We have seen in our previous lecture that 
atmospheric pressure is equal to a
hundred and one thousand three hundred
and twenty-five Pascal’s. Now it is quite
a significant amount, but where do you
think this figure is actually coming
from or in other words how do you think
is the atmospheric pressure calculated?
The atmospheric pressure was first
calculated by a scientist who is known
as Evangelista Torricelli. Evangelista
Torricelli used a particular instrument
for measuring the atmospheric pressure.
This instrument that Evangelista Torricelli 
used was invented by him only.
This instrument is known as the
barometer and with the help of barometer, he was
able to calculate the atmospheric
pressure. So let us find out what a
simple barometer looks like and with the
help of it, how we can calculate
atmospheric pressure.
So this is what a simple barometer
looks like.
It consists of a long thin tube. Now
this tube is closed at one end and open
at the other and it is hundred
centimetres or one metre long.
Now initially, this tube is filled with pure
mercury in such a way till the brim
so that no air bubbles exist inside the tube.
After filling it the open end is closed
with the thumb and inverted into a trough 
of pure mercury. Now while inverting it care
should be taken that no air bubbles
escape and move into the tube.
Now once the tube has been inverted and
kept in the trough of mercury, it will be
noticed that the level of mercury in the tube
falls to a certain extent. If we measure
the height from the surface of mercury
in the trough till the point where the
liquid has fallen in the tube, that
particular height or length will be
equal to 76 centimetres. So let us find
out why in the first place
liquid is flowing out, that is, mercury
is flowing out from the tube and into
the trough.
Over here we consider both the initial
and the final condition.
This is the initial condition
and this is the final condition. Now in
the initial condition if you notice,
the liquid that is mercury is completely
full
in the tube and in this manner it has
been inverted into a trough of mercury.
So if you now observe closely,
the pressure that is acting on the trough 
of mercury, that is, on the outside on
the surface
is the atmospheric pressure. Now
initially while performing the experiment,
that is, absolutely at the beginning I
had mentioned that mercury should be
filled in the tube such a way so that no
air exists inside the tube. So what does
this mean? This means that inside mercury 
there is absolutely no air. So as a result what
happens? At this particular point which
is inside the tube atmospheric pressure
is not acting, instead pressure
due to mercury is acting at this
point. So let me call this, so this point
let us consider A and the point outside
C.
So now we found that liquid is flowing from
inside to the outside. So what does
this mean? This means that pressure must be
greater inside the liquid as compared to
the outside. If we compare these two
points now, that is, point A and point C,
we will find that the pressure of the
liquid column that is 76 centimetres in
height from point B is acting at point A
and at point C atmospheric pressure is
acting. Obviously when liquid is flowing
out from the tube and into the trough of
mercury, there is a change in level of
mercury in the trough and a change in
level of mercury in the tube. So now let us
find out why liquid flow is stopping.
So initially mercury flowed from the
tube to the container, that is, from the
tube into the trough and we have studied
earlier that liquid always flows from one
region to another region only when there
is a pressure difference in between
those two regions.
So since liquids or fluids for that
matter flows from the region of high
pressure from a region of low pressure, what can
we say?
We can say that initially pressure
inside the tube was higher than pressure
outside or in other words we can say
that initially pressure inside the tube
was higher, that is, pressure due to mercury
than pressure due to the atmosphere. Now
when the liquid stops moving and becomes
stationary at point B, that is the time
when there is no more pressure
difference in between C and A.
Now at point C, atmospheric pressure is
acting
because point C lies on the trough of
mercury.
However at point A there is no atmospheric pressure,
instead of acting a point is only due to
the liquid column or the mercury column
AB which is 76 centimetres in length.
So what can we say? We can say that this
pressure being exerted by the mercury
column will be equal to the
atmospheric pressure that is acting on
the surface of mercury in the trough. So
as you can see from this equation
pressure exerted by mercury column
that is this, at this point will be equal
to the atmospheric pressure. So now let
us find out how we can calculate the
atmospheric pressure with the help of
this information that we obtained.
So this height, that is 76 centimetres
from A to B has a particular name. This
height is known as the barometric height.
The vertical height of the mercury
column from the mercury surface, that is,
the trough level to the point where it
becomes stationary is 76 centimetres
and this is known as the barometric
height and it is this barometric height
which we will be using in our
calculations for atmospheric pressure.
So consider this scenario where the
barometer, the simple mercury tube of
barometer is taken to 2228 meters
above sea level.
What happens in that case? Since we are
moving up or the altitude is increasing
the atmospheric pressure will decrease. So
as a result when atmospheric pressure is
decreasing it means that pressure inside
the tube is greater than pressure
outside. As a result more mercury will flow
out from the tube and into the trough.
The reverse happens if we go down below sea
Level. Now in this picture we find that
the Dead Sea has been shown.
The Dead Sea is a sea on earth which is
actually four hundred and twenty nine
metres below sea level.
Quite interesting, isn't it? That is, sea
actually exists below sea level. So if
one were to go to the Dead Sea with a mercury
tube barometer, the pressure exerted
on the trough on the surface of the
mercury in the trough would be more as
compared to the pressure inside the tube.
So as a result what happens?
Mercury will flow from high pressure to low
pressure and since the altitude has
decreased, we have gone below the sea
level and pressure has increased on the trough,
liquid, that is, mercury flows into the tube
from the trough.
So this is how the level of mercury in
the tube changes as we go up or down as
compared to sea level.
So now let us find out how we can calculate
the pressure that is being exerted by 76
centimetres of mercury column.
Earlier we found out that this pressure
being exerted by 76 centimetres of mercury column
is equal to atmospheric pressure
because when the mercury becomes
stationary at point B, point B is at a
height of 76 centimetres from the
surface of mercury in the trough and this
is exactly equal to the atmospheric
pressure, that is, the pressure exerted by
the mercury column will be equal to the
atmospheric pressure. So if we can find
out the pressure exerted by mercury column, we
can find out the atmospheric pressure.
So earlier we had found what is the
pressure exerted by a liquid column of 
density rho and height from top H. So over here we
write the density of mercury as rho,
the height of mercury column from the
top as H and
acceleration due to gravity as G.
Now the identity of mercury is 13,600
Kg’s per meter cube.
The height of mercury column is 76
centimetres, so converting it into SI unit,
it will be 0.76 metres and
the acceleration due to gravity is 10
m/s square.
So thus we have this equation as I showed you.
Over here, for more accurate calculation,
the acceleration due to gravity is not
taken as 10 m/s square, instead its
actual value, that is, 9.8 m/s square. So once
this calculation is done, what is the
result we're obtaining? Firstly we find
that
two metres over here are getting cancelled
and one unit metre remains, so the
resulting unit that we get will be Kg
per metre per second square
and the quantity that we get is a
101325 Pascal’s or a 101325
Kg/m per second square. Does this
value seem familiar to you? That's right,
this value is nothing but the pressure that
is exerted by the atmosphere which in turn is
equal to the pressure exerted by 76
centimetres of mercury column. Thus
with the help of barometer, we were able
to calculate the pressure exerted by the
atmosphere, which is equal to
101325 Pascal’s or a hundred and one
thousand three hundred and twenty-five
Pascal’s.
So since the atmospheric pressure is 101325
Pascal’s, we can also say that
this pressure is equal to the pressure
exerted by 76 centimetres of mercury
column at sea level or we can also say
that it is equal to pressure exerted by 760
millimetres of mercury column at sea
level. It's nothing but converting
centimetres to millimetres by multiplying 10.
Now with the help of this barometer, we can
actually use it to forecast the weather.
Weather forecast to an extent is quite
possible with the help of a barometer.
Let us find out how.
So when there is an increase in
temperature of air,
the kinetic energy of the air molecules will
increase. Why, because the kinetic
energy of molecules is directly
dependent on the temperature of the
medium. So due to this, they start to vibrate
more vigorously with a greater speed
because kinetic energy is directly
influenced by the speed of the molecules.
So it is due to this reason that air
expands due to an increase in its
temperature.
As a result, what happens is the volume
increases but however the mass of air
does not change. So density, that is,
is equal to mass by volume, since mass
is not changing but the volume is
increasing the density is decreasing and thus
since density is decreasing, we can say
that the atmospheric pressure also
decreases. Why, because the height from
where we are considering the air column
is not changing. We are considering a
particular height from the top, that
is, the attitude remains constant.
Acceleration due to gravity is also not
changing but density of air is
decreasing and due to that reason the
atmospheric pressure is decreasing.
Conversely, if there is a decrease in temperature
of the air due to contraction of air, the
atmospheric pressure will increase.
Now let's see what happens if there is an
increase in humidity or the moisture
content of air. Now when there is an
increase in the moisture content of air,
that is, when water vapour amount is
increasing, it means that per unit volume,
there is more water vapour as compared to
oxygen and nitrogen and water vapour happens
to be a relatively light gas. So what
happens?
When water vapour content by unit volume is
becoming more as compared to oxygen and
nitrogen, the mass is decreasing.
So if we consider a fixed volume
and if mass decreases, then density will
increase. So since density is decreasing,
in the equation P equals to rho into H
into G. H and G remains the same but
rho is decreasing and thus the
atmospheric pressure, P atm will
decrease.
The converse is also true, that if there is a
decrease in moisture in the air or if
there is a decrease in humidity, the
atmospheric pressure will increase
because then per unit volume there will
be less amount of water vapour
and more amount of oxygen and nitrogen.
So thus let’s see what happens when
temperature increases. We found out that
when there is an increase in temperature
the barometric height will fall. Now
due to this increase in temperature the
barometric height fall will be a sudden
one due to which the pressure will also
fall suddenly. So if there is a sudden
fall of barometric height, it indicates
that there has been a sudden fall of
atmospheric pressure and it usually
indicates the onset of a cyclone.
On the other hand if there is a gradual
increase in moisture, it will mean that
the barometric height will gradually fall,
It will not be a sudden fall. This
indicates that the atmospheric pressure
has also fallen gradually and this
indicates a possibility of rainfall. As
you might be knowing that an increased
quantity of moisture in the air
means possibility of rainfall and this
is how the barometric height is helping
us predict it.
The converse is also true. Let’s say the
moisture content in air is decreasing.
So due to this decrease in moisture content, if it is
gradual then the barometric height will
gradually increase, which will signify
an increase in pressure. So it means that
dry weather is about to come on or the onset of
dry weather is at hand. On the other hand,
if there is a sudden decrease in moisture 
content of air, it will mean that
the barometric height will rise suddenly, which
in turn means that the pressure of the
atmosphere will have undergone a sudden
increase. This means that extremely dry weather
is about to be coming on in the
next few days.
However, if there is no abrupt change in
the barometric height,
that is, if the height of the mercury column
in the tube remains the same, then it
means that normal weather conditions
will be
encountered in the next few days or in
other words there will not be any heavy
rain or cyclonic weather or even dry or
extremely dry weather. Normal weather
conditions that had been prevailing so
far will be there for the coming few
days and this is how we can use
barometric height in order to predict
weather conditions. So taking a quick
recap, we studied about the barometer. The
barometer was an instrument that was
first invented by Evangelista
Torricelli which we use to measure
atmospheric pressure. We found the same, that
is, how we can use the barometer in
order to measure atmospheric pressure.
Atmospheric pressure is equal to the
pressure exerted by the liquid column,
that is, the mercury column when its height from
the trough to the point where it becomes
stationary is 76 centimetres. Under
such a circumstance no mercury is flowing
into
out from the tube and this is why the
atmospheric pressure and the mercury
pressure inside the tube is equal and
this is how we found out the value of
atmospheric pressure, that is, from the
value of pressure exerted by the mercury
column. And we also found out how we can
use the barometer and the barometric
height in order to predict changes in
weather.
