Climatology – Part 1 – Geography

Adiabatic Heating & cooling closed system PV = K So , P decreses , V increases , T decreases
The rising parcel of air expands , as lower pressure outside ( when air ascends ) alows air molecules to push out on the parcel walls
Since , it takes energy for parcel molecles to ” push-out ” on the parcel walls , so they use some of their internal energy in the process
As inernal energy gets used , internal energy decreases , so parcel of air cools . ( temperature of air body is proportional to molecular internal energy ) ( here , internal energy has been used in molecular motion to increase the volume of air .. )

That’s why clouds expand on cooling… Clouds surface area much more than initial surface from where evaporation take place .

Insolation Solar constant = Amount of energy per unit area wavelength 1/ temperature

Factors affecting distribution of Insolation

Angle of Incidence Energy received at surface = Energy * Projected area = Energy * Actual area * cos theta
Also , as angle of incidence increases , the travelling distance increases , causing more scattering , reflection and absorption

Duration of sunshine

Nature of surface albedo – It is the reflecting power of a surface . It is the ratio of reflected radiation from the surface to incident radiation upon it – Click

Solar constant Though almost constant , but varies due to sunspots , etc.
The solar constant, a measure of flux density, is the amount of incoming solar electromagnetic radiation per unit area that would be incident on a plane perpendicular to the rays, at a distance of one astronomical unit.

Dist btn eart and sun Perihelion ( Jan 3 ) – nearer to earth
Aphelion ( July 4 ) – fartherest to earth

Earth is relatively closer to sun during northern hemisphere winter

Transparency of Atmosphere Reflection from dust particles , cloud , gases , vapour particles
Turbidity factor – A measure of the atmospheric transmission of incident solar radiation. Turbidity is the cloudiness or haziness of a fluid caused by large numbers of individual particles that are generally invisible to the naked eye, similar to smoke in air.

Heat energy loss processes

When sun rays , consisting of light & heat waves of diff wavelength pass thru molecules of air , watervapour and dust , prism effect takes place
wavelength inversely proportional to scattering – See reyleigh formula
Wavelength of blue less than red , so scattering more of blue colour , so sky is blue – in day
During sunrise and sunset , blue gets too much scattered before reaching our eyes , while red undergoes less scattering and reaches our eyes . That’s why morning sunise apperas red

Condition for Scattering – Dia of molecules wavelength of light , Here no scattering , only diffusion
diffusion of light in all possible direction gives sufficient working light bfore sunrise / after sunset
Diffusion causes solar radiation loss by cloud , dust particles

defined as process in which incident radiation is retained by substance and is irreversibly converted to some other form of energy

When a gas molecules absorbs light waves , this energy is transformed into internal molecular motion which causes a rise in temperature

O2 + 1/2 O2 ( in presence of sunlight forms Ozone – Here , solar energy is absorbed to form ozone , especially the UV radiation of solar energy
water vapour is the most imp & efficient absorber of UV radiation of larger wavelength
CO2 also absorbs incoming solar radiation

* After gases absorb incoming solar radiation , they gets heated by conversion of radiant energy into heat – moecular motion increases by heat addition
Higher the temperature of a source , shorter is the wavelength emitted by the source .
Thus these gases start emitting the heat as long wavelength radiation
Cloud drops , have no absorption , only scattering and diffusion .. Why ?

Green house effetct
Loss of solar radiation is proportional to water vapor quantity ( as more water vapor , more absorption of heat )
water vapour opaque to longer IR , (which is emitted by the earth’s surface – long bcoz temp is low ) and is transparent to shorter IR
So , water vapour is the most effective regulator of surface temperature . – Bcoz water vapour / water has high coeffienct of lateral expansion compared to other gases , so absorbs more heat than CO2 , etc..

Also , as water vapour absorbs much heat , so diurnal range of temp. in humid areas is less than diurnal range of temp. in arid areas.

Reflection Albedo – See above

More cloud – thick cloud – more albedo
Obliquity of sun’s rays – more oblique rays – more albedo
Nature of surface – Ice ( maximum albedo ) > sand – > grass – > dry earth – > wet earth

Heat Budget TBD


Condensation It is a process by which water vapour passes into liquid state or water
Sublimation Ice to vapour or vapour to ice directly , without passing through liquid state
Melting solid to liquid
Freezing liquid to solid
Transpiration moisture loss from vegetation
Evapo – transpiration combined effects of evaporation &transpiration from vegetation covered land areas.

The surplus of precipitation over evaporation found in equatorial & (40-70 N/S ) – So , huge amount of heat energy is gained in the atmosphere thru latent heat of condensation of the vapours to form clouds ( some may doubt , ki less evaporation mean less vapour , but here , moisture is very high bcoz of high ppt , so even if evaporation is low comaprtivly, more moisture causes cloud formation releasing latent heat of condensation ) ..
( this means more precipitaion than evaporation , which means more cloud formation which means more release of latent heat of condensation , so atmosphere warms )
The surplus of evaporation over precipitation found in (10-40 N/S )
( this means more evaporation than precipitation , which means heat energy of atmosphere transformed into latent heat of evaporation so atmosphere cools )

However , planetary winds , cyclones , ocean currents and other atmospheric disturbances removes the latitudinal heat imbalances

Water holding capacity of air increases with increase in temperature , bcoz space btn air molecules increases , thus more water molecules can fit in the space , and vice-versa

Water is said to be saturated at a given temperature , when the water vapour content in air is = capacity of air water can hold { at a given temp. }
The amount of water vapour a given mass of air can hold at a given temp. is known as its capacity

If air reaches saturation , then on further cooling , it condenses . In absense of a condensation nuclei , condensation starts at a temp < Dew point – See cloud theories .. For clear mechanism of condensation

Dew point – The temp. to which air has to be cooled in order to reach saturation Still needs more clarity

Dew point of ascending air decreases with altitude @ 2 degree C per km ———— Why ?
Since water vapour holding capacity of air increases with increasing temperature , the air in equaotorial belt has a higher dew point than that in higher latitudes — ———– Why ?

Howver , to reach a water without lowering its temp can be achieved by increasing moisture .

Relative Humidity Ratio of amount of water vapour in air to the amount of vapour air can hold at that temperature

If air is saturated , then relative humidity is 100%

Relative humidity = ( Vapour pressure / Saturated vapour pressure ) * 100

As temp decreases , volume decreases , but as water vapour is constant , so RH will increase

Water holding capacity increases with increase in temp….
Maximum RH as equator , as air is very moist . At 30-40 – subsiding air , dry , so RH decreases . Btn 40-60 , T goes on decreasing , RH increases gradually . Beyond , RH decreases
In tropical regions , summer wet & winters dry , so RH in summer more than RH in winter . In higher latitudes , humid winters record higher % of RH )

In morning , min temp . And maximum RH . In mid-afternoon , maximum T & min. RH


Evaporation is a cooling process – Why ? Bcoz when when evaporation occurs , latent heat of vaporization is used to evaporate a given liquid , thus energy of the system containing liquid decreases , thus temperature also decreases .
So , more evaporation , more cooling , more comfortable we feel . That's why Dogs protudes their tongue to feel cooler in summer .

Evaporation rate is high in drier areas than in humid areas , where Relative humidity is high .
That’s why , we feel comfortable if we are away from coastal areas , then in areas which are nearer to the coast . Ex : Patna vs Kolkata ……. In kolkata , evaporaion reduces bcoz relative humidity is very high .. So sweat don’t dry up .

During evaporation , water molecules absorb energy which gives them energy/motion required to escape from surface of liquid and becomes a gas . As energy is used of the liquid , thus the energy of liquid decreases , thus it becomes cooler .

The vapour temperature increases , as energy of liquid is transferred to the gas , so gas gets hotter and surrounding water body cooler , while in condensation , the energy of vapour is transferred to surrounding air , causing increase of surrounding gas and formation water droplets

Heat absorbed during Evaporation is k/as latent heat of vaporisation

The reservoir of energy to maintain earth's surface temperature against cooling effect of evaporation is supplied by solar radiation

Factors affecting Evaporation

Amt of wate available i.e Quantity ( oceans vs continent )
Temperature – As temp increases , RH decreases , so evaporation increases
Relative Humidity – As RH decreases , vapour pressure decreases , evaporation increases ——– Bcoz as RH increases , vapour pressure increases , so rate of evaporation decreases , bcoz vapour pressure is pressure of air exerted by its molecules over evaporating body . So if VP increases , more pressurised evaporating body – so molecules in water have to gather more energy to escape the water body .
Wind speed – As WS increases , it decreases vapor pressure as turbulence in air drives away vapour , so as speed increases , evaporation increases
Surface Area – As SA increases , evaporation increases
Air pressure – Low pressure , low vapour pressure , more evaporation
Water composition – dirty water , less evaporation

Potential Evapo- transpiration
evaporation and transpiration from plants, together called evapotranspiration, transfers water to the atmosphere
It is an idealized condition in which there be enuf rainfall to provide sufficint moisture for all possible evapo-transpiration in an area .
In places , having surplus of ppt over evapotranspiration are marked by surplus water for storage .
In areas , where ppt less than evapotranspiration , no water for storage . Stored water evaporates , so soil becomes dry & vegetation gets parched up

Condensation Process of water vapor changing to liquid state

If a given air body is cooled below its dew point , water vapor becomes liquid . { in presence of a condensation liquid , super cooled water freezes , else remains super cooled water }

Doubt – In childhood , learnt that water evaporates at 100 degree C & freezes at 0 degree C at standard pressure and temp , . But here it is saying that tempearture can change for condensation . .. My view is that after reaching reaching 100% Relative humidity (by decreasing upto Dew point ) .. when T is decreased further , P is constant , so volume must decrease , so molecules gets compressed , which eventually converts gas to liquid , even though temp is not 100 degree C or 0 degree C
That’s why , only after reaching dew point , crystallisation / condensation can occur

So , comndensation can occur by ,
addition of further water vapour to saturate the air , and further adding vapour beyond the saturation point .. So in moist areas , dew point is higher
reduction of its temperature beyond dew point — In dry area , to achieve condensation , vapour must be cooled to a greater extent i.e. dew point is much lower , bcoz RH is low .. Ex – rajasthan , in nights , water condense – as nights are very cold …. as RH is low..

Latent heat of condensation Condensation releases the energy of vapour to vapour to surrounding air , thus surrounding air gets warmer on condensation.. As energy of vapour is transferred to surrouding air – From this , arises the concept of dry and wet adiabatic lapse rate
This addition of heat lowers the cooling of air / surrounding air

**If air contains small dust particles , condensation starts aftr air has cooled much below its dew point.. Doubt –I think , presence of hygroscopic nuclei must ease the process of condejnstion

For clouds , saturation of air ( by decreasing temp upto dew point or increasing RH) and presence of hygroscopic nuclei – sea sals or SO2 or SO3 or nitric oxides


Beyond 80 km Gas don’t mix so
N2 at bottom
O2 above N2
Helium ( He ) above O2

upto the height of 80 Km gas mixes

Based on Gases M for Mesosphere / M for middle

Thermosphere / Ionosphere + Exosphere Temp increases with increasing height , as sun effect is more pronounced than adiabatic lapse rate Beyond

Mesosphere Temp decreases with increasing height upto 80
In the mesosphere, the temperature decreases with height again, because there is very little ozone to warm up the air.

Stratosphere Temp increases with increasing height , bcoz of Ozone layer upto 50
Upto 50 km at both Equator and poles

Tropopause Isothermal Zone – No change in Temp.
2 km width

Troposphere / Ionosphere Temp decreases with increasing height 0 – 8/18
50% of atmospheric gases lies in first 5 km due to gravity , so convection , conduction , sublimation occurs here
Earth is blue planet , bcoz of Troposphere as it regulates T of earth & gives energy to eart , as most atmospheric gases are here
Equator – upto 18km / Poles – upto 8 km
More loss of temp at equator as width was 18 km.. So Temp at Equator = -79 degree C
Less loss of temp at poles , as width was 8 km .. So Temp at poles = -58 degree C

Based on layers

Pressure Belts

Principles : Covective instability + radiational cooling + corollis + Geostropic + subsidence = anticyclonic
As temperature increases , Gas gets heated up , volume expands , density decreases , so moves up as it becomes lighter
On rising , air cools , loses heat due to release of latent heat of condensation (+ adiabatic heating – I think )
Cold air now starts moving towards heat , and gets cooled by radiational cooling , thus density increases and it becomes heavier .
Also corollis force gets stronger with increasing distance from equator . Which produces a blocking effect and causes subsidence

Thermally induced
Equatorial trough of Low Pressure

Location 7 N – 5 S – ( more heat in norhtern hemisphere ) . In july , extends upto 20 N in Africa
Temp. Very high temperature due to insolation..
Pressure Low pressure , as air rises up as it becomes lighter on being heated causing expansion
Rainfall Heavy rainfall due to convective instability { rising moist air condenses – releases latent heat of condensation in upper layers , gets saturated , and causes rainfall..}
Insolation Insolation is 100 %
Names Zone of convergence / Doldrums { Zone of convergence as trade winds converge , Doldrums as keeps on shifting with shifting of the sun.. Just like drum rolls. So does it..
Corollis Zero
Winds Air rises up + trade winds converge – winds are light and variable with frequent calms..
Mechanism Low pressure is formed due to high insolation as sun rays are vetical , thus very high temp + Tropical warm air being supplied by constant prevailing trade winds

Polar high Pressure Belt

Location 75-90 – ( more heat in norhtern hemisphere )
Temp. Very low temperature
Pressure Fomation of anti-cyclones – Very high pressure due to heavy subsiding air as well as low temp
Insolation Insolation is less than 40%
Names High Pressure polar cap
Corollis Maximum
Winds cold winds subside as winds outflow of cold region –
Mechanism Though corollis is maximum causing thinning of air , but since the temp is extremely low , so thinning effect is -ve , helped by cold subsiding air..

Dynamically induced

Sub – tropical High Pressure belt

Location 25-35
Temp. Air cools due to radiational cooling and becomes heavier , thus subsides after blocked by corollis effect . At surface , temp is high ( btn 25-35 , but subsiding sir is more powerful.
Pressure Subsiding air increases the pressure of the region
Rainfall Descending air is dry , so no rainfall .. Also moisture holding capacity increases as air descends .
Names Horse latitude – bcoz of calm winds
Corollis Corollis force keeps on increasing causing blocking effect and thus convergence
Winds winds after spreading from equator , when spread towards poles is acted upon by geostrophic effect , thus they become westerlies .. On descending forms anticyclonic condition
Mechanism geostratic force + radiational cooling + corollis force

Sub- Polar low pressure belts
Location 60-70
Temp. Temp is low
Pressure Though temp induces high pressure winds , but ascending air due to forced convergence of air , creates low pressure
Rainfall Very low , bcoz of located over land in northern hemisphere

For hemisphere wise distribution of pressure belts – See Savinder singh – Pg 452
For shifitng of pressure belts , see Savinder singh – Pg 453

Planetary wind system But as far as I know , angular momentum at equator is zero..

Equatorial Westerlies angular momentum

Location Found in the doldrum area btn 5N – 5S / btn the ITCZ
Temp. Very high temp..
Pressure Extremely low pressure
Insolation With apparent movement of sun , it also moves – as thus is a fluctuating pulsating line
Corollis Nearly zero , so winds are not deflected , but winds produced due to angular momentum
Mechanism Apart from angular momentum , their origin is the convergence and uplifting of air which is compensated by westerlies flows..

ITCZ The low pressure area – due to meeting of trade winds .
shifts with the apparent shift of the sun.
NITCZ Convergence of equatorial westerlies and NE trade winds btn 5-10 N
SITCZ Convergence of equatorial westerlies and SE trade winds

Trade Winds
Location originate from Horse latitudes . In south hemisphere , more wide spread, constant and prevalent through-out year due to ST high pressure belt
Names Trade wind bcoz they are constant in direction and velocity thru-out the year . The name derived from trade.. To keep track using same direction
Corollis In NH , deflected right forming north easterly , and in the south – deflected left.. Forming south easterly Trade winds

Subtropical Winds

** there is an inversion layer in atmosphere which is not a favourable factor for condensation.. So hot deserts are found on western margins btn 20-30 latitude– 100 % nhi aaya samajh mei..

belt of calm known as Horse latitude

anti-cyclonic condition with clear and dry weather…

Location 35-60/65 – the poleward boundary fluctuates with seasons..
Winds From horselatitude to subpolar low pressure belt -A large number of travelling cyclones and anti-cyclones make the belt in this belt highly variable.. Highly variable winds as travels thru varying continental features .
Mechanism creation of Polar front – Bcoz , During the winter season , the poleward boundary of westeries in the vicinity of sub-polar trough of low pressure experiences great thrusts of polar air …It is here that the great polar air masses moving equatorward clash with relatively warmer air masses from low latitudes . The result is creation of a surface of discontinuity called polar front..

Polar winds

Tricellular Meridional Circulation Horizontal transport of Heat and angular momentum

Potential energy generated by unequal heating of earth…and its atmosphere is continuosly being transferred into kinetic energy by ascent and descent of heated and cold air respectively.

Tropical cell Hadley cell Convective instability + Latent heat + 10 to 30
The meeting of the trade winds in the equatorial region forms the Inter-tropical Convergence Zone (ITCZ). 
The trade winds pick up latent heat across the warm tropical oceans, and are forced to rise by violent convection currents. – bcoz of convective instability
The unstable warm moist air is rapidly cooled adiabatically to produce towering cumulonimbus clouds, frequent thunderstorms and low pressure characteristics of the equatorial climate.
It is these upward currents that form the 'powerhouse of the general global circulation' and which turn latent heat intosensible heat and later into potential energy.

As rising air cools to the temperature of the surrounding environment air, uplift ceases and it begins to move away from the Equator. 
Further cooling by radiational cooling , increasing density, and diversion by the Coriolis force cause the air to slow down and to subside forming the descending limb of the Hadley Cell

The poleward outflow of air from the equator is called the antitrades..
While moving from low to higher latitudes , these upper tropospheric winds are subjected to progressively increasing coriolis force , as a result of which they are deflected and become geostrophic westerlies
high degree of Angular momentum is being transferred

According to Thermal theory , latitudinal temperature difference btn the tropics and the higher latitude is the main driving forve
According to Dynamic theory , instability of equatorial masses ( convective instability ) is the driving force
However , both theories complement each other.

Ferrel cell Polar front cell 30-60 35-65

The surface airflow is directed towards the pole , and bcoz of coriolis force the winds blow almost from west to east
the cell is characterised by multiple weather conditions .
Bcoz of the upper air convergences , a jet stream or high velocity westerlies are created at ht. of 10-12 kms
The horizonta surface movement of the cell is in the form of westerlies which completes the circulation
The prevailing westerlies are disrupted frequently by migratory extra-tropical cyclones and anti-cyclones
D S Lal * A westerly flow exists in the upper troposhere in mid-latitudes .
If we take into account the conservation of angular momentum , then the upper air flow in this cell should be easterly…
But according to Rossby , the westerly momentum is transferred to middle latitudes from the upper branches of the cells in high and low latitudes
Didn’t understand – ———-The cause of upper air westerlies is the poleward decrease of temperature . In winter , when meridional temperature gradient is steepest , the upper air westerlies are most intense.

These upper-troposhere weesterelies are characterized by long waves and jet streams
Refer pg 114 D S lal for doubts The upper air westerlies play a signifant role in transfer of both air and energy

Polar or sub-polar cell

thermally induced cell
Subsidence near the poles produces a surface flow that , while moving towards the equator , comes under corollis force and becomes polar easterly
The cold polar easterlies clash with warm warmer westerlies of temperate regions .
The zone of contact btn these air-flows of contrasting nature is called polar-front
Then , sir is up-drafted and at a altitude of 6-8 km , there is upper air divergences & anti-cyclogenesis and part of air is deflected towards the pole and subsidises near 75-90 & reinforcing High pressure system
This complete the entire cycle
When upper-air divergence is resent , another jet stream is present k/as Polar Jet stream . This is k/for the creation of Rozby waves

Not understood It is characterized by horizontal turbulent mixing at all levels .
Here , heat transport is accomplished by waves in the westerlies .
To sum up , in tropical regions the exchange of heat and momentum is accomplished by direct circulations
In middle to high latitudes , the transfer of mean potential energy to man kinetic enrgy is effected thru the energy of cyclones and anti-cyclones

Significance of meridional circulation the term " meridional exchange " is imp….

most effective channel of meridional exchange of heat and momentum .
The areas of convergence and divergence play dominant roles in meridional circulation

Ekman theory explains the theoretical state of circulation if water currents were driven only by the transfer of momentum from the wind. In the physical world, this is difficult to observe because of the influences of many simultaneous currentdriving forces (for example, pressure and density gradients). Though the following theory technically applies to the idealized situation involving only wind forces, Ekman motion describes the wind-driven portion of circulation seen in the surface layer.[3][4]
Surface currents flow at a 45° angle to the wind due to a balance between the Coriolis force and the drags generated by the wind and the water.[5] If the ocean is divided vertically into thin layers, the magnitude of the velocity (the speed) decreases from a maximum at the surface until it dissipates. The direction also shifts slightly across each subsequent layer (right in the northern hemisphere and left in the southern hemisphere). This is called the Ekman spiral.[6] The layer of water from the surface to the point of dissipation of this spiral is known as the Ekman layer. If all flow over the Ekman layer is integrated, the net transportation is at 90° to the right (left) of the surface wind in the northern (southern) hemisphere.[2]

Jet stream TBD

Defined as swift geostrophic air streams in upper troposhere that meander in relatively narrow belts
These are strong cores of upper level westerly winds which follow a meandering path

The jet streams are relatively narrow bands of stronger winds bounded by slower moving air.
thousands of KMs in length , a few 100 KMs in width and two or more KMs in vertical thickness

It is almost entirely a thermal wind and its strength is proportional to the T contrast thru the whole layer below

Fog and Clouds

Both are same in respect of appearance and structure .
Howver Fog forms near the surface , whereas clouds form at a much higher altitude.
Fog form as a result of radiation cooling or movement of air over a cold surface , while clouds are formed when air rises , expands and cool adiabatically – Note , We were taught that clouds form when water condense due to cooling at upper altitude alround a nuclei and when its capacity is full , it rains… But if , adiabatic cooling doesnt takes place , clouds arent formed.. kyunki cooling se fog banta hai… 🙂
Fog may also form thru saturation of the of the air by increasing its water vapour content


Fog is said to exist , only when visibility is less than 1 km
Fog occur during calm or light wind conditions
Generally whitish , but in polluted cities , it looks dirty yellow or grey

Fog occurs during calm or light wind conditions
It is more common in the vicinity of ocean where there is abundant supply of misture
Common duruing winter


The cooling process that produces fog, frost, and dew is either radiation or advection. On the other hand, clouds usually develop from a cooling process that results when a parcel of air on Earth’s surface is lifted into the atmosphere.

The rising parcel of air will expand as it encounters decreasing atmospheric pressure with height. This expansion allows the air molecules to spread out, which causes the parcel’s temperature to decrease. This is known as adiabatic coolingand occurs at the constant lapse rate of approximately 10°C per 1000 meters At Dry adiabatic lapse rate
However, the rising and cooling parcel of air will eventually reach its dew point—the temperature at which water vapor begins to condense out, forming cloud droplets.
From this point on, the adiabatic cooling of the rising parcel will decrease as latent energy released by the condensation process is added to the air. k/as wet adiabatic lapse rate
*On the other hand, the warming temperatures of descending air allow it to hold greater quantities of water vapor.In other words, as the air temperature rises farther above the dew point, condensation will not occur, so the heat of condensation will not affect the rate of rise in temperature. Thus, the temperature of air that is descending and being compressed always increases at the dry adiabatic rate

Clouds also play an important role in the heat energy budget . This is because clouds absorb a part of incoming solar radiation . They reflect some of the incoming solar radiation back to space and also diffuse some of it . Cloud also absorb a part of terrestrial radiation and then re-radiate it back to the surface . Cloud , like a black body , radiate heat continously in proportion to their temperatures .
So , in absence of clouds , days wud have been much warmer and nights much colder ( tabhi salimpur mei ppl say , aaj badri kail hai.. Isliye garmi bhut hai aaj.)

Maximumcloudiness btn 30-60 latitudes, as there are cyclones and fronts

Classification of Clouds


strato= low level alto = middle level cirro (meaning high-level cloud )

cumulus= a rounded shape stratus = layered shape

nimbus,meaning precipitation (rain is falling )

High – 5-13km Middle – 2-7km Low -0 – 2 km
Cirrus Alto-cumulus Strato -cumulus will form drizzle & most common type of clouds
Cirro-cumulus Alto-stratus Cumulus Foound over oceans
Cirro-stratus Cumulo-nimbus vertical clouds , produced by convection
Nimbo-stratus horizontal – produced by mixing
High clouds never rain & they aree made up of Ice-crystal
Cumulus – mass of H2O My observation : cumulus clouds over stratus , bcoz round shape , high bouyant force – rises
White cloud never rain
Dark clouds rains


It refers to the fall/release of moisture in the form of water droplets or ice crystals , under the influence of gravity

Geographical Adv : Atmospheric circulations are primarily inolved in transfer of moisture and heat . The differences in the moisture and heat brings about the changes in meterological condtn , thus precipation plays a role in transfer them .
Precipitation also plays an imp. Role in running the hydrological cycle which is essential cause of all the life cycles & geo-material processes .

Processes Insolation Evaporation

Gravity falls Condensation

Melting / Sublimation Crystallization

Though all clouds contain water , some produce precipitation
Even when precipatated moisture does fall from clouds , it gets evaporated in atmosphere before actually reaching earth's surface

Colloidal Stability Rain doesn’t occur

After condensation when water droplets have been formed Bcoz in some cases , condensation do not takes place
and the water droplets are of uniform size
and the cloud is stable

When water vapour is cooled down adiabatically , condensation takes place among solid nucleii k/as hygroscopic nucleii .
Condensation means water vapour into ice-crystal or tiny water droplets ranging less than 10 micro-meters in diameter
Condensation takes place at temp. which is k/as Dew point while freezing point of water is 0 degree C – i.e water changes to ice

When clouds are unstable , and the size of the droplets are heterogenous , then colloidal instability occurs & thus causes precipitation

* Only when relative humidity is 100% , the air column is said to be saturated & only after saturation , condensation can take place

Ice – Crystal Theory by Bergeron

Background A water remains in liquid state in super cooled state without freezing , even below 0 degree C
These supercooled droplets freeze when they come in contact with solid nuclei

B vapour diffuses rapidly to ice crystals , so that ice -crystals begin to grow at the expense of water droplets ( I understand that more ice is formed than water , bcoz of Relative humidity concept – See D S Lal pg 181 .. I cudnt understand the concept clearly )

So , When a solid nuclei is introduced into a cloud of supercooled water droplets ( or in an air column saturated with Relative humidity 100% – both mean the same ) ,then condensation takes place forming 1st ice crystal . The diameter of solid nuclei can vary from 1-10 micromater

After forming the first ice-crystal , the ice crystal rapidly grows **************************************This abrupt change i.e sublimation is caused by diff. vapour pressures existing over super-cooled water droplets and ice crystals at the same time.. D s lal pg 181 pptn occur after formation of ice-crystals

The growth of ice-crystals is rapid enough to generate crystals large enough to fall under gravity. When ice crystal grows more than 10 microeter , it begins to fall

***********Due to gravity , it is shattered and again thrown back into the upper layer due to convection . Thus tiny fragments of ice-crystals become the nucleii for further condensation & crysatllisation – Alok ranjan . When cloud is saturated with water , relative humidity factor is 100% and when cloud is saturated with ice -crystals , RH is 115% at 10 degree C

While falling from cloud , the ice crystals grow by intercepting cloud-droplets that freeze upon them . A chain reaction takes place . D S lal

These ice crystals by COAGULATION grow further in size to become snowflakes before leaving the cloud

When clouds are saturated with ice-crystals , the colloidal instability is more that result is their fall.When they enter the warm air crystals are melted and rain droplets are formed

Snowflakes generally melt before reaching the ground and fall as rain

But , for lower clouds . i.e. at an altitude of 500 to 2 km , freezing point is not seen
For cumulus clouds above the ocean , ice crystal theory fails
They best explains clouds for temperate and polar regions

Collision- Coalesence Theory

For cumulus clouds above the ocean , ice crystal theory fails
It is not dependent on formation of ice-crystals

Above the sea surface and coastal areas in upper air , sea salts are in suspendedposition .
This sea salts have diff charges or ionisation . Thus they are mutually attracting each other

Condensation takes place even at a higher temp than freezing point . The oceanic clouds are saturated even at 7 degree C , bcoz the saturation depends on the supply of moisture
Thus condensation are dependent on 2 factors
Vapour Pressure
Mutual attraction btn salts and water vapour
Once saturation takes place & condensation begins tiny water droplets are formed .

since the rate of fall of these unequal particles is diff. , they collide with each other within the cloud , and the larger drop grows

The forms of condensation mutually collide and coalesce , thus larger droplets are formed more than 200 micrometer and fall as precipitation

The colloidal instability is very often reached in the clouds above sea , bcoz at lower height , turbuence is greater

Types of Precipitation TBD

Convectional Precipiation

Orographic Precipitation

Cyclonic or Frontal Precipitation

Distribution of Rainfall TBD

Question Answers

Which of the following climates is associated with dry summer and rainy winter? Read the following statement with respect to temperate cyclone. Read the following statement with respect to Polar Vortex. The temperature and rainfall of a Match List Iwith List IIand select the
(a) Mediterranean 1. They are extra tropical cyclone. 1. It is large scale cyclone located near the geographical poles of the earth. meteorological station are given below: correct answer using the codes given
(b) Tropical 2. They are located at mid-latitude belts. 2. In Northern Hemisphere it circulates in clockwise direction Temp. Rainfall (°C) (cm) below the lists: List I List II (Local wind) (Region)
(c) Tundra 3. They follow easterly path. 3. The circulation of Polar Vortex is due to Coriolis effect. J 9.4 12.2 A. Fohn 1. Argentina
(d) Semi-arid 4. They rotate in clockwise direction. Which of the following statement is F 10.6 9.1 B. Samun 2. Kurdistan
a Which of the following statements with respect to temperate cyclone is correct? correct? M 11.7 9.7 C. Santa Ana 3. California
(a) 1,2 (a) 1,2 A 12.2 2.5 D. Zonda 4. Alps
Where is the doldrums belt located? (b) 1,2,3 (b) Only 1 M 13.3 1.8 Codes:
(a) Near the Equator (c) 1,4 (c) 1,3 J 13.9 0.3 A B C D
(b) Near the Poles (d) All of the above (d) All of the above J 13.9 – (a) 2 4 1 3
(c) Near the Tropic of Cancer a c A 14.4 – (b) 4 2 3 1
(d) Near the Tropic of Capricorn S 15.6 0.8 (c) 2 4 3 1
a O 15.0 2.5 (d) 4 2 1 3
N 13.3 6.1 b
The most important activity of the D 10.6 11.7
Tundra region is Average temperature: 12.8°C
(a) Fish farming (b) Cattle rearing Average rainfall: 54.9 cm per annum
(c) Hunting (d) Cropping Identify the region having the above climatic pattern from amongst the following:
c (a) Mediterranean region
(b) Monsoon region
(c) Steppe region
(d) NW European region


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