Wednesday, 24 February 2021

RE: ozone depletion - how does the extra Ultraviolet B radiation affect clouds?

That's a question to which I have found no obvious answer, but I assume that if there is a bit of extra high intensity UV-B hitting the atmosphere, it will evaporate some of the clouds, i.e. turn water droplets back into water vapour. The wavelength of UV-B is orders of magnitude less than that of infra red, so the chances of it being absorbed by a molecule in a tiny water droplet is commensurately higher. And 'absorbed' just means that radiation energy is converted to some other form of energy.

This can lead to a disproportionate effect on surface and atmospheric temperatures. This theory might be totally wrong of course but it seems plausible to me. The effect must be warming, however slight. I've not put numbers on the effect because you have to make far too many assumptions so that would 'prove' nothing. This is a wait and see operation. If I live long enough to see the 'ozone hole' repair itself (perhaps by the middle of this century?) and temperatures fall again even though CO2 levels have increased (and they will), then that would support the theory but not really 'prove' it either way:

1. Starting position pre-ozone depletion

Some sunlight hits the surface, most of it hits clouds and is partially reflected: 2. There is now more UV-B (imaginatively coloured violet, even though it is invisible)

Some hits the surface; most of it hits clouds: 3. Cloud cover is reduced

Some of the energy in UV-B evaporates water droplets and so is converted to latent heat of evaporation (no measurable temperature increase). That thins the clouds slightly and reduces the amount of cloud cover. This allows more sunlight at all other wavelengths through to the surface.

So it's not so much the bit of extra UV-B which warms the surface; it is all the other sunlight that isn't reflected and that now gets through. An average reduction in cloud cover of 2% reduces albedo and increases the amount of sunlight getting through by about 1%, sufficient to cause about 1 degree of surface warming: 4. At night, the water vapour condenses into clouds again

The energy converted to latent heat of evaporation during the day turns back into extra thermal energy when the water vapour condenses again (or the rate of cooling is lower than it otherwise would be). This warms the atmosphere slightly. The surface is also slightly warmer. The pink arrows denote the extra infra red and warmth generally:


Bayard said...

You do realise you will be hauled up in front of the Inquisition for this heresy, don't you?

Mark Wadsworth said...

B, for sure. But I'd rather risk being in the minority and wrong than being in the majority and wrong.