Archive for May, 2014

My mission to the MARS simulator

May 27, 2014
An apparatus for simulating the atmosphere on MARS. We are going to use it test a new apparatus for measuring the atmosphere on Earth.

An apparatus for simulating the atmosphere on MARS. We are going to use it test a new apparatus for measuring the atmosphere on Earth.

Next Saturday (31st May) I will be setting off to Denmark to carry out an experiment in the MARS simulator at the University of Aarhus.

Inside the gigantic red planet sausage chamber, we will be able to change the pressure, temperature and humidity of the air to simulate the conditions in the upper atmosphere and stratosphere of Earth rather than Mars

My aim is to test a new type of combined thermometer and hygrometer.

There are at least two clever things about the new device

  • The first clever thing is that it measures the temperature of the air without ‘touching’ it – it is a non-contact thermometer.
    • It does this by measuring the speed of sound in a volume of relatively unperturbed air. From the speed of sound we can relatively directly infer the temperature.
  • The second clever thing is that it simultaneously measures the humidity in the air, again without making contact with the air.
    • It does this by shining a laser through the same volume of air. The frequency of the laser is adjusted so that exactly matches a frequency of molecular vibration in water molecules.

But inventing the device is not enough: every invention needs an acronym. So after playing with the acronym generator, I have baptised the device NCTAH (pronounced nectar) which stands for Non-Contact Thermometer And Hygrometer.

And why does it matter? 

Measuring air temperature is difficult. For example at NPL we will happily calibrate a good thermometer with an uncertainty of around 0.001 °C, if it is to be used in contact with a liquid or solid. But when used in air we have to give an uncertainty more than 50 times larger!

And one place where the measurement of temperature and humidity is particularly important is when we try to determine how close a particular sample of air is to saturation. In other words when we ask:

“How close is a particular sample of air to forming water droplets or ice crystals?”.

This is the basic process of cloud formation in the atmosphere and it is extremely difficult to study. Assessing how close air is to saturation requires accurate measurement of both the amount of water present in the air and the temperature of the air.

When conventional instruments ascend to the upper atmosphere (where it is very dry) from the lower atmosphere (where it is relatively wet) they carry up moisture with them which affects their slow-responding humidity sensors.

Additionally the temperature readings from contact thermometers frequently lag the true air temperature.

Both these effects make it difficult to know how close the air in the upper atmosphere and stratosphere is to saturation and our instrument could make a significant improvement.

Making it work.

Developing and testing this device has required experts from

  • the Gas Analysis team (Tom Gardiner and Andrew Finlayson),
  • the Humidity Team (Stephanie Bell and Jenny Wilkinson),
  • and the Temperature team (Robin Underwood and myself).

We have tested the device at temperatures from -40 °C to + 40 °C at atmospheric pressure. And we have separately tested it at room temperature at a pressure of less then a twentieth of an atmosphere – equivalent to an altitude of about 25 km.

But the MARS chamber tests in Denmark will assess the whole instrument together at the extremes of its operating range.

The facility is expensive to hire, and the logistics of moving the experimental team and all the monitoring equipment to Denmark are challenging.

So I am excited – but nervous. I will try and let you know how it goes.

This is my colleague Robin Underwood holding the Non Contact Thermometer and Hygrometer (NCTAH). Tiny microphones and speakers measure the speed of sound in the space between the parabola on the left and the acoustic 'mirror' on the right.

My colleague Robin Underwood holding the Non Contact Thermometer and Hygrometer (NCTAH). Tiny microphones and speakers measure the speed of sound in the space between the parabola on the left and the acoustic ‘mirror’ on the right.

 

Who paid for this?

You did. The work is funded in part by the UK government and in part by the METEOMET project funded by the European Metrology Research Programme of the European Union.

 

What’s that in degrees Celsius?

May 24, 2014
Table describing how various temperatures 'feel'. Click for a larger version.

Table describing how various temperatures ‘feel’. Click for a larger version.

Here are some simple guidelines for using degrees Celsius to describe every day temperatures: enjoy 🙂

Weather (Simple Guide)

  • -20 °C: The coldest it gets in the UK
  • -10 °C: A bitterly cold winter’s day
  • 0 °C: Ice melts and water freezes
  • 10 °C: A typical day in winter
  • 20 °C: A nice warm day
  • 30 °C: A very hot summer’s day

Weather (More complicated guide)

  • -20 °C: The coldest it gets in the UK
  • -10 °C: A bitterly cold winter’s day
  • 0 °C: Ice melts and water freezes
  • 5 °C: A typical cold day in winter
  • 10 °C: A typical day in winter
  • 15 °C: A warm winter day or a cool summer day
  • 20 °C: A nice warm day
  • 25°C: A hot summer’s day
  • 30 °C: A very hot summer’s day
  • 35 °C: The hottest it ever gets in the UK

Indoors

  • -20 °C: Your freezer should be this temperature
  • 0 °C: Ice melts and water freezes
  • 5 °C: The bottom of your fridge
  • 10 °C: Condensation forms if walls are colder than this
  • 20 °C: Nominal ‘Comfortable’ indoor temperature
    • What feels comfortable depends on air flows and varies from person to person typically in the range 18 °C to 23°C
  • 25°C: A warm room
  • Temperature sensation varies from person to person but typically
    • 35 °C: Bath water feels cool
    • 40 °C: Bath water feels hot
    • 45 °C: Scalding water
    • 65 °C: A hot cup of tea
  • 100 °C: Water boils

Where were you on World Metrology Day?

May 21, 2014
On World Metrology Day 2014 I was attending a meeting of working group 4 of the Consultative Committee on Thermometry at the BIPM in Paris

On World Metrology Day 2014 I was attending a meeting of Working Group 4 of the Consultative Committee on Thermometry at the BIPM in Paris

Where were you on World Metrology Day?

I was in Paris, attending a meeting at the International Bureau of Weights and Measures (BIPM).

The Bureau is reached by turning off the busy main street through Sevres, and turning up a narrow cobbled path to the historic Pavillon de Breteuil

As I entered the splendid gardens of the Pavilion on this bright Tuesday morning, I found myself taking coffee with the creme-de-la-creme of world experts in every aspect of temperature measurement.

They were gathered for the 27th meeting of the Consultative Committee on Thermometry (CCT), and I was there to update my colleagues in Working Group 4 of the CCT on the progress of my research.

Personally, I find the formality of these meetings a little intimidating, and I guess everybody else does too. But it is hard to imagine that things would be very different in any organisation with the stated aims of BIPM

…to ensure world-wide uniformity of measurements and their traceability to the International System of Units (SI).

And what gives BIPM the right to do this? The Convention of the Metre, a diplomatic treaty between fifty-six nations. It is the first signing of this treaty in 1875 which is commemorated each year on 20th May.

Despite it’s name, the convention applies does not just apply to measurements of length.

So when a thermometer in Japan agrees with one in Australia, this has not happened by coincidence. It is has happened because of ongoing active collaborations mediated formally and informally via the BIPM.

This coherence of measurement brings benefits to everyone at very little cost or inconvenience. However because BIPM’s goal of world-wide uniformity of measurements has been substantially achievedpeople just don’t notice that this coherence is a positive achievement, and that it needs active ongoing attention for it to be maintained.

As I whisper to myself during the long hours of our meetings, “We worry about the millikelvins so that you don’t have to”

=========

By the way, I feel obliged to mention that my wife, Stephanie Bell, sits on the CCT itself and is thus metrologically more important than me: I only sit on a Working Group.

But because of our shared involvement in CCT, these triennial committee meetings represent a significant childcare problem for us – a problem which is probably unique in the history of the CCT.

So this week Stephanie travelled to CCT on Sunday, returning to the UK on Monday to make sure the children were up for their exams on Tuesday morning. She then returned to Paris on Tuesday afternoon for more meetings – and I hope she will be back home on Friday

My own travel arrangements were relatively straightforward. I left the UK on Monday for a Tuesday morning meeting and then returned immediately afterwards.

But by good chance, our paths crossed late on Monday afternoon at the Gare du Nord as I travelled to BIPM and she travelled home.

So for 10 precious minutes we stood still amongst the hurly burly of the moving world, and shared a beer. And then we went on our respective ways. Santé

Me and Steph at the Gare du Nord

Stephanie and I cross paths at the Gare du Nord.

 

Why you shouldn’t use the ‘F’ word

May 13, 2014
Some people use the F-word with consideration for how offensive its use may be to there people.

Some people use the F-word without consideration for how offensive its use may be to other people.

As a parent, I obviously watch my language around my children. And I promise you that they did not learn to use the ‘F-word’ from either myself or my wife.

But we can’t control their whole lives. And at school or on the internet they encounter all kinds of bad language. And being children, they love to use shocking words to shock.

And so before we knew it the children were routinely referring to temperatures in ‘degrees Fahren***t‘. They hardly knew what the word meant, but it was shocking nonetheless.

How could our children behave this way? we asked. Where had we gone wrong?

And initially of course, we blamed ourselves. Had we been too strict? But we have come to terms with it now, and we think it is ‘just a phase’.

We had some long difficult ‘conversations’ with the children where we explained the importance of using the correct words to describe ‘difficult’ ideas about which they may feel embarrassed.

And we are past it now – but I am writing this because some people use this word all the time. Its use was common in earlier times and indeed, as a child I often heard people using the word without any embarrassment at all! And I blush to think I even used it myself.

Even now some older people think its just ‘political correctness‘ to describe temperatures in degrees Celsius or kelvin. But its not.

To use ‘Fahren***t‘ in the modern world is not just an idiosyncrasy: it is an insult to other people – and yes America, I am talking about you.

The US National Measurement Institute (NIST) is the biggest and best in the world, but I guess they feel a little embarrassed about the US’s idiosyncratic insistence on continuing to use – of all things – ‘Imperial’ measurement units! If the United Kingdom can slowly wean itself of these units then so can you ‘revolutionaries’!

And the reason it matters is that at the heart of the scientific endeavour is the process of measurement. And the International System of Units – the SI – is the cumulative achievement of humanity in being clear about precisely what we mean when we express a value of a physical quantity in measurement units.

Expressing temperatures in degrees Celsius or kelvin is simply being polite.

It means acknowledging that people outside one’s own cultural whirlpool may be interested in what we have measured.

It means acknowledging that we belong to one world and that the SI is our shared achievement and inheritance: a language of science through which all humanity can communicate clearly.

So, now you know: stop using f***ing Fahren***t!

UPDATE: Here is a friendly post for those of you who have never had the chance to learn how to use degrees Celsius.

 

 

 

 

Between weather and climate: the predictability gap

May 9, 2014
Illustration of the region of the Pacific Ocean in which the El Nino climate event is focussed. The event affects global climate and measurably affects the rotation of the Earth.

Illustration of the region of the Pacific Ocean in which the El Nino climate event is focussed. The event affects global climate and measurably affects the rotation of the Earth. However we cannot predict this vast event even 6 months in advance.  Picture is from Ars Technica

[UPDATE: Monday 12th May 2014: Richard Gilham wrote to highlight the Met Office’s work in trying to fill the ‘predictability gap’ – which they refer to as ‘seasonal to decadal forecasting‘. ]

The ease with which the future may (or may not) be predicted is a subject of continuing personal fascination.

When it comes to weather and climate I recently noticed there is a curious blindness in our forecasts in between the ranges of weather forecasts and climate forecasts.

  • A weather forecast involves saying (for instance) whether it will rain at a particular time and in a particular place. Advanced measurements and advanced computer models allow weather forecasts over the UK to be accurate over a range from a few days (when things are complicated) to a week or two (when things are simple).
  • However even though climate forecasts are more complex than weather forecasts and reach out much farther into the future, they are in some ways easier than weather forecasts because there is no need to predict exactly when or exactly where it will rain. So, for example, we are confident that in 10 years time, the average daily temperature for July in the United Kingdom will be close to 15 ºC.

[Aside: Do take a look at this excellent Met Office page which summarises ‘normal’ UK climate and has data for the last 100 years of so!]

This ‘predictability gap’  is at the heart of the questions about the ‘hiatus’ in global mean temperature. Of course the reason for the gap can be easily stated:

  • Weather forecasts begin with a very detailed picture of the existing weather and use  the laws of physics to run this detailed picture forward in time. Eventually, the lack of detailed knowledge of the initial state makes the forecast inaccurate.
  • Climate forecasts are based on models of the entire Earth including many layers in the oceans and the land and of course many layers in the atmosphere. Also included are the changing distance from the Sun and known solar cycles. We expect Climate models to predict the average of quantities such as the average July temperature – not the temperature in any particular year.

So there is a gap which I sometimes find shocking given the advanced state of development of both weather forecasts and climate models. And this can be  confusing when some specific weather events take on special significance. This year I am keeping my eye on two weather/climate events:

  • The extent of Arctic Sea Ice at its minimum value in September
  • The possibility of an El Nino event in the Southern Hemisphere summer.

Arctic Sea Ice Extent after the summer melt

Chart showing the average amount of sea ice (black line) versus the month of the year. The grey region shows the typical variability and the  green line shows this years data: what will be the value in September 2014? It's impossible to say.

Chart showing the average amount of sea ice (black line) versus the month of the year. The grey region shows the typical variability and the green line shows this years data: what will be the value in September 2014? It’s impossible to say. Click the image for larger version or click here for a live version of the graph

We can confidently predict that the extent of Arctic Sea Ice will reach its minimum extent in late September. We predict this not based upon climate models or weather forecasts but on statistical analysis of previous behaviour.

However, we cannot predict whether it will reach a new record minimum, even though we are sure that the trend will mean that in less than 50 years, the summer minimum of Arctic Sea Ice extent will be zero.

El Nino

The El Nino event  is the largest ‘regular’ climate event on the Earth, strongly affecting rainfall and temperatures across the Pacific Ocean and having detectable affects across the entire planet.

But even though we now monitor the globe hour-by-hour, and day-by-day, we really don’t know even six months in advance if this year will be an ‘El Nino’ year.

Climate models do predict an El Nino-like behaviour – a prediction which emerges from the models rather than being programmed into them. But they predict that it will occur typically every two to seven years – not which year will happen

Currently NOAA estimate the chance of an El Nino Event at the end of this year is just greeter than 50% but really, we just don’t know.

 


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