Heat rises. Everyone knows that, right? It’s absolutely true. Heat does rise. The trouble is that sometimes people say this together if the circulation of warm is pushed by that is wanting to rise. It’s not. Heat deserve to move up, down, or sideways, depending upon the situation. What the regulations of thermodynamics tell us is that warmth moves from locations of greater temperature to locations of reduced temperature. Placed a torch to the top of a steel pole, and heat will travel downward through conduction. So, temperature difference is yes, really what drives warm to relocate in any type of given direction.

You are watching: When hot and cold air meet, the hot air rises to the top. which process causes the hot air to rise?

When you’re managing fluids, you need to account because that density and buoyancy together well. Air is the fluid we live in, and also this time that year we spend a the majority of money pumping warmth into that in ours homes and also workplaces. When we heat air, the molecules jiggle and also zip around faster, which causes them to spread out. As soon as a mass of air takes up more space, it has a lower density. As soon as you have actually a lower thickness fluid immersed in a higher density fluid, the lower thickness fluid rises and also the higher density fluid falls.

Think of air bubbles in water, as shown in the photo above. Think of a helium balloon. Think the a hot air balloon. Now, imagine things with greater density immersed in a fluid. Put Wile E. Coyote’s anvil in the air above his head, and it transforms him right into a pancake.

The suggest here is the it’s basic to get confused by heat in the building science of air movement. Heat air rises as soon as it’s surrounded by cold air because of its lower density. Yes, that’s as result of heat, but density is the main element causing the movement here. The name for this phenomenon is stack effect. 2 factors affect how much stack effect a building experiences:

Temperature difference between inside and out (because thickness depends on temperature)Height the the building

The trouble with stack effect in structures is that structures aren’t vacuum chambers. Lock leak. Obviously, a house isn’t walking to start floating up into the air like a balloon (although i recall with great fondness the Disney movie of mine childhood that verified such miracle events). But the low thickness air inside the residence will move up and out right into the cold, thick winter wait when offered the chance.

Try this experiment if friend don’t believe me. Open your pull-down stairs or scuttle hole to the attic ~ above a cold day once your house is warm. Rise up into the attic and then placed your challenge over the hole. You’ll feel the stack effect pushing too many of warm air into the attic.

So, in winter, the warm, low thickness air inside your house wants to rise…if that can. If her house has no leaks, the warm air can’t escape and also do the thing. There’s quiet a pressure difference across the building envelope, however that’s yes sir if the wait barrier’s good. Hopeful pressure within the house with i do not have anything to go because there are no pathways.


Ah, warm air falls! warm sinks. The old expression, “Heat rises,” is not a an easy truth after all. As with many facets of building science, you have to look in ~ the full context to recognize what’s going on.

See more: How Is A Personal Fact Sheet Used, (Steps And Template Included)

Followup Article

Who Knew the stack Effect can Be so Controversial?

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Photo that water bubbles by Christian Haugen from flickr.com, used under a creative Commons license.