why does the warm air rise up ?
Answers
Answer:
because it is lighter than the cold air.....
Answer:
rises. Everyone knows that, right? It's absolutely true. Heat does rise. The problem is that sometimes people say this as if the flow of heat is driven by its wanting to rise. It's not. Heat can move up, down, or sideways, depending on the situation. What the laws of thermodynamics tell us is that heat moves from areas of higher temperature to areas of lower temperature. Put a torch to the top of a steel pole, and heat will travel downward by conduction. So, temperature difference is really what drives heat to move in any given direction.
When you're dealing with fluids, you have to account for density and buoyancy as well. Air is the fluid we live in, and this time of year we spend a lot of money pumping heat into it in our homes and workplaces. When we heat air, the molecules jiggle and zip around faster, which causes them to spread out. When a mass of air takes up more space, it has a lower density. When you have a lower density fluid immersed in a higher density fluid, the lower density fluid rises and the higher density fluid falls.
Think of air bubbles in water, as shown in the photo above. Think of a helium balloon. Think of a hot air balloon. Now, imagine an object with higher density immersed in a fluid. Put Wile E. Coyote's anvil in the air above his head, and it turns him into a pancake.
The point here is that it's easy to get confused by heat in the building science of air movement. Warm air rises when it's surrounded by cold air because of its lower density. Yes, that's due to heat, but density is the main factor causing the movement here. The name for this phenomenon is stack effect. Two factors affect how much stack effect a building experiences:
Temperature difference between inside and out (because density depends on temperature)
Height of the building
The problem with stack effect in buildings is that buildings aren't vacuum chambers. They leak. Obviously, a house isn't going to start floating up into the air like a balloon (although I recall with great fondness the Disney movies of my childhood that showed such magical events). But the low density air inside the house will move up and out into the cold, dense winter air when given the chance.