Distribution Of Rainfall Video Lecture - Class 9

rainfall let's take a quick look at the
stages in the formation of rain the
first hand is nucleation during which
water collects on condensation nuclei
and forms tiny cloud drops this type of
nucleation occurs in the tropics in the
second stage the crystal droplets
continue to grow in size through
condensation if the drops of crystals
grow large enough they start falling the
amount of rainfall depends on the
moisture content of the air which
depends on three main factors the first
factor is land and sea contrast
more rainfall occurs among coastal
regions since the source of water vapor
the sea is closer than in the interior
of continents the second factor is the
direction of the prevailing winds
onshore winds from the sea
bring more rainfall than winds flowing
from the land which are offshore winds
presence of mountains also affect the
rainfall when mountains obstruct moist
winds they are forced to give rain on
its side of the mountain facing of wind
known as the windward side the side of
the mountain facing away from the wind
known as the leeward side remains dry
the leeward side of the mountain is also
known as the rain shadow since it
receives very little or no rain beside
these factors convection cyclones or the
meeting of warm and cold air masses are
the causes of rainfall
based on these causes there are three
types of rainfall the first type is
conventional rainfall convection
currents are normally set up in the
atmosphere
due to local heating discouraged rise up
and the center and a drawn up from the
sides the very rapid rising of this warm
moist air results in the formation of
cumulonimbus clouds which gives heavy
rain usually accompanied by thunder and
lightning but the rainfall does not last
long these are called thundershowers
convection rainfall is very common in
the equatorial regions where it occurs
almost every day in the afternoon hence
it is also called the four o'clock wave
the second type of rainfall is a relief
or autocratic rainfall which is the most
widespread form of rainfall when
moisture bearing wind encounters a
mountain background it is forced to rise
as the wind rises of the mountains it
cools to a saturation point in further
cooling rain occurs Mahabaleshwar
receives more rain than who name this is
because mahabaleshwar lies on the
windward side of the Western God's while
Kooning lies on the leeward side
the third type of rainfall is cyclonic
of frontal rainfall when a warm air mass
and a cold air must meet the warm air
being lighter is forced over the heavier
cold air the warm air cools on rising
the moisture in it condenses on cooling
beyond the saturation point forming
cumulonimbus clouds which bring heavy
rain cyclonic rainfall is common in
middle latitude where the warm
westerlies meet the cold polar air as
the name suggests this rainfall is
associated with cyclones and depressions
which are centers of low pressure the
distribution of rainfall on the earth
depends on location and the season the
equator region receives conventional
rainfall throughout the year this is
because the rate of evaporation is high
since temperatures in these regions
remain high throughout the year eastern
margins of continents in the subtropical
latitudes come under the influence of
the trade winds throughout the earth
that brings plenty of rain to the
eastern margin
Western margins of continents between
latitudes 40 degrees and 60 degrees come
under the influence of the West Indies
throughout the year which rings plenty
of grain to the Western marjorie's thus
these Western modules the greater region
and the eastern margin in the
subtropical latitudes are the regions of
heavy rain let's take a look at regions
of scattered rainfall polar regions
receive very scanty rainfall this is due
to extremely low temperatures bringing
about very little evaporation and
precipitation
Western margins of continents that come
under the influence of trade rates is
another region of scanty rainfall this
is because trade winds blow from the
eastern direction and they are dry by
the time they come to the western margin
tropical deserts are located on the
western margin
Eastern margin of continents that come
under the westerlies belt is also the
region of less rainfall since westerlies
flow from the western margin they become
dry by the time they reach eastern
margin tempered deserts are located on
eastern margin of continents
terms heavy moderate scanty in case of
rainfall our relative terms in tropical
regions evaporation is high and hence
rainfall of around 200 centimeters and
more annual is considered heavy rainfall
between 100 to 200 centimeters annually
is considered moderate rainfall between
35 to 200 centimeters annually is poor
and rainfall below 35 centimeters
annually is considered scanty in
temperate regions there is relatively
less evaporation and rainfall therefore
a rainfall of hundred centimeters
annually is considered to be heavy
rainfall between 35 to hundred
centimeters is considered to be moderate
whereas rainfall below 35 centimeters
annually is said to be scanty
when the annual rainfall of a place is
less than 25 centimeters it is called a
desert there are many regions in the
world that are identified as desert
the average monthly or annual
distribution of rainfall of a particular
region is generally shown either by
rainfall maps or by either hires a
rainfall map is divided into various
rainfall regions showing different
amounts of rainfall such as heavy
moderate or scandium these regions are
colored or shaded differently isohyets
are lines of equal facilitation that is
the lines to join places having the same
average or monthly rainfall these lines
are drawn at uniform intervals
temperature and rainfall are the two
most important elements of climate not
only temperature but also rainfall of a
place is affected by various factors
which ultimately affect the climate the
amount of solar radiations receive
increases with latitude therefore the
temperature decreases from the equator
to the poles leading to a reduction in
the rate of evaporation hence the amount
of rainfall also decreases
the Earth's atmosphere is heated by the
energy absorbed by the molecule of gases
and other impurities with an increase in
altitude temperature decreases and more
condensation takes place thus the amount
of rainfall increases the direction of
the wind also affects the amount of
rainfall warm winds make the place to
blow into warmer and the cold winds make
the place that blow into colder entre
winds contain more moisture than
offshore winners
Landen see contrast is another factor
that affects rainfall man heats up and
cools down faster than the sea that is
why landed warmer than oceans in summer
and cooler than seas in winter places
near the sea are cool in summer and warm
in winter while interiors of continents
are hot in summer and cold in winter
therefore margins of the continents get
more rainfall than the interiors of
continents mountain ranges also bring
about contracts in rail with the
windward side receiving heavier rainfall
than the leeward side the Himalayas
prevent the cold winds of northern Asia
from flowing into India that is why
ganga plane is warmer in winter than
southern China on the same latitude
ocean currents also show their effect on
rainfall this effect is especially felt
in the coastal areas which have on
children's when these winds blow over a
warm current they pick up water vapor
and bring more rain to coastal regions
winds that blow over a cold current pick
up very little water vapor and give less
or no rainfall at all vegetation reduces
temperature by releasing moisture in the
air which makes the place cooler and
thus it is also a factor because of
which rainfall increases have you ever
seen a rain gauge it is used to measure
the amount of grain also snow fog or
hail is measured by melting the same and
including it in the total rainfall of
the place the simplest type of rain
gauge is a funnel resting on top of a
metal cylinder
the stem of the funnel bosses into a
glass bottle placed inside the metal
cylinder this entire paratus is
protected by placing it inside another
metal casing the rain gauge is placed in
an open area away from trees and
buildings buried in the ground such that
30 centimeter of it remains above the
ground level this prevents splashing and
evaporation of water collected in the
jar
when it rains the rain water passes
through the funnel and trickles into the
glass water at the end of 24 hours the
rainwater is emptied into a graduated
cylinder the graduation on the cylinder
is accurate to one hundredth of a
millimeter the reading the stain
indicates the depth of the rain that has
fallen over an area equal to 100 of the
top of the funnel
the depth of the snow is measured for
the help of a measuring rod the depth of
the snow is measured at several places
and it's average is taken the equivalent
length is calculated by considering 10
centimeter of fresh dry snow equal to
one centimeter of rain effect such as
rainbows thunderstorms lightning thunder
are associated with rainfall let's take
a quick look at them a rainbow is seen
in the sky only when we are facing the
rain and the Sun is behind us
rainbows are caused by refraction and
total internal reflection of sunlight
each drop of rain acts as a prism and
breaks up with a white light into the
primary colors such as violet indigo
blue green yellow orange and red the
second special effect is in the form of
thunderstorms which are severe local
storms accompanied by thunder and
lightning within a thunderstorm cloud an
intense electric field develops with
negative charges collecting in the lower
part of a cloud and positive charges in
the upper part a positive charge is
induced on the ground for several
kilometers around the storm
lightning occurs when the two charges
build up enough to overcome the
resistance affair and a bolt of
lightning is produced Thunder is also a
special effect which is caused by the
rapid heating and expansion of the air
through which lightning passes all the
Thunder is more prominent than lightning
it is not dangerous to conclude we may
say that the manifestations of nature in
the form of humidity and rainfall
brought about by the phenomenon of
hydrologic cycle are imperative for the
survival of the planet that we live in
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