Tuesday, 19 October 2010

Inversion

Flying west was an interesting experience earlier last week. Initially, the visibility into sun was about zero. Seriously, the view ahead was the same as being in solid IMC. The ground was still visible, but I wouldn't have fancied navigating by vertical reference (thanks Mr Garmin). Climbing made things a lot better, and by the time I got to 3,000' the scene was stunning.

I obviously wasn't the only person to notice this, and Simon Keeling who runs The Weather School offered this explanation on the FLYER forums.

Reason for the inversion layer is that in anticylonic conditions the air is sinking slowly. As this happens, the air is warmed adiabatically (it becomes dry and so warms at 3C/1000ft as it sinks. At the surface the air is colder, in this case due to the northeast flow moving in off a warmer North Sea. So, the cold, dense air sits at the surface, whilst the warm, dry and less dense air above quite happily sits atop. Hence the reason that temperatures rise with height in the lowest layers.
You might then ask why the cloud forms in the inversion layer? Well, for any given temperature the air can only hold X amount of water as an invisible vapour. Warm air can hold more water vapour than cold. So, on the boundary where these two layer of air meet condensation takes places as the one cannot hold the water as a vapour and hence condensation must take place. We see this as stratus or stratocumulus cloud. As air cannot rise due to the sinking motion described above, it stagnates below the inversion and visibility is reduced. Hope that helps, tried to explain it as best I can.

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