Unstable weather

By Marcus Wilson 20/02/2012

Unfortunately I didn’t get to the cricket last night – I wasn’t feeling well – and missed Richard Levi’s demonstration of how to hit sixes Would have been fun to see.

It occurred to me at the weekend that I’ve managed to avoid the rain in the last week. While its poured down in Cambridge, I’ve been at work in Hamilton – and Hamilton has managed to get a soaking outside normal work hours while I’ve been in Cambridge or elsewhere. Sooner or later the thunder is going to catch up with me. In the meantime, I’m not complaining.

That’s one of the features of this kind of weather pattern – you just don’t know where the storms are going to pop up. The met-service people can tell us that they will pop up, and someplace is going to get drenched,  but just where and when is anyone’s guess. The atmosphere is ‘unstable’ – and that word means something very specific in physics. Specifically, the equilibrium state, where the heat, moisture, airflow etc  is shared out equally over an area (i.e. everywhere in Waikato, say, is a similar temperature and humidity) isn’t going to stay like that. If we start with the equilibrium state, and change it slightly, it encourages further changes to occur. So we end up with pockets of rising air, leading to rain clouds forming, and pockets of falling air, which give us dry conditions, rather than all the air sitting still. 

There are many examples of unstable that we can see – for example, the ridges on metalled roads, patterns in sand on a beach, and bunching of buses along bus routes so that you really do wait ages for one and then three come at once.  In our work on computer modelling of the cortex, we can also see spatial and temporal (time-varying) instabilities in the firing rates of collections of neurons. This can lead to patches of a cortex having greater activity than other patches (i.e. the equilibrium state, with activity shared out equally over all areas of the cortex, doesn’t occur) which is similar to what is seen in practice – for example with electroencephalography and MRI.

So when something has an equilibrium position, we have to be careful as it doesn’t mean that the equilibrium will be the state that the system takes.

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