What is a seiche?

In 2013 I posted about an observation that the wind seemed to be driving the water of a lake to one end - I wondered about more water piling up at one end than the other. http://wigtonphysics.blogspot.com/2013/12/what-happens-to-all-that-water.html I was reading the wonderful old natural history book above and got a clue. They say that when the wind stops blowing, you get a low frequency oscillation of water backwards and forwards. The signifance in the book is that this mixes up fixed temperature layers in the water in a way that is significant to the organisms that live in the water. A bit of research shows that the phenomenon has a name: SEICHE https://oceanservice.noaa.gov/facts/seiche.html#:~:text=Seiches%20are%20typically%20caused%20when,for%20hours%20or%20even%20days. I think that the bit referred to in the book is probably an "internal seiche" https://link.springer.com/referenceworkentry/10.1007%2F978-1-4020-4410-6_160#:~:text=An%20internal%20seiche%20is%20a,opposite%20direction%20(Figure%201).
because this directly wobbles the layer that divides the upper mixed layer from the calm deep water beneath.

Ductile near Birmingham

Ductile has been a word for which I've seen different definitions over the years. One was that it meant that a material could be pulled into a wire. Another was that it could undergo plastic deformation before breaking, which is important in enginnering because the material can absorb quite a bit of energy before breaking. There is some warning. https://en.wikipedia.org/wiki/Ductility mentions that this is due to metallic bonds and the way that atoms can slide past each other. But cast iron is brittle for that reason and steel has carbon inserted so that the layers can't slide over each other as easily. https://www.reliance-foundry.com/blog/difference-cast-iron-wrought#gref talks about cast iron having a heterogeneous internal structure which creates stress points. It talks about it having a high carbon content. So I've learned somehting new. Cast iron has more carbon in it than steel.

This week's updated Covid graph

I wasn't expecting to get data this week because of controversy about the figures being reported but it turns out that the Office for National Statistics (ONS) and the government department have different criteria for what to record. I have been using the ONS data. They explain the difference here https://www.ons.gov.uk/peoplepopulationandcommunity/healthandsocialcare/causesofdeath/articles/comparisonofweeklydeathoccurrencesinenglandandwales/uptoweekending03july2020#comparisons My log graph is still impressively straight.

What's a riffle? Ben Gill, Ennerdale

We were on Crag Fell in Ennerdale when I found this notice at the top of a precipitous stream called Ben Gill.

 A bit of research found this https://www.therrc.co.uk/2011%20Conference/Poster%20-%20Quinlan.pdf Section 3 mentions sensors being put into a riffle. I'd never heard of one. I looked it up and found that it seems to be a shallow section in the flow of a stream, probably with a pool behind it. I'm wondering whether I found one that they had artifically created. However, this is shallow and narrow rather than shallow and wide so is perhaps a nozzle instead. So not a riffle but at least I know to look out for them now.

Weekly Covid log graph update and the upcoming data correction

I added the data point for the week that ended on 15 July. A 7-day rolling mean of 77 kept my line straight. Yesterday it was announced that new data was being suspended because of a controversy over what was being reported. Today I read in The Times that the deaths that might be removed from the statistics might account for 10% so I multiplied the numbers by 0.9 and replotted the graph - orange line above. Of course the gradient remains unchanged. If those deaths represent a fixed fraction then the numbers will not come down at a faster rate; they will just reach the bottom sooner because they started from lower. Since the issue is about people who had Covid and died over 28 days later, it won't quite be a constant factor though. More should have entered the statistics as time has gone by. This might change the gradient part way through.

Effective R number

It turns out that the R number that the government puts out is not the number who can potentially be infected by one person but the number who are actually being infected on average. I posted about the intrinsic "basic reproduction number" Ro and realised that this can never go below 1. Then I found this very clear explanation, not peer reviewed but written early in the outbreak. https://www.cebm.net/covid-19/when-will-it-be-over-an-introduction-to-viral-reproduction-numbers-r0-and-re/ It made a lot of sense to me. So there is an "effective reproduction number" Re and this is what the government are publishing. It gives an equation linking the two R numbers:
Re=Ro(1-Pi)
Where Pi is the proportion of the population who have developed immunity.
So if Ro=2.4, as has been assumed and Re is about 0.8 at the moment, you get Pi = 0.67. In other words, it would give two thirds of the population as being immune. This surely can't be the case. It must mean that the various distancing methods have worked to make the numbers be the same as if we were immune - a sort of virtual herd immunity. I think in Physics we'd call that an unstable equilibrium. It is nicely balanced but the removal of a key prop keeping it balanced would upset the current decline. Makes me keen to maintain the distancing measures.

Using LED lights to set gel nail polish

 I was intrigued by this machine which uses light emitting diodes to make gel nail polish set.

 It turns out that the trick is that they emit ultra-violet light.

 https://www.miladypro.com/home/b/nails/archive/2017/11/10/the-difference-between-uv-and-led-nail-dryers#:~:text=LED%20lights%20work%20similarly%2C%20but,cure%20much%20faster%20than%20UV.
The UV has the energy to get the molecules in the gel to turn into long polymer molecules - molecules made of repeating identical sections. This makes them turn from small molecules in a liquid to hard molecules in a solid. No mention of whether or not it is a cross-linking polymer but a little consultation of Google suggests it probably is.

Sadly not my nails! Thanks, Lydia.