13 Mar 2012

To shine or not to shine?

Whilst the forecast for most parts of the UK has gone to plan during the last few days, for some places it hasn’t.

The difficulty at the moment is forecasting exactly where cloud breaks are likely to develop and thus how much sunshine there will be – something that has a huge impact on how warm it feels at this time of year.

You probably think that the weather is really easy to forecast when there’s a big area of high pressure across the UK, but actually it can provide some of the greatest challenges to us weather forecasters, as well as the weather computer models that we use.

The weather computer models work by dividing our atmosphere into grid boxes, within which complex calculations are made to determine the weather conditions likely at each point around the globe.

In simple terms, these calculations focus on the state of three fundamental things that influence the weather on our planet: pressure, temperature and humidity.

Air pressure determines the weight of air above a particular location. If the pressure is high, then the air sinks and the weather is likely to be settled. If the pressure is low, then air is more likely to rise and form clouds and unsettled weather.

Temperature not only affects the nature of what falls from the sky, but also drives the jet stream. Contrasts in temperature horizontally and vertically in the atmosphere help create wind and aid the transfer energy from one place to another.

Humidity relates to how much moisture is in the atmosphere. Whilst in some parts of the world, such as the tropics or deserts, it doesn’t change much over time, in more temperate latitudes it varies hugely. This is what makes the weather across the UK some of the most changeable in the world.

So why do weather computer models struggle to get the cloud detail right when the weather is so quiet?

The reason is that cloud cover is parameterised, meaning that a number of assumptions and simplifications have to be made as to how cloud cover is worked out in each of the grid boxes that I mentioned a little earlier.

The Met Office NAE model that covers the UK has grid boxes that are 12km by 12km in size.

In the current weather set-up, there’s a lot of cloud in the lower part of the atmosphere and not much wind to stir things up. As a result, the computer model can have difficulty working out exactly what’s happening with the cloud cover in each 12km by 12km grid box.

For example, the model may have calculated that in one grid box there’s full cloud cover, when in reality the cloud has broken up, leaving just 50 per cent cloud cover.

This may not sound like a big deal, but when an incorrect assumption is plugged into the bigger, evolving picture, it can give a significantly different forecast compared to what is happening in reality.

So the next time you hear me saying that cloud amounts are uncertain and will tend to vary on the TV forecast, you know why!

Don’t forget you can get the latest five day forecast on the Channel 4 Weather website and you can contact me on Twitter – @liamdutton

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