Let us start by looking at a couple of examples.

First of all you take a pie out of the freezer and put it in a hot oven. After about 20minutes you check its temperature – it is still frozen in the middle and at 0ºC. After another 20 minutes you check its temperature and you find that it is well over 70ºC. What has happened is that for the first 20 minutes all you are doing is melting the ice. Its temperature will not go beyond 0ºC while there is ice there.

Another example boiling a pan of water. As you heat it up you will see the temperature rise until it reaches 100ºC. At that point it does not matter how much you heat it up all you do is boil off more water and the temperature does not go above 100ºC.

All the extra energy you are putting in is just converting the liquid water into gaseous water. If you put a lid on then this gaseous water will condense on it giving up some of this energy (the pan lid gets hot) and the hot water goes back into the pan. It takes less energy to boil a pan with the lid on since you get some of the energy back when the water condenses on the pan lid.

Greater Than 100% Efficient

You will sometimes see the efficiency rating of boilers quotes as over 100%. The reason for this is that there are two ways that you can measure the heat you get from burning natural gas (methane). The first is where the water produced is in the gas phase:

If you have a condensing boiler then the water is condensed into a liquid state.

In this case you not only get the heat from burning the methane but also the heat released when you condense the water.

This is often called the latent (hidden) heat of vapourisation. There is also something called the latent heat of fusion which is the hidden heat when you melt or freeze something.

For water the heat of vapourisation (2265KJ/Kg) and fusion (334KJ/Kg) are very large.

The Melting Arctic

As I said at the beginning of this post it takes the same amount of energy to melt one kilogram of ice as it does to heat that water from 0ºC to 80ºC. This is one of the reasons that we should be concerned about the melting of the Arctic ice sheets. This is not to say that we would expect the sea in the Arctic to suddenly warm to 80ºC after the ice has gone but the affects will be very profound.

At the moment the Arctic acts as a air-conditioning unit for the Northern Hemisphere. It is a large mass that cannot go above 0ºC no matter how much energy it absorbs. All that happens is that more ice melts.

Not So Hot In The Tropics

At lower latitudes we have a related effect. Not all the extra heat we get from the sun goes into heating the air or the ocean. Some of it results in increased evaporation of the water.

As with the pan lid this energy is not lost but will be given back again when the water vapour condenses. Rainstorms produce their own heat.