Magnetic field on Mungrisdale Common

Mungrisdale Common might look like an idyllic upland pasture in the bottom photograph but it's an awful swamp. Not sure why Wainwright put it in his Northern Fells book but I'm guessing it was to add a chapter to a thin volume. So this was our third visit. On Friday I remembered that I had access to a portable magnetic field strength meter that hadn't been used for years. So I'm going to start taking readings in different places. The reason for choosing Mungrisdale Common was that it is 5 miles from the nearest civilisation. Surely that will give an unpolluted reading of the Earth's magnetic field strength. The black plastic thin is a coated Hall Probe. I moved it around and got a maximum reading of 8.0 milli Teslas. I'm getting half that in my front room. Hmmm. I need to take more readings in different places. A weird place to go but it has a great view south to the highest fells in England.

Loch Cluanie and hydro-electric power

We stopped for lunch beside Loch Cluanie. You can see the dam in the top picture. Apparently the dam is 675m long and 40m high. It feeds the Ceannacroc power station which is 20MW but this is also fed by Loch Loyne. Given that the wind farm at Bothel close to Wigton is quoted as 10MW this hydro-electric scheme doesn't seem that much. It must surely have been more expensive than the 10 turbines at Bothel. It could be argued that it has disfigured the landscape as much. It's advantage is that the energy supply is more consistent. I do need to look further into these figures because 20MW doesn't sound very much.

Knife-edge effect diffraction and Flodigarry Island

These are pictures of Flodigarry Island off the north end of Skye taken from the Quaraing. The diffraction of the waves bothered me and I have had to do a lot of research to explain it. I teach that waves will only diffract around an object if their wavelength is the same size as the object. That's why FM radio reception is poor in mountainous areas. But look at the waves in the picture. Their wavelength is tiny in comparison to the length of the island and yet they diffract round both ends of the island. The answer is called the knife-edge effect and owes a lot to Huygens' understanding of how waves propagate. He came up with idea that you divide the wavefront up into points that act as sources of semi-circular waves. Where these semi-circles overlap, you get the new wave. On a plane wave (a flat fronted wave) the points are so close together that the result is actually another plane wave. But a knife edge can act as a distinct secondary source of waves with none inside it. The key to my example is that the island is not round. It has pointed ends and these pointed ends are about the same width as the wavelength. So the ends of the island act as secondary sources, diffracting circular waves into the space behind the island. A mountain does not act as a knife edge to FM radio waves. The size of the mountain is too big for that.

Rotation and phase difference

I use this experiment to show the phase difference in cyclical motion. I rotate the turntable and the shadow of the ball moves from the middle to the side, back to the middle across to the other side and back to the middle. If we set up a pendulum as well, its shadow would move in the same way. So motion is a circle is linked to pendulum motion, and is linked to wave motion, because waves go middle to side to middle to other side and back to middle. One quarter of a rotation moves the shadow from the middle to one side. So I say that the phase difference between the middle and the side in a wave motion is 90 degrees or pi/2 radians. Half a rotation will bring the shadow back to the middle but it will be moving in the opposite direction. So the half way point in a wave motion is 180 degrees or pi radians out from the starting point.

The Quaraing

The Quaraing is an amazing area at the north end of Skye. If you look at the bottom photograph you will see that it is made of tilted layers. It is the result of basalt lava flows from a massive volcano where the Cuillin mountains now stand, 20 miles to the south. These can just be seen in the far distance on the right of the picture. My earlier pictures from the Old Man of Storr show that the lava was very gassy because you can still see the bubbles. With the gas being less dense that the liquid, the bubbles float to the top. I have read that this makes the top of each successive layer more crumbly. There was a massive landslide resulting in the weird features like The Table, which is hard to find but just as good as it looks. The top picture shows the miniature features resulting from the landslide. The rock has been split in two perpendicular planes. The corners of the original rock are still sharp because they were not transported and subject to erosion. Instead, mineral rich fluid penetrated the cracks, The water evaporated and left the mineral veins. The rock was frighteningly crumbly and bits were coming off in my hand as we negotiated that gully!

Amygdales in Trotternish

We found rocks with white flecks all over them on the way up to the Old Man of Storr. I knew that we were crossing old lava flows and figured out that the holes were where there had been gas bubbles in the lava. I know also that when you get mineral layers in rocks it is because very hot water with dissolved minerals inside go down the cracks where the water evaporates causing the minerals to crystallise out. I was pleased to find out that I was basically correct but found also that when it is in bubbles in basalt lava flows they have a technical term. They are AMYGDALES. It comes from the Greek word for almonds because they are that shape. The same is true of the amygdala, part of your brain.

Coastal convection at the Old Man of Storr

 

We were on the Isle of Skye and visited the amazing rock pinnacles of Trotternish, including the Old Man of Storr. We were caught in a shower when the photographs were taken and sheltered behind a rock because the wind was driving the rain in. I looked up to see which way the clouds were moving and noticed that they were moving in the opposite direction to the rain. The wind seemed to be going in the opposite direction higher up. I wondered if this was evidence for coastal convection that I remember learning about at school. The sun heats the land faster than the sea during the day because the sea has a much higher specific heat capacity. The land becomes hotter than the sea, so warm air rises over the land and cold air sinks over the sea. This sets up a cycle that is completed by air moving off the sea onto land at ground level and off the land out to sea at a higher level. This was roughly what I observed.