Why Does Hot Air Rise Above Cooler Air?

[DaveC426913]

With the exception of a couple of recent blips, this thread is 8 years old.

 

[JakusLarkus]

I'm aware that this question has been very successfully answered, but allow me to post a more equation-based contribution;
In accordance with P=m/V, if we lower the density of a subsystem (here, the hot air), then providing that its mass does not change, the volume of the subsystem increases. This causes the mass of the hot air to become lower in proportion to the volume.This, due to the weight law W=mg causes a lower overall gravitational effect on the subsystem. Hence, the effect of the Earth on the hot air (lowered in density by an increase in Ek) becomes less significant and so it rises above the cooler air (which is more greatly affected by gravitation).
Hope this helps ^.^

 

[JakusLarkus]

when air gets heated it starts rising because on heating it's molecules gets charged in a way that they repell cooler molecules around them and gets repelled from the Earth as well

Mmh... not technically true, but a very interesting principle. As previously stated, in accordance with General Relativity, a particle's mass increases with its energy. Unfortunately this has no effect on the overall charge of a particle or indeed an atom as the relative electrical energy inside the atom remains proportional. In fact, there is no effect.
For example, the mass of an electron is 9.11x10^-31kg and its overall charge is deemed '-1'. A particle experiencing a change in Ek (due to heating) would be subject to a change in mass, but not a change in overall charge. Were this incorrect, particles would behave entirely differently in relation to modern observations; they would have a stronger repulsive effect on some particles and a stronger attractive effect on others (possibly infinite when traveling at c).
There's also no reason why only cooler molecules would be repelled, nor why they would be necessarily repelled towards a gravitational field (i.e. downwards).
I hope I've provided a valid insight =)

 

[Philip Wood]

Just think about an imaginary bubble enclosing the gas to be heated. V is proportional to T at constant p, so the gas in the bubble expands, becomes less dense and rises (if it is surrounded by cooler, denser gas).

 

[dougy]

I have an answer that I think hasn't been given yet.

Consider a pocket of hot air surrounded by colder air. The cold air is slightly less dense above the pocket than it is below it, because there is less weight pushing down the air above than the air below (and so the air molecules above the pocket are less compressed together than the air molecules below the pocket). By definition the hot air molecules have higher velocity than the cold air molecules, but since the cold air molecules above are less packed together than the cold air molecules below, the hot air molecules going upwards will be able to go further than the hot air molecules going downwards. Since statistically there are in the pocket about as many hot air molecules going upwards than going downwards, on average the hot air molecules will rise.

 

[klimatos]

By definition the hot air molecules have higher velocity than the cold air molecules, but since the cold air molecules above are less packed together than the cold air molecules below, the hot air molecules going upwards will be able to go further than the hot air molecules going downwards. Since statistically there are in the pocket about as many hot air molecules going upwards than going downwards, on average the hot air molecules will rise.

It doesn't work that way. When a mass of air moves, there will be more molecules moving in the direction of movement than in the opposite direction, but the relative proportions are still pretty equal. The differences in mean molecular speeds in each direction are negligible at normal wind speeds. An air mass will move from A to B only if the molecular flow rates (not speeds) are greater from A to B than from B to A. As I showed in an earlier post (#60) in this thread, molecular flow rates increase with a decrease in temperatures under isobaric conditions. Therefore, winds blow from cooler to warmer areas under those conditions.

 

[mfb]

This thread is from 2004 and the answer has been given long ago, it is time to let the thread rest.