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Cromarty, Forth, Tyne, Dogger... what does the shipping forecast tell us about our planet?

Helen Czerski Helen Czerski | 19:40 UK time, Wednesday, 13 July 2011

Distance travelled ~ 499'123'200 km

The absolute magic of the shipping forecast is hard to explain to people outside of the UK. Four times a day, a slow soothing voice reads out 350 words which almost no-one understands and almost everyone loves. But hidden in this swirl of familiar words is the story of giant swirls in the atmosphere, mixing air up and moving energy north. There is always another shipping forecast tomorrow, because there is always more energy from the Sun to take that journey.

Let's imagine the shipping forecast as if we were sailors out at sea. First of all, the General Synopsis gives us the position of a region of low pressure, for example in "Irish Sea". Then area forecasts work clockwise around the British Isles, giving the conditions in every section of the sea, and we can put it together like a jigsaw puzzle. We can get the map out and as we listen, we can draw the wind direction and speed on the map for every section. As we draw on the wind arrows, we see that they point anticlockwise around the pressure low. The words have painted a picture of the weather, and it's a swirl. But what's going round what and why?

The UK is in an interesting place from a meteorological point of view, because we're right in the path of a huge boundary in the atmosphere, the boundary between warm air in the south and cold polar air in the north. The Sun heats Earth most at the equator, and all our weather is just the atmosphere's way of moving this energy towards the poles. The poles lose energy to space faster than the equator does, so it's a bit like a conveyer belt for heat. But it's not a smooth process.

swirl of tea

Next time you add some milk to your tea, mix it just a little bit and then watch. Rather than the milk slowly diffusing into the tea, the mixing all happens in the swirls. And the same is true in the atmosphere, but the swirls can be a thousand miles across. These are the cyclones and anticyclones, always moving towards the east, which make up the bulk of UK weather patterns. Where warm and cold air meet, at around our latitude, they get mixed up in swirls. At the centre of each swirl is either a low pressure or high pressure region, and weather fronts (boundaries between warm and cold air) are moving around that region to mix everything up. Warm air moves north, cold air moves south, and overall, energy travels towards the north pole.

So the soothing tones of the shipping forecast are telling the story of the movement of energy on a planetary scale. Next time you're listening to the shipping forecast at bedtime, imagine the swirls in your cup of tea as you drift off to sleep and listen: "Northerly or northwesterly 5 to 7. Moderate or rough. Mainly fair. Good. "


  • Comment number 1.

    Why do the poles lose energy faster than at the equator? Does it have something to do with the spinning action of the earth?

    You talked about swirls. What do you think is going on with the fact that everything spins, cells, electrons, hurricanes, tornadoes, whirlpools, the earth, sun, galaxies? What's going on with this spinning? Is it a way of harnessing or perhaps creating energy? Perhaps reorganizing energy?


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  • Comment number 2.

    Hello Todd_B,

    The poles lose energy faster because they're not getting any very much extra energy from the sun to balance their loss. All areas of the Earth are radiating heat out into space. The areas around the equator are absorbing huge amounts of solar energy, and they can't lose it as infrared energy as quickly as they gain it. So, on balance, they're absorbing more than they lose. Since that situation makes the poles colder than the equator, heat energy will tend to move from the equator to the poles.

    Swirling is indeed a very common phenomenon and in all the cases you listed (except the electrons) it represented a near-balance between forces pulling "stuff" towards a central point, and the forces which limit that. There are lots of combinations which can cause that balance, so there are lots of swirls. An alternative answer is that there are lots of situations where a fluid is pulled towards a central point, but if it can go straight there, the force vanishes, so we don't see a swirl. For example, if you pull an object out of a pool, leaving a hole behind, the water just rushes straight into the hole and there is no swirl. But you do get a swirl in any situation where there's something acting to stop the fluid rushing straight into the hole. A satellite has momentum carrying it forwards, even as it's being pulled towards the Earth, so although it keeps falling, it never reaches the centre. Hurricane winds are being pulled towards a low-pressure area at the centre, but they can't get there because of the Coriolis force. So any time you see a swirl, it's probably caused by this tug of war (inwards versus some other direction) being close to equal.


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