Climate Science FAQs from CPDN, University of Oxford

These FAQs relate to the science that underpins the modelling in climateprediction.net (CPDN). See our Technical FAQs for help with running BOINC software.

If you have any climate questions that you would like answered in this FAQ, please contact us.

• Why is the weather so unpredictable?
• Is making an accurate weather forecast, or climate prediction, a hopeless cause?
• How do clouds and rain form?
• Does the distribution of land and sea affect temperature variation?

Why is the weather so unpredictable?

Well, first of all, it is important to understand that weather is not random, in fact we call it chaotic. If weather was random, it would mean there was no possible way of knowing what it was going to do next, whereas the weather does obey the laws of physics, every effect has a cause. The problem is that since there are so many possible causes, we can’t possibly know about them all. You may have heard of the butterfly effect (which originated with Ed Lorenz in the 1960s) where a butterfly flapping its wings in the Amazon rainforest might, through a long line of unlikely, but possible, consequences, cause a storm over Texas.

Is making an accurate weather forecast, or climate prediction, a hopeless cause?

The answer is no! We need to get an idea of all the possible ways the atmosphere could develop, and what the likelihood, or probability, of each possible way is. The way we do this is by running ensembles of Global Climate Model (GCM) runs. An ensemble is a collection of runs of the same GCM, which differ very slightly in their initial conditions, or their parameterisations. For example, there might be a 1% difference in wind speed over Oxford between the different models.

Ensemble sizes vary hugely. The European Centre for Medium-Range Weather Forecasts (ECMWF) currently uses an ensemble of 50 members to make the weather forecast. In climateprediction.net, we’re hoping for ensembles with millions of members. It will then be possible to build up statistics for how many ensemble members produced each possible outcome.

If we were trying to predict temperature change, our best guess at what the temperature is actually going to do is the one that most runs predicted, i.e. the one with the greatest probability.

How do clouds and rain form?

Clouds and precipitation (rain or snow, depending on where you are) form mainly when warm, humid air is forced to rise. As it rises, it expands and cools and the amount of water vapour that can be held is reduced. Any surplus water vapour condenses and forms droplets, which we see as clouds, and may become large enough to fall to the Earth’s surface as rain or snow.

In the tropics, the ascent is vigorous and huge cumulonimbus clouds (thunder clouds) form, reaching over 10km high and frequently grouping into clusters. They form preferentially over the oceans, where there is a large source of warm water to evaporate. However, in the mid-latitudes, ascent is more localised and less deep, resulting in more shallow, individual cumulus.

Does the distribution of land and sea affect temperature variation?

The distribution of land and sea distorts the simple picture of global circulation; land heats up and cools down faster than water, leading on a large scale to the Asian monsoon but on a smaller scale to sea breezes, a common phenomenon at the coast, where winds blow from the sea during the day, but from the land during the night. The presence of continents which break up the ocean obviously disrupts the ocean circulation. The presence of mountain ranges deflects the atmospheric flow (for example, the Himalayas affect the monsoon pattern), while patterns of precipitation are determined to a large extent by land-sea contrasts, continental land masses, mountain ranges and so on.

There is a lot less land in the southern hemisphere than in the northern hemisphere, so the atmospheric circulation is a lot simpler. For example, the storm tracks are more continuous around the Earth.