The main reason that hot air rises is because sinking cold air pushes it up. However, other things like mountain slopes can also cause hot air to rise. When any substance is hot, its molecules are farther apart than when it is cold, reports History for Kids. This makes hot air less dense than cold air and lighter per cubed square foot. These air currents also create storms, including hurricanes and tornadoes. Hot air rising and colliding with cold air is what creates thunderstorms.
Cumulus clouds look like fluffy clouds. The University Cooperation for Atmospheric Research reports that cumulus clouds usually have a flat base and are sometimes only feet above the ground. These clouds typically grow upward and can grow into thunderstorms. Violent tornadoes are also associated with cumulus clouds. Hot air rises because when you heat air or any other gas for that matter , it expands.
When the air expands, it becomes less dense than the air around it. The less dense hot air then floats in the more dense cold air much like wood floats on water because wood is less dense than water. This floating effect in a less dense medium is called a buoyant force or a displacement force.
We can model what I just said with a little physics. Consider the air to be an ideal gas this is a good approximation which neglects the interaction of air molecules with each other. Say you start with an amount of air N at pressure P, volume V 1 , and Temperature T 1 and heat it in a balloon we will neglect the mass and elastic properties of the balloon for simplicity to temperature T 2.
Now the force of gravity on the hot air F g is given by the mass of the hot air times the gravitational constant g 9. To be picky I try to avoid saying that heat rises for other reasons. I say that heat moves toward cold. This is particularly obvious with conduction and convection. Your observation that in the warm months convection currents can draw warm air into the conditioned space from an attic is an important and often neglected point.
The issue of warm air rising has created much confusion among energy professionals and home owners. In an effort to clear the air, so to speak, I would like to discuss better wording that may help to correct this long ignored science.
Warm air moving up is indeed what we see, but warm air has no internal energy that makes it move up or down. It is entirely controlled by gravity. Much more to be said. Armand M. Bud P. Buoyancy is the relevant principle here, and Archimedes figured it out thousands of years ago. What is the better wording that you suggest?
Since warm cannot rise of its own accord, there is no pulling effect behind it and thus the implied negative pressure. Attic ventilation is an example where you often hear that the warm air rises to exit the upper vents and pulls in its replacement air. The objective here is to dispel the notion that warm air is leading the way in the process of convection, when in fact it is the cold air that initiates the process.
It seems like semantics until you try to calculate the moving forces behind stack effect and find there is no equation for the mythical upward force from the warm air. Buoyancy is indeed the principle involved and it is the difference in the weigh of warm vs cold air. Allison, well put.
I always Allison, well put. Also, I have a link to an excellent photo of a house-shaped hot air balloon. At the bottom, the pressure difference is reversed positive outside, negative inside.
Leaks at both the top and bottom are necessary for air to keep moving. If the top is airtight, the pressure difference at the bottom goes to zero. If the bottom is airtight, the pressure difference at the top goes to zero. If air leaks out through the top of the house, more warm air rises inside the house to take its place. Great article Allison. I will Great article Allison.
I will share it, so some of my green building buddies can read it. Funny thing about the semantics conversation is that usually when this is brought up, it leaves out the comprehension level of the average home owner. Which is whom I serve. I already see the blank stares on their face when I try to explain air infiltration based on pressure planes and differentials and the stack effect.
I disagree. I just finished I disagree. I just finished reading Dr. But I found the article clear and easy to understand. The pressures at the top will be identical because each will have an identical amount of air above them. As the cold air moves due to the difference in pressure at the bottom it creates a reduced pressure at the top resulting in the warm air being pulled in the replace it.
It is obvious that both must move, but the warm air cannot rise by itself and therefore cannot be credited with pulling the cold air in behind it.
Convection is also the movement of air and since it is the increased pressure that determines the direction of that movement, I give credit to the cold air.
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