Why does a flame burn upwards?

Why does a flame burn upwards?

We are familiar with how flames burn. Let it be a tear shaped candle flame or a forest fire, it always burns upwards. We will look into the details later, but the following image released by NASA says it all - 

An illustration of how candle burns on earth and in zero gravity

Courtesy: http://science.nasa.gov/

Gravity does the magic: 

Its obvious from the above illustration that gravity has a role to play. However, one can't help but wonder why a flame would go upwards where there is strong gravitation pull like on earth and remain spherical where there is no gravity like on a space station (Is there really no gravitational pull in space?). The answer is buoyancy.

When you start a fire, fuel combines with oxygen to release heat, light, carbon dioxide, water vapor, soot etc. The heat energy generated in this process heats up the air around the flame which in turn reduces its density** (a good analogy is water vapor which is hotter and less denser as against water which is colder and heavier).

This hot air around the flame then starts rising up and the surrounding colder heavier air rushes down into its place accelerating the hot air upwards which in turn causes the flame to shoot up (a good analogy here is a wooden log dipped in water. the buoyancy pushes the log to the surface). Basically the buoyancy shoots the flame up. It is this hot air rushing upwards that also causes fire to flicker.

The following image shows what happens to the air around a flame.

Hot air around the flame rises up shoots the flame upwards

However, in a micro gravity environment, there is no reason for the heavier air to fall down and rush into the place of the lighter air. So the flame just remains a spherical blob as you would imagine. 

So there you have it. A combination and gravity and gravity induced buoyancy causes the hot air to rise up causing the flames to point upwards here on earth.

**Just in case you are curious as to why hot air is less denser than cold air in the first place - the heat gives the molecules in the gas higher (kinetic) energy. So these molecules can move faster and further apart because they now have the energy needed to overcome their binding forces. The molecules being farther apart means that the material is less dense.