Archimedes's Principle

If hot air balloon rises, it needs a larger force pulls it up to counter its  weight that pulls it down. This upward force is called buoyant force. That’s why the balloon can rise and float?

This buoyant force concept comes from a famous principle called 'Archimedes Principle'. Now, let’s analyse how can we can relate it to this principle...

Archimedes Principle
If an object is fully / partially immersed in a "FLUID" the buoyant force is equal to the weight of the fluid displaced.


THEN according to Archimedes principle..we can deduce a formula like this:

Buoyant force  = Weight of AIR  displaced  ( since W=mg)
Buoyant force  = mg  [(mass of air displace ) x (g)]
(since density ρ = m / V ,  so m = ρV)

Therefore,
Buoyant force  = ρVg (air displace)
                          ( ρ =density, V =volume)

From the formula, Buoyant force depends on the air density.
We can say that the bigger the density, the larger the buoyant force...

Early in the morning, the air is cooler than in the afternoon. Cooler air is denser than hot air, i.e. it has higher density. According to the relationship above, cooler air will result in bigger buoyant force. So the balloon has a larger buoyant force to counter its weight. Thus the balloon will be easier taking off in a cool temperature rather than during hot temperature. This explains why the organiser choose those two session, i.e. early in the morning and in the evening.

Another point, when buoyant force is bigger, so we can carry more loads. Thus if the same no of 'burners' is used, more participant can be carried in the morning compared to afternoon.