Just how mild has this December been?

A great deal of rain has fallen on some parts of the UK this month (December 2015)

A great deal of rain has fallen on some parts of the UK this month (December 2015) (Source : BBC)

It’s been exceptionally warm this month.

We will have to wait a day or two for the various agencies to compile their annual reports, but let me précis the results for you:

  • 2015 has been ‘warm’.

And this  warmth has been evident where I live and work, in Teddington.

Analysing the data from my weather station a day early, I have compared my local results with the ‘climate normals’ for nearby Kingston upon Thames.

‘Climate Normals’ are the average values of various meteorological quantities over (typically) 30 year periods, in this case from 1981 to 2010.

Graph showing the 'Climate Normals' for the daily maximum and minimum temperatures for Kingston upon Thames. Also shown are data from my weather station for September, October, November and December.

Graph showing the ‘Climate Normals’ for the daily maximum and minimum temperatures for Kingston upon Thames. Also shown are the averaged data from my weather station for September, October, November and December. Click the graph for a larger version

My data shows that locally:

  • The average minimum temperature for December was higher than the expected average maximum temperature!
  • And the average minimum temperature is more characteristic of May or June, than December!

Averaged over the whole of the UK, the differences from the climate normals are reduced.

However the rainfall rates in particular places – thankfully not Teddington – have been shockingly extreme.

The image at the head of the article suggests that in North Wales, 50% more rain fell in December than in the previous record breaking December. FIFTY PERCENT!

It is well-understood that this extreme rainfall is linked to the general  warmth of the air. Why?

The amount of water vapour that can be held in air is much larger than one might at first imagine, and increases from around 5,000 tonnes per cubic kilometre of air at 0 °C, to nearly 18,000 tonnes per cubic kilometre of air at 20 °C (Link to Data or see the graph below).

Thousands of tonnes of water? Yes. One cubic kilometre of air has a mass of approximately 1.7 million tonnes and water vapour makes up about 1% by mass at 20 °C.

Humidity versus Temperature

The mass of water in tonnes in one cubic kilometre of saturated air is surprisingly large. Every rise of 1 °C results in an increase in ‘holding capacity’ of around 6%.

So air which is 5 °C warmer than normal can hold roughly 50% more water than normal – and hence has the potential for increased rainfall.

And if the winds bring a stream of this warm, moist air across highland regions, then extreme rainfall is inevitable. And breaking rainfall records by large amounts becomes not only possible, but likely.

I am aware that not all air is saturated, and that air temperature varies with height, and that there are complex mixing patterns in the atmosphere. And I am aware that this rate of ‘moistening’ does not apply globally.

But for air travelling over the ocean to the UK, every one degree rise in air temperature corresponds to an increase in water-holding capacity of around 6%.

At it’s heart, it actually is that simple: if the wet air striking the UK was cooler, rainfall rates would not be so extreme.

Personally

I have found it thrilling that my weather station has produced meaningful quantitative data about the local weather and climate.

Although the station is very basic, it has allowed me to go beyond qualitative comments. This is the essence of scientific observation

And in a meditative sense, the simplicity of the endeavour brings me great pleasure, especially when compared with the near impossible things I am trying to do at work at the moment!

Happy New Year and all that.

Further Reading

NOAA’s ‘State of the Global Climate’  analysis for November.

The UK monthly weather summary.

NOTE: This post was updated on January 9th 2016 to reflect the fact that I got the water holding capacity of air wrong by a factor 1000! Air holds THOUSANDS of tonnes of water per cubic kilometre

5 Responses to “Just how mild has this December been?”

  1. telescoper Says:

    Reblogged this on In the Dark and commented:
    In case you hadn’t noticed…

  2. Clever Daffodils | Protons for Breakfast Blog Says:

    […] In a previous post I explained how anomalous the weather was in December 2015.  […]

  3. davecl Says:

    Reblogged this on Disturbing the Universe and commented:
    Over a metre of rain in one month – that’s a lot!

  4. Kevin Mosedale Says:

    Hello – could I ask where you got the data for the tonnes of water in one cubic kilometre graph? I’d like to use a copy of the graph in school (with credit of course) but without your very helpful annotations. Thanks.

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