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Name: Christopher S.
Status: educator
Age: 50s
Location: N/A
Country: N/A
Date: 2001-2002


Question:
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.


Replies:
Chris,

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
Physics Instructor
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 sinks.

Richard E. Barrans Jr., Ph.D.
Assistant Director
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 place.

Tim Mooney


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.

Best regards

Alcir Grohmann


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 the height.

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.

Vince Calder


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 sit in 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 differential of 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) are equal.

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.

Larry Krengel


Chris,

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 surface.

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!

Ric Rupnik


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.

Dick Plano



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