BBC | 23 Degrees

The Sun and the wonders of light

Mon, 28 Mar 2011 12:33:00 +0000

d ~ 223'833'600 km: day 87

The sun - that giant ball of boiling gravity-driven nuclear reaction - has been blasting us with electromagnetic radiation for billions of years. "Radiation" is a word loaded with bad connotations, but actually we live surrounded by it and depending on it every day. We can see and feel some of that radiation - yet our eyes and skin experience only a very small range of the sun's rays down here in the murky depths of our atmosphere.

Electromagnetic radiation travels from the sun as waves and there's a whole spectrum of wavelengths, going from nano-meters short to kilometers long. Each wavelength totally changes how we experience that radiation and what we call it. The smallest waves we get from the sun are x-rays, which are small and energetic enough to pass through your flesh but not quite through your bones. A little larger than x-rays are ultraviolet rays which we know from all the professional advice on sun-blocking creams can be damaging to your skin cells. Next up we have our good friend visible light, followed by infrared. Microwaves are the next biggest and finally we come to radio waves - a really broad category with wavelengths up to tens of kilometers long.

Why, bathed in all this variety, have our eyes developed to see only visible light? Just as objects around us are coloured because their molecules absorb or scatter certain wavelengths of light, different particles and molecules in the atmosphere also do the same. We don't see this because visible light lies in what is called an "atmospheric window" - it can travel relatively freely through the atmosphere without being absorbed or scattered along the way. Having said that visible light doesn't pass through all atmospheric objects easily; water droplets are particularly good at scattering visible light, which is why we can see clouds. But if our eyes were tuned to a wavelength which was not in an atmospheric window, we would just be surrounded by a permanent fog of light scattered by the atmosphere. And that would be pretty useless for getting around.

There are other atmospheric windows, though and you might wonder why we haven't chosen one of these? Light travels easily in the "middle infrared" for example - the wavelength of heat. If we could tune our eyes to see in this window then we would live in a dimmer world where warm things shine in their own light. Without temperature contrasts, objects of the same temperature would tend to blend into one and cold-blooded animals like snakes or crocodiles may well be perfectly camouflaged. There is also a third major atmospheric window at the wavelength of radio waves, which is why we use this for communicating. If our eyes saw through this window then we would see an alien world - lit up by a dim sun, radio transmitters and the odd pulsar.

We use the visible because here the sun is at its brightest and it is brighter than anything else on Earth or in the sky. This gives us the best chance of seeing our landscape clearly. We can walk around our world safely, avoiding objects as visible light is scattered off them. And this means that - among other things - we can devote a bit more time to enjoying the beauty of light and the things it allows us to see.

Can an earthquake shift the Earth's axis?

Mon, 14 Mar 2011 18:11:12 +0000

d ~ 187'814'400 km: day 73

The short answer to this is yes. But the effects of such a shift are tiny. The Earth's tilt and rotational spin on its axis as it travels around the Sun causes our seasons. The earthquake in Japan moved the axis of rotation by around 16 cm. That might sound like a lot, but it's small compared to the size of the Earth. 1 degree change to the tilt of the axis of the Earth would mean moving it by around 110 km.

But the quake's interference with our axis doesn't stop there. The Japanese landmass was moved around by as much as 4m. This redistribution of mass on the surface changes our moment of inertia. In order to conserve angular momentum, the changes in inertia are compensated by changes in the rate of rotation of the Earth about the axis. After the earthquake it's quite possible that our days will be 1.8 millionths of a second shorter because of this shifting.

We can see differences in the average length of the day due to other changes in the Earth and atmosphere. The plots below show that there is a significant seasonal variation, with the day length (speed of rotation) being shortest (fastest) during the boreal summer. This happens because the northern hemisphere winds slow down in the summer and the momentum they lose - half the momentum of the atmosphere - is transferred to the Earth. This increase in momentum makes the Earth spin faster and our days become slightly shorter by 1-2 milliseconds.

So while the changes brought to our planet by the earthquake are unique and collosal enough to affect the Earth; they aren't big enough that we will notice them any more than we notice the milliseconds we lose each summer.

St David's day..celebrate the Welsh microclimate

Tue, 01 Mar 2011 17:01:41 +0000

d ~ 154'368'000 km: day 60

Today is St David's day. A day for celebrating all things connected to Welsh life and culture - eisteddfod, rugby and mountains to name the most obvious. But there is another aspect to Wales that we could well be celebrating.

Not many countries could be said to have a national weather-type but certainly for people journeying from the south of England the weather type culturally associated with Wales is rain. Rainwater seeps down the mountainsides and gushes along rocky stream beds watering lush grasses on steep slopes and down into fertile mountain valleys.

But why so much rain? Well it's caused by the mountains themselves. They act as a physical barrier to the prevailing westerly wind and air is forced upwards over the craggy peaks. High up there temperatures are cooler and so water vapour in the air condenses turning from an invisible gas into millions of droplets of liquid water or clouds. As more and more water condenses the droplets become bigger and heavier and often will eventually fall out of the cloud as rain.

But why do many holiday makers from the east find this Welsh rainfall so particularly unpleasant? That's all to do with typical weather in the east. After this air has passed over the mountains it descends again and moves towards the east of the country. But even though clouds may form it's much less likely that rain will fall now - like wringing out a sponge the journey up over the mountains has dried out the air and there simply isn't enough water left in it to give the same amounts of rain. Land which lies downwind of mountains like these is said to be in their "rain shadow". London lies in the rain shadow of the Welsh mountains so the wet welsh weather actually keeps our capital dry - drier than Rome in fact.

Every day is different of course but this is the typical setup of rain over the southern half of UK brought about because of our landscape. Even smaller-scale variations in geography across the country can lead to localised tendencies in the weather called "microclimates". The particular location of a hill may mean that one town is often flooded and rainy while another is usually dry. These microclimates exist all over the UK.

So if you're considering enjoying some lavabread or barabrith tonight then take a few moments to think about whether you yourself live in a microclimate and what the cause of your local climate may be. If there was ever a day for celebrating our glorious tapestry of regional weather variations (and that life-giving rain in particular) then St David's day could well be it.

Dydd Gwyl Dewi hapus