat given off by Earth [AS (Pre) 2001]

•  Which type of cloud has the greatest vertical extent which brings conventional rain accompanied by lightning and thunder?

- Camalonimbus [LAS (Pre) 2005]

• The line joining points with equal cloud cover is called

- Isomeph [Odishi PSC (Pre) 2018]

Self Check

1. The relative humidity of a region is low when MPSC (Pre) 2014

(a) the wet and dry bulb temperatures read the same.

(b) the difference between the reading of the wet and dry bulb thermometer is large.

(c) the temperatures are high.

(d) the temperatures are low.

2. Why are dewdrops not formed on a cloudy night?

IAS (Pre) 2019

(a) Clouds absorb the radiation released from the Earth's surface.

(b) Clouds reflect back the Earth's radiation.

(c) The Earth's surface would have a low temperature on cloudy nights.

(d) Clouds deflect the blowing wind to ground level.

3. Which one of the following is the most important factor for the formulation of smog?

(a) Long winter nights.

(b) Formation of an 'inversion lid'.

(c) Presence of many air pollutant resources.

(d) Rapid fall in temperature with increasing height above the sea level.

4. Consider the following statements. UPPSC (Pre) 2007

1. High clouds primarily reflect solar radiation and cool the surface of the Earth.
2. Low clouds have a high absorption of infrared radiation emanating from the Earth's surface and thus, cause a warming effect.

Which of the statements) given above is/are correct?

(a) Only 1

(b) Only 2

(c) Both 1 and 2

(d) Neither 1 nor 2

5. Match List I with List II and select the correct answer by using the codes given below the lists.

UPPSC (Pre) 2012

List 1

(Clouds)

1. Cirrus
2. Stratus
3. Nimbus
4. Cumulus

List II (Characteristics)

1. Rain giving
2. Feathery appearance
3. Vertically growing
4. Horizontally spreading

Codes

(a) 3 142

(b) 3 412

(c) 24 13

(d) 2 1 4 3

6. Which one of the following statements is correct?

UPPSC (Pre) 2004

(a) Cirrus clouds are composed of ice crystals.

(b) Cirrus clouds exhibit a flat base and have the appearance of rising domes.

(c) Cumulus clouds are white and thin and form delicate patches and give a fibrous and feathery appearance.

(d) Cumulus clouds are classified as high clouds.

7. Consider the following climate and geographical phenomena.

IAS (Pre) 2002, MPSC (Pre) 2004

1. Condensation
2. High temperature and humidity
3. Orography
4. Vertical wind

Thunder cloud development occurs due to which of these phenomena?

(a) 1 and 2

(b) 2, 3 and 4

(c) 1, 3 and 4

(d) All of the above

8. During a thunderstorm, the thunder in the sky is produced by the

IAS (Pre) 2013, UPPSC (Pre) 2013

1. meeting of cumulonimbus clouds in the sky
2. lightning that separates the nimbus clouds.
3. violent upward movement of air and water particles.

Select the correct answer by using the codes given below.

(a) Only 1

(b) 2 and 3

(c) 1 and 3

(d) None of these

9 . Which one of the following is not a favorable occurrence of a Condition for thunderstorms? 

(a) Conditional and convective instability.

(b) Adequate supply of moisture in the lower atmosphere.

(c) Advection of cold dry air in the lower troposphere and warm moist air in the upper troposphere.

1. d) A synoptic situation of low level upper level divergence.

10. Which one of the following statements with regard to cloud burst is not correct?

(a) Cloud burst is a localised weather phenomenon ) Cloesenting highly concentrated rainfall over a small area in a short duration of time.

(b) Cloud burst occurs due to upward movement of moisture-laden air with sufficient rapidity to for cumulonimbus clouds.

(c) Cloud burst occurs only in hilly areas.

(d) There is no satisfactory technique till now for predicting cloud bursts.

11. Match List I with List II and select the correct answer by using the codes given below the lists MPSC (Pre) 2012

Codes

(a) 3142

(b) 2 1 3 4

(c) 4 2 13

(d) 24 13

Know Right Answer

1 (b)

2 (b)

3 (c)

4 (d)

5 (c)

6 (a)

7 (d)

8 (d)

9 (c)

10 (c)

11 (d)