Weather school: Conduction, convection and latent heat

It takes lots of heat to evaporate water, so it releases lots of heat when it condenses

This is one of those months where the end of the month and my deadline just don’t match up. With the volatile temperatures we’ve seen during March, I just couldn’t try to do a monthly weather roundup with five days still left in the month. So, the weather roundup and the look ahead will have to wait another week. While it might be spring break, for us, it’s still school time — weather school.

So far in weather school we have looked at solar energy, seasons, the composition of the atmosphere and the absorption and reflection of solar energy. This week we will take a look at just what happens with the solar energy that is absorbed — that is, we’ll look at how this absorbed energy is transferred from one area to another.

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When solar energy is absorbed by an object, the molecules in that object are excited, which causes them to vibrate quicker. The faster they vibrate, the warmer the object. So, it is fairly apparent why objects will get warm, but now the question is, how does this heat energy transfer from the object to the atmosphere?

The answer lies within four different processes: conduction, convection, advection and latent heat transfer.

Potential energy

Conduction is the simplest process to understand, as it is the transfer of energy from one molecule to the next. As solar energy strikes a surface, the molecules in that object gain energy and begin to vibrate faster and faster; the object warms up. If you were to put your hand on that object, the molecules on the surface of the object would be vibrating next to the molecules on your hand and some of that energy would be passed on to the molecules in your hand. Now because the molecules in your hand are vibrating faster, your hand will begin to feel warm. This is conduction of heat.

Solar energy, in the same way, strikes the Earth’s surface, causing molecules to vibrate and heat up. The molecules in the air immediately over the ground surface begin to vibrate faster as they come into contact with molecules in the ground, thus they heat up. This process will only be able to heat about the bottom two centimetres of the atmosphere, so now the question is, how does this heat get transferred throughout the atmosphere?

You guessed it; this is where convection, advection and latent heat transfer come in.

Basically, convection and advection are very similar. They both refer to the physical mixing of the atmosphere. Convection is when the mixing occurs primarily in a vertical direction and advection is when it occurs in a primarily horizontal direction.

How convection takes place has to do with density and the fact that less dense objects are more buoyant. When part of the atmosphere is heated through conduction, molecules are vibrating faster — and that faster motion also means they need more space around them. Taking up more space means there will be fewer molecules in a given area. Since density is calculated dividing mass by volume, fewer molecules (less mass) in a given volume of air would result in a lower density of that air.

Now, because the air is less dense than the air above it, that air will begin to rise up, taking heat energy from Earth’s surface and moving it into the atmosphere.

The final process for moving heat energy around is by latent heat transfer. The term latent means something potentially exists but is not currently in existence or realized. In latent heat, we have heat that exists but is not actually present as heat… yet.

So how does this work? It has to do with water and the fact that it takes heat energy to turn water from a liquid to a gas. As water absorbs solar energy, its molecules are getting excited and vibrating faster and faster. Eventually, the molecules at the surface of the water will vibrate fast enough to break free from the rest of the water molecules and float away. The liquid is now a gas — water vapour. What’s interesting is that the heat energy it took to cause the liquid water molecule to become a gas molecule is still contained within the gas molecule; the potential energy is there.

The evaporated water molecule floats away from where it acquired its heat energy and at some point, it will begin to lose some of its energy and cool down. As it cools, it will eventually condense back into a liquid and at that point it will release all the heat energy it took to evaporate the water in the first place. Since it takes a lot of heat to evaporate the water, it releases a lot of heat when it condenses. We will explore this more in the future, especially when we talk about thunderstorms.

About the author

Co-operator contributor

Daniel Bezte

Daniel Bezte is a teacher by profession with a BA (Hon.) in geography, specializing in climatology, from the U of W. He operates a computerized weather station near Birds Hill Park.

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