How To Forecast The Weather

The Naked Scientists spoke to Alex Hill, Head of the London Met Office
09 April 2006

Interview with 

Alex Hill, Head of the London Met Office


Chris - Tell us, what goes in to making a weather forecast?

Alex - Information. Vast amounts of information just about sums it up really. We have everything from satellites to men, to aircraft to get a description of how the globe is behaving at one particular time, so at the start of the day for example. Then the great number cruncher in the sky which we call our Met Office computer then integrates over large numbers of differential equations for thousands of spots and thousands of feet up in the air. So we essentially describe the atmosphere in a computer.

Chris - But what sorts of things are you measuring to do that?

Alex - Actually just two things principally: just wind and temperature. Everything else can be derived from that. You get an idea of wind and temperature and you get an idea of humidity from satellite pictures and clouds, that sort of thing. Once that's all in the model, the machine has a kind of description for how it's likely to behave and then we run that model. It runs very very quickly.

Chris - But you're continuously refining this model presumably.

Alex - Oh yes.

Chris - Who came up with it?

Alex - The first one was way back in the early parts of last century. Two chaps called Richardson and Fry worked out a lot of the basic mathematics of it. They reckoned that we didn't really see the first computer working on it until well into the fifties, and that computer if you ever see a picture of it is very different form what we use nowadays. It was the size of a four storey house and was full of valves and probably had less computing power than you're average pocket calculator these days.

Chris - So basically you have a model that says these are the readings we have across the country and the world and these readings usually add up tot he following happening. So that's going to be our prediction for tomorrow. Is that how you do it?

Alex - That's a fairly simplistic way of looking at it. It's a lot more complicated than that. Basically what the model does is describe the atmosphere first of all. This gives you an idea of what the atmosphere is behaving like. It then breaks it down into blocks, and within those blocks and the layers in the upper atmosphere and indeed down into the depths of the ocean and how the land behaves, we have that model as well in the computer.

Chris - Where are you getting all this data from? The temperature and the wind speed: where's that coming into you from? Who's measuring it?

Alex - Everyone. It's a totally international operation.

Chris - What are they doing? How are they getting that data?

Alex - Lots of it is just a man going out and measuring the data and sending the message in on a teleprinter. Or you can get readings from satellites or aircraft. You can get direct readings from ships and oil rigs. There are lots of different ways of getting the information into the machinery. We also have radio balloons that carry instrument packages into the top of the atmosphere, and that's a regular occurrence. It happens virtually every six hours.

Kat - So in terms of wetness and humidity, how can you tell by looking at the clouds how wet they are? Or do you measure humidity as well?

Alex - Well we do measure humidity, but it's a derived product. It's not a direct measurement in many cases. You simply take a parcel of air and cool it. If you cool it enough you eventually form a cloud. That's the start point. That's the basic science behind it.

Kat - I'm pleased that we've got loads of weather experts because there's something I've always wanted to know ever since I was a kid seeing mackerel sky. What causes it and what does it mean?

Alex - There are lots of these but mackerel sky is just a very chaotic part of it after a cold front has gone past. It's just the remnants of the cold front with the sun going through it.

Kat - It's a beautiful regular pattern of clouds. Thank you.

Alex - I don't know if you've seen all the fuss recently in the newspapers about cloud watcher.

Chris - They go looking for noctiluminescent clouds.

Alex - Yes, they go looking for lots of different clouds these days.

Chris - Can I just ask you one quick question that just popped into my head. I've heard the old explanation red sky at night shepherd's delight plotted out. It's that in a country, the weather principally comes from one direction towards the other. Seeing as the sun always rises on one side and sets on the other, if you've got clouds going away from you towards where the sun is setting, then that must mean that there is a clear sky where the sun will be rising the next morning. This explains why the sky is red at night, because there are clouds there, and explains the shepherd's delight because there is a clear sky the next day. Is this true?

Alex - No.

Chris - Why is it then?

Alex - Well I say no, but a lot of these things are fun. In certain situations they will work. If the wind is in the west and the weather is coming from the west, then fair enough. If you have a red sky at night, the chances are that you may have a reasonably fine day the following day. If, however, the wind is in the east and you get a red sky chances are you're in for a cold wet day the next day. So you've got to be careful that everything matches up. The other thing of course is that they say farmers and shepherds, but I have a farmer friend up in Scotland and he hates it. It usually means in March that if you get a red sky at night with a westerly wind, he gets very cold which is dangerous for the lambs. So not all shepherds will be pleased.


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