Fun Science Experiment

We are told that water vapour is a greenhouse gas for the same reasons as CO2, i.e. it can absorb and re-emit infra red radiation. We can’t arbitrarily vary CO2 levels, but absolute humidity can be anything between zero and thirty-thousand mg/l (cold, dry days v warm, humid days). So the amount by which temperatures fall after dusk should be inversely proportional to absolute humidity (i.e. the insulating ‘greenhouse effect’ should be proportional to absolute humidity).
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So we could produce strong evidence to support or contradict the MMGW theory quite easily and relatively cheaply, as follows:

Set up measuring devices, which record the following variables:
- temperature at dusk,
- temperature six hours later (or slightly shorter than the shortest night at that latitude),
- absolute humidity (milligrams H20 vapour per litre of air),
- cloudiness, and
- windspeed.

Ideally you’d do measurements at many different locations, the more readings you have the better. And ideally, you’d have three sets of measuring devices at each location – one at the bottom of a tall structure like a TV mast, one half-way up and one at the top, and average the three readings for each variable.

At the end of (say) one year, you:
- filter out the ‘cloudy’ nights. We know that clouds warm things up at night, but clouds are not water vapour (gas), they are water droplets (liquid) or ice (solid) and clouds have a quite different and much stronger effect than water vapour. Maybe you can measure ‘cloudiness’ by using a telescope to see what percentage of stars are clearly visible, in which case you filter out measurements for nights where fewer than (say) 95% of stars were visible for at least 95% of the time?
- filter out the ‘windy’ nights, because the air blowing in might be warmer or colder than the air at that location at dusk, so they could be entirely random results.
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Having done your readings and filtering, you interpret the results.

1. Calculate the average fall in temperature over the six hours after dusk (to show the insulating effect) on the most humid (but cloud-free nights) when absolute humidity was high, (say) 20,000 mg/l, and set this as the baseline level.

2. IF the MMGW theory is correct and the relationship is logarithmic, at 10,000 mg/l the fall in temperature should be 1C* more than the baseline, and so on. On very dry (and cloud-free) nights with (say) 312.5 mg/l, the fall in temperature should be about 6C* more than the baseline.

So you would get a chart that looks like this:

I have not shown the scale on the x-axis because I have no idea what the baseline will be or what the gradient will be – that is what the experiment is supposed to find out.

NB – One mg/l is approx. equal to one part per million by volume (we can adjust this to the true value for ease of comparison).

*  I believe that H20 is considered to be a far stronger ‘greenhouse gas’ than CO2 as it can absorb and re-emit infra red at far more wavelengths, I think about ten times as many, but don’t quote me on that. Clever scientists can work backwards from the findings to establish what sort of insulating effect (aka ‘greenhouse effect’) there is if CO2 levels double from ‘pre-industrial levels’ of 300 ppm to 600 ppm.
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Personally, I expect that the result will be within the margin of error of the calculations, but I’m open-minded about all this. You don’t do experiments to ‘prove’ or ‘disprove’ things, you do the measurements and then interpret them as honestly as possible.
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The bonus here is that you can do a similar plot of fall in temperature vs cloudiness, with clear nights on the left and cloudy nights on the right. I expect that this will show a very strong correlation and a steep gradient. But I’m open minded on this as well.