Just as precise measurements are necessary in science, it is equally important to be able to make rough estimates of quantities using rudimentary ideas and common observations. Think of ways by which you can estimate the following (where an estimate is difficult to obtain. try to get upper bound on the quantity) :
(a) the total mass of rain-bearing clouds over India during the Monsoon
(b) the mass of an elephant
(c) the wind speed during a storm
(d) the number of strands of hair on your head
(e) the number of air molecules in your classroom.

Explanation

(a) Let the average rainfall recorded during monsoon = h= 100 cm =1 m
Let Area of our country =A
Volume = V= `A xx h`
density of water = p
Here, A = 3.3 million sq km
`=3.3xx10^(6)km^(2)= 3.3xx10^(12)m^(2)`
`p= 1000 kg"/"m^(3)`
Mass of rain-bearing clouds = Aph
`=3.3xx10^(12)xx1000xx1= 3.3xx10^(15)kg`
(b) To find the mass of an elephant, take a boat of area A.
Let depth of boat in water `=d_1`
Volume of water displaced by boat `V_(1)=Ad_(1)`
Let the elephant moves in the boat
Now, depth of boat inside water `=d_2`
Volume of water displaced by boat and elephant `=V_(2)=Ad_(2)`
Volume of water displaced by elephant `=V= V_(2)-V_(1)`
`=A(d_(2)-d_(1))`
density of water = p
Mass of elephant `=Ap(d_(2)-d_(1))`
(c ) The wind speed during a storm can be calculated by using a gas filled balloon.
Let OP= Normal positions of balloon when there is no wind.
Suppose the wind starts blowing towards right, the balloon shifts to position Q in one second.
Let `theta` be angle of shift.
h= height of balloon from ground
Distance travelled by wind in 1 sec. = PQ= x= `h theta`
This is wind speed.

(d) Let area of our head = A
Let thickness of hair =2t (diameter)
area of cross section of hair `= pi t^(2)`
No. of strands of hair, n
`=("total area")/("area of cross section of each hair")= (A)/(pi t^2)`
(e ) Let volume of room = V
Let one mole of air occupy a volume of 22.4 litres `=22.4xx10^(-3)m^(3)`
No. of molecules in `22.4xx10^(-3)m^(3)= 6.023xx10^(23)`
No. of molecules in `Vm^(3)=(6.023xx10^(23))/(22.4xx10^(-3))xxV`
`=2.68xx10^(25)V`.