Which Best Describes How Air Moves During Convection

Simple Answer

Warm air rises during convection. Perhaps better said, natural convection of air is the rising of less-dense warm air relative to the more-dense cold air.

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This is a simple matter of buoyancy. Gravity pulls everything downward, but the pull is proportional to mass and items immersed in a fluid that have lower mass per unit volume than the fluid will tend to float, i.e. experience a buoyant force up as gravity pulls harder downward on the surrounding fluid.

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Causes and Explanation

Everyone says “Warm air rises” and that is an example of natural convection. Convection requires a fluid, heat source and gravity. The heat source causes a temperature gradient in the fluid so that buoyancy of the warmer fluid causes it to rise.

Buoyancy is the reasons bubbles rise and hot air balloons float and anything light, like a piece of wood, raises to the surface. The gravitational force experienced by a volume of material is just its weight, i.e. mass times gravity. Mass is the density of the material times its volume. So, if the object (like wood) has a lower density than water, the force of gravity on the object is less than the equivalent volume of water. The buoyant (upward) force is the difference in the wight of the object and the weight of the same volume of liquid.

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Light objects experience the upward buoyant force and if the fluid (air or water or any fluid) is heated and expands, then it has a larger volume and less density and it also experiences the upward force.

The natural buoyancy of warmer fluids causes the force that moves the fluid and that movement is what we call natural convection.

Other Aspects

1. Convection is why we say “heat rises.” And, an interesting side observation is that when water cools near the freezing point, it actually gets lighter as the temperature drops from 4 C to 0 C, hence causing the cooler water in that range to be less dense and more buoyant.
2. There is also something called forced convection where fluid is moved by artificial means such as air pushed by a fan. Most home heating occurs with “forced convection” but years ago natural convection resulting from buoyancy of hat air was the cause for air circulation.
3. Convection is a main driver of our weather, but locally and globally. Surface air warmed when the sun sticks the Earth and is changed to heat which gets absorbed by the air. Surface air tends to rise and takes surface moisture with it. If there is enough of the moisture, it comes back as rain somewhere else. Globally, the oceans give up most of the moisture we later experience as rain and the warmest area of the planet, near the equator, produces large scale convection of air that is essential to global weather patterns.

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