(cake analogy at the bottom)
Weather & climate
Illustrative analogies for the difference between weather and climate are quite common nowadays. The simplest description of the difference is that weather is the conditions produced by the atmosphere, and climate is the statistics of weather. ‘Statistics’ could be simple averages, or some much more sophisticated techniques.
The World Meteorological Organisation says that ‘climate’ statistics must be calculated over a period of 30 years or longer, but this is merely a convention. The period depends on the physical phenomenon we are thinking about. The goal of climate statistics is to essentially get rid of the random variation of weather, so the period we choose depends on how much random variation we need to get rid of. For example, I don’t need to watch the Sun rise too many times before I decide that the Sun is probably going to be doing that for the foreseeable future. There is no variation, so I don’t need to watch it for very long.
Using climate to make predictions
Let’s say we want to work out what the maximum temperature on July 14th 2013 here in Reading, UK. The average maximum temperature in Reading in July is 22 C, so I’ll stick my neck out and say that the maximum temperature on July 14th 2013 in Reading is going to be 22 C. A number of things could go wrong here. We might get a weather system coming through which brings down cold air from the north or warm air from the south and this would ruin my prediction. We don’t know what the weather will be doing, but at least we can say that July is likely to be about 22 C. It’s certainly more useful estimating based on past experience than just guessing. For example, I would be willing to bet the maximum temperature isn’t going to be 5 C.
This method only works if we work out the average temperature over a long enough period. If I just take the average of, say, July 2011 and July 2010, I wouldn’t have much confidence in the number I get. That’s because all it takes is a few weather systems changing where the wind comes from to knock the temperature away from what is expected. If we take 30 Julys, though, most of these random variations will average out. The cold Julys will cancel out the warm Julys and we’ll end up with a good expression of the usual temperature.
So climate statistics give us some power in prediction. But we are assuming climate stays the same. Usually, July temperatures in Reading hit a maximum of 22 C. Climate change means that usual temperature changes. If the Earth warms up it doesn’t mean it will never be as cold as 22 C in Reading in July again. We still might get cold spells of weather when the wind blows down from the Arctic. It just means that it will be more usual than before to see a higher maximum temperature.
Analogies for weather & climate
Many analogies are used to express the difference between the weather experienced right now and climate. There are two I especially like:
- A man walking a dog. The man will follow a path and walks with purpose. He knows the route. The dog will dash about, distracted by other animals or curious smells, either side of the path. He can’t go to far from the man because he is being held with a lead. The dog is the weather, and the man is climate. This is the analogy presented in the video at the top of this post.
- Sea level and waves. Consider a boat moored at a harbour. Climate is the level the boat is at. Every now and then a swell comes in and lifts the boat a bit. But it will soon go down again. Most people wouldn’t say the boat is getting higher just because the waves lift it. But if the tide comes in (climate change), the boat will be lifted and keep on going up. But the pattern of swells means there might be times when the boat is going down.
Some have asked whether climate change ’caused’ October’s Superstorm Sandy and the damage it did to the north-east USA. The reality is far more subtle. Climate change does not ’cause’ weather events. The atmosphere causes weather events to happen. Climate change alters the atmosphere, so it stands to reason that it will affect weather, but it won’t cause it. My analogy for this involves cake (because everyone likes cake!).
Let’s say a storm (weather) is like a cake. A storm has a specific set of ingredients, which the atmosphere provides. If we don’t get the ingredients, we’re not going to make a cake. A storm needs air masses to collide and rise, allowing water to condense and fall out as rain. A cake needs a combination of flour, eggs, butter and sugar.Now what happens if we change the ingredients slightly? Say we use wholemeal flour, which makes the cake dense. Or we use more sugar, which makes it sweeter. We’ll still make a cake, but it will be different from before. Changing the ingredients doesn’t change the fact that it is you who has to make the cake. This is what climate change does. It doesn’t make the cake, but it changes the ingredients! The cake will turn out a bit different.
The atmosphere is the only cause of weather. But the atmosphere is changing.