Hot Air Rising
Name: Christopher S.
Every High School text book I have ever read says that hot
air rises because it is less dense than the surrounding air. This makes
no sense to me whatever. If, presumably, the motion of the particles in
the hotter air are random, where does the "up" direction come from? Does
it not make more sense to say that the cooler, more dense, fluid is
pulled down thus "pushing" the less dense fluid up? The first action is
the down action of the more dense fluid, FOLLOWED BY the warmer fluid
rising, rather than the other way around, which, as I say, is the way ALL
textbooks I have read explain it.
What you say is an excellent observation, except for one detail. Both occur
at the same time. Motion of fluids (liquids and gasses) is in fact a
molecule by molecule process.
Dr. Ken Mellendorf
Illinois Central College
You are exactly correct. There is no reason for any substance, hot or cold,
to independently move in opposition to gravity. Heat does not rise; cold
Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois
Your explanation is correct. You do not need the "FOLLOWED BY" part,
though. Cold air cannot come down unless hot air rises to take its
Hi, Christopher !!!
Well, let us think of a balloon, as our system.
he gas inside it has a random movement and
is less dense than the air outside our balloon.
Let it free to move. The balloon flows upward. Why ??
Well, as you know, the gravity force, acting downward,
results in a force from the bottom to the top, and the
balloon goes up !! Over the superior face of the balloon
you have a great mass of air, acting upon it. But,
against it, under the inferior face of the balloon there
will act a bigger mass of air, that weights more. The
difference in weights results in a force upward.
And the balloon goes up.
If there were no gravity force, there will be no forces
acting upon the balloon.
The same happens with our hotter air surrounded
by the colder air. The random movement of the hotter
air - considered as a system - has no effect upon the
resulting force. And the colder air flows down, to the
place where the hotter air was before ( otherwise
a vacuum will be produced...what impels the colder
air to flow and fill that empty place ). I did not mention
here the action of winds or other atmospheric phenomena
just to make things easier.
It seems to me that saying that "hotter air is less dense" or "colder air
is more dense" is equivalent. That is, symbolically: HH
assuming the usual rules of numbers. Nonetheless, the "up" is provided by
gravity. The potential energy being given by:
rho*g*h where rho is the density, g is the gravitational constant and h is
Perhaps a better way of visualizing the difference is to apply Archemedes
Principle that a body (hot air) is buoyed up by a force equal to the volume
of the displaced fluid (cold air) in contrast to a body (cold air) is buoyed
up be a force equal to the volume of the displaced fluid
(hot air). Since a volume of cold displaced fluid weighs more than an equal
volume of hot displaced fluid, the buoyancy force on the hot air will be
greater than the buoyancy force on an equal volume of cold air.
There is truth in what you say, but the real answer is a relative
one. Here is
how I would describe a hot air balloon rising.
An object in a fluid - liquid or gas - experiences a buoyant force because of
the differential pressure of the fluid at differing depths of the fluid (i.e.
the deeper you go the greater the omni-directional fluid force). As you
your chair, there is a buoyant force because the lower part of your body has
greater air pressure pushing - including up - than the upper part of your body
where there is less air pressure pushing - including down. The
these two pressures lifts you up in the fluid, but the force is not enough to
overcome the force of gravity... your weight. You do not rise in the air.
If you are immersed in a denser fluid, the differential pressure becomes
greater and therefore the buoyant force is greater. When it becomes great
enough, you are forced up until the forces (differential pressure vs. gravity)
In the case of a hot air balloon what is changed by the heating of the bag is
the mass of the bag and therefore its weight. When the buoyant force is
greater than the force of gravity, the balloon rises.
Hope that thinking helps.
The way I view it, the warm air is in fact lighter and
does rise compared to the heavier cooler air which
then occupies the area below the warmer air. The
whole thing has to do with gravity, so there is in
fact an "up" and "down" component to the movement.
A good example of this is what happens on a typical
summer day over land. The sun's energy heats the
ground which radiates heat to the air near the ground.
This warmer air (less dense) is lighter than the
cooler air around it and begins to rise compared to
the cooler, heavier air. As the heating and rising
continue there is motion in the atmosphere which
ultimately results in the formation of clouds, the
transfer of energy into them and the formation of
strong convective forces with resultant thunderstorms.
If warm air were not lighter and did not rise, our
weather would not resemble its current profile. The
clouds would not form, because the warm air currents
which can hold more moisture and carry that moisture-
laden air upwards would not form; temperatures at the
ground would increase, and increase, and increase.
Life itself would likely take the form of organisms
which could tolerate a wild swing in
temperature.....low temps at night, hot hot hot
temperatures during the day. Wind would cease to
exist because there would be no lateral movement of
cool air to replace rising warm air at the ground's
I hope this presents a view on motion in the
atmosphere and its cause...the differential heating
and rising of air compared to surrounding cooler air.
Thanks for using NEWTON!
Your understanding is very good! However, I would not time order the
factors. It is, of course, just Archimedes Principle that the less
dense hot air "floats" on the denser cold air. Both the hot air and
the cold air are pulled down by gravity. However, the pressure at any
depth is greater in the cold air than in the hot air and it is this
difference in pressure that causes the net force that causes the hot
air to rise.
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Update: June 2012