Posts Tagged ‘Weather’

Cloud in a bottle!

March 22, 2019

One of the best parts of the FREE! ‘Learn About Weather‘ course, was the chance to make a cloud in a bottle. Here’s my video!

The demonstration involves squeezing a bottle partly filled with water and then letting go. One can see a cloud form as one lets go, and then disappear again when one squeezes. Wow!

But there is a trick! You need to drop a burning match into the bottle first!

Heterogeneous versus homogeneous nucleation

How does the smoke make the trick work? It’s to do with the way droplets form – a process called nucleation.

There are two ways for droplets to nucleate. An easy way and a hard way. But those words are too short for scientists. Instead we call them heterogeneous and homogeneous nucleation!

  • Heterogeneous nucleation‘ means that the water droplets in a cloud form around dust or smoke particles. The ‘hetero-” prefix means ‘different’, because there is more than one type of entity involved in forming droplets – dust and water.
  • Homogeneous nucleation‘ means that the water droplets in a cloud form spontaneously without any other type of particle being present. The ‘homo-” prefix means ‘the same’, because there is just one substance present – water.

The experiment shows that hetero-gen-e-ous nucleation is dramatically easier than than homo-gen-e-ous nucleation. And in reality – in real clouds – practically all droplet formation is heterogeneous – involving dust particles.

The reason is easy to appreciate.

  • To form a tiny droplet by homogeneous nucleation requires a few water molecules to meet and stick together. It’s easy to imagine three or four molecules might do this, but as new molecules collide, some will have higher than average energy and tend to break the proto-droplet apart.
  • But a dust or smoke particle, though small by human standards (about 0.001 mm in diameter), is roughly 10,000 times larger than individual molecules. So its surface provides billions of locations for water molecules to stick. So when the average energy of the water molecules is at the appropriate level to form a liquid, the water molecules can quickly stick to the surface and cause a droplet to grow.

How big is the temperature change?

Squeezing the bottle compresses the air quickly (in much less than 1 second) and so (because the air is a poor conductor of heat), there is no time for the heat of compression to flow from the gas into the walls and the water (this takes a few seconds) and the air warms transiently.

I was curious about the size of the temperature change that brought about this cloud formation.

I calculated that if the air in the bottle changed volume by 5%, there should be a temperature change of around 6 °C – really quite large!

Squeezing the bottle warms the air rapidly – and then over a few seconds the temperature slowly returns to the temperature of the walls of the bottle and the water.

If one lets go at this point the volume increases by an equivalent amount and the temperature returns to ambient. It is this fall which is expected to precipitate the water droplets.

To get the biggest temperature change one needs a large fractional change in volume. I couldn’t do the calculation of the optimum filling fraction so I did an experiment instead.

I poked a thin thermocouple through a bottle top and made it air tight using lots of epoxy resin.

Bottle

I then squeezed the bottle and measured the maximum temperature rise. The results are shown below.

Delta T versus Filling Fraction

The results indicate that for a bottle filled to around three quarters with water, the temperature change is about 6 °C.

But as you can see in the video – it takes a few seconds to reach this maximum temperature, so I suspect the instantaneous change in air temperature is much larger, but that even this small thermocouple takes a couple of seconds to warm up.

Happy Experimenting

The Met office have more cloud forming tricks here.

 

 

 

Learning about weather

March 17, 2019

I have just completed a FREE! ‘Learn About Weather‘ course, and slightly to my surprise I think I have learned some things about the weather!

Learning

Being an autodidact in the fields of Weather and Climate, I have been taught by an idiot. So ‘attending’ online courses is a genuine pleasure.

All I have to do is to listen – and re-listen – and then answer the questionsSomeone else has selected the topics they feel are most important and determined the order of presentation.

Taking a course on-line allows me to expose my ignorance to no-one but myself and the course-bot. And in this low-stress environment it is possible to remember the sheer pleasure of just learning stuff.

Previously I have used the FutureLearn platform, for courses on Global WarmingSoil, and Programming in Python. These courses have been relatively non-technical and excellent introductions to subjects of which I have little knowledge. I have also used the Coursera platform for a much more thorough course on Global Warming.

So what did I learn? Well several things about about why Global Circulation Cells are the size they are, the names of the clouds, and how tornadoes start to spin. But perhaps the best bit was finally getting my head around ‘weather fronts’.

Fronts: Warm and Cold

I had never understood the terms ‘warm front’ and ‘cold front’ on weather forecasts. I had looked at the charts with the isobars and thought that somehow the presence or absence of ‘a front’ could be deduced by the shapes of the lines. I was wrong. Allow me to try to explain my new insight.

Air Mixing

Air in the atmosphere doesn’t mix like air in a room. Air in a room generally mixes quite thoroughly and quite quickly. If someone sprays perfume in one corner of the room, the perfume spreads through the air quickly.

But on a global scale, air doesn’t mix quickly. Air moves around as ‘big blobs’ and mixing takes place only where the blobs meet. These areas of mixing between air in different blobs are called ‘fronts’

Slide1

In the ‘mixing region’ between the two blobs, the warm – generally wet – air meets the cold air and the water vapour condenses to make clouds and rain. So fronts are rain-forming regions.

Type of front

However it is unusual for two blobs of air to sit still. In general one ‘blob’ of air is ‘advancing’ and the other is ‘retreating’.

This insight was achieved just after the First World War and so the interfaces between the blobs were referred to as ‘fronts’ after the name for the interface between fighting armies. 

  • If the warm air is advancing, then the front is called a warm front, and
  • if the cold air is advancing, then the front is called a cold front.

Surprisingly cold fronts and warm fronts are quite different in character.

Warm Fronts 

When a blob of warm air advances, because it tends to be less dense than the cold air, it rises above the cold air.

Thus the mixing region extends ahead of the location on the ground where the temperature of the air will change.

The course told me the slope of the mixing region was shallow, as low as 1 in 150. So as the warm air advances, there is a region of low, rain-forming cloud that can extend for hundreds of kilometres ahead of it.

Slide2

So on the ground, what we experience is hours of steady rain, and then the rain stops as the temperature rises.

Cold Fronts 

When a blob of cold air advances, because it tends to be more dense than the warm air, it slides below it. But sliding under an air mass is harder than gliding above it – I think this is because of friction with the ground.

As a result there is a steep mixing region which extends a little bit ahead, and a short distance behind the location on the ground where the temperature of the air changes.

Slide3

So as the cold air advances, there is a region of intense rain just before and for a short time after.

So on the ground what we experience are stronger, but much shorter, rain events at just about the same time as the temperature falls. There generally follows some clearer air – at least for a short while.

Data

I had assumed that because of the messy nature of reality compared to theory, real weather data would look nothing like what the simple models above might lead me to expect. I was wrong!

As I was learning about warm and cold fronts last weekend (10 March 2019) by chance I looked at my weather station data and there – in a single day – was evidence for what I was learning – a warm front passing over at about 6:00 a.m. and then a cold front passing over at about 7:00 p.m.

  • You can look at the data from March 10th and zoom in using this link to Weather Underground.

This is the general overview of the air temperature, humidity, wind speed, rainfall and air pressure data. The left-hand side represents midnight on Saturday/Sunday and the right-hand side represents midnight on Sunday/Monday.

Slide4

The warm front approaches overnight and reaches Teddington at around 6:00 a.m.:

  • Notice the steady rainfall from midnight onwards, and then as the rain eases off, the temperature rises by about 3 °C within half an hour.

The cold front reaches Teddington at around 7:00 p.m.:

  • There is no rain in advance of the front, but just as the rain falls – the temperature falls by an astonishing 5 °C!

Slide5

Of course there is a lot of other stuff going on. I don’t understand how these frontal changes relate to the pressure changes and the sudden rise and fall of the winds as the fronts pass.

But I do feel I have managed to link what I learned on the course to something I have seen in the real world. And that is always a good feeling.

P.S. Here’s what the Met Office have to say about fronts…

Weather Station Comparison

January 7, 2019
img_7898

My new weather station is on the top left of the picture. The old weather station is in the middle of the picture on the right.

Back in October 2015 I installed a weather station at the end of my back garden and wrote about my adventures at length (Article 1 and Article 2)

Despite costing only £89, it was wirelessly linked to a computer in the house which uploaded data to weather aggregation sites run by the Met Office and Weather Underground. Using these sites, I could compare my readings with stations nearby.

I soon noticed that my weather station seemed to report temperatures which tended to be slightly higher than other local stations. Additionally, I noticed that as sunshine first struck the station in the morning, the reported temperature seemed to rise suddenly, indicating that the thermometer was being directly heated by the sunlight rather than sensing the air temperature.

So I began to think that the reported temperatures might sometimes be in error. Of course, I couldn’t prove that because I didn’t have a trusted weather station that I could place next to it.

So in October 2018 I ordered a new Youshiko Model YC9390 weather station, costing a rather extravagant £250.

Youshiko YC9390

The new station is – unsurprisingly – rather better constructed than the old one. It has a bigger, brighter, internal display and it links directly to Weather Underground via my home WI-FI and so does not require a PC. Happily it is possible to retrieve the data from Weather Underground.

The two weather stations are positioned about 3 metres apart and at slightly different heights, but in broad terms, their siting is similar.

Over the last few days of the New Year break, and the first few days of my three-day week, I took a look at how the two stations compared. And I was right! The old station is affected by sunshine, but the effect was significantly larger than I suspected.

Comparison 

I compared the temperature readings of the two stations over the period January 4th, 5th and 6th. The fourth was a bright almost cloudless, cold, winter day. The other two days were duller, but warmer, and all three days were almost windless.

The graphs below (all drawn to the same scale) show the data from each station versus time-of-day with readings to be compared against the left-hand axis.

Let’s look at the data from the 4th January 2019

4th January 2019

Data from the 4th January 2019. The red curve shows air temperature data from the old station and the blue curve shows data from the new station. Also shown in yellow is data showing the intensity of sunshine (to be read from the right-hand axis) taken from a station located 1 km away.

Two things struck me about this graph:

  • Firstly I was surprised by the agreement between the two stations during the night. Typically the readings are within ±0.2 °C and with no obvious offset.
  • Secondly I was shocked by the extent the over-reading. At approximately 10 a.m. the old station was over-reading by more than 4 °C!

To check that this was indeed a solar effect I downloaded data from a weather station used for site monitoring at NPL – just over a kilometre away from my back garden.

This station is situated on top of the NPL building and the intensity of sunlight there will not be directly applicable to the intensity of sunshine in my back garden. But hopefully, it is indicative.

The solar intensity reached just over 200 watts per square metre, about 20% of the solar intensity on a clear midsummer day. And it clearly correlated with the magnitude of the excess heating.

Let’s look at the data from the 5th January 2019

slide2

Data from 5th January 2019. See previous graph and text for key.

The night-time 5th January data also shows agreement between the two stations as was seen on the 4th January.

However I was surprised to see that even on this dismally dull January day – with insolation failing to reach even 100 watts per square metre – that there was a noticeable warming of the old station – amounting to typically 0.2 °C.

The timing of this weak warming again correlated with the recorded sunlight.

Finally let’s look at data from 6th January 2019

slide3

Data from 6th January 2019. See previous graph and text for key.

Once again the pleasing night-time agreement between the two station readings is striking.

And with an intermediate level of solar intensity the over-reading of the old station is less than on the 4th, but more than on the 5th.

Wind.

I chose these dates for a comparison because on all three days wind speeds were low. This exacerbates the solar heating effect and makes it easier to detect.

The figures below show the same temperature data as in the graphs above, but now with the wind speed data plotted in green against the right-hand axis.

Almost every wind speed reading is 0 kilometres per hour, and during the nights there were only occasional flurries.  However during the day, there were slightly more frequent flurries, but as a pedestrian, the day seemed windless.

slide4

Data from 4th of January 2019 now showing wind speed on the right-hand axis.

slide5

Data from 5th of January 2019 now showing wind speed on the right-hand axis.

slide6

Data from the 6th January 2019 showing wind speed against the right-hand axis.

Conclusions 

My conclusion is that the new weather station shows a much smaller solar-heating effect than the old one.

It is unlikely that the new station is itself perfect. In fact there is no accepted procedure for determining what the ‘right answer’ is in a meteorological setting!

The optimal air temperature measurement strategy is usually to use a fan to suck air across a temperature sensor at a steady speed of around 5 metres per second – roughly 18 kilometres per hour! But stations that employ such arrangements are generally quite expensive.

Anyway, it is pleasing to have resolved this long-standing question.

Where to see station data

On Weather Underground the station ID is ITEDDING4 and its readings can be monitored using this link.

The Weather Underground ‘Wundermap’ showing world wide stations can be found here. On a large scale the map shows local averages of station data, but  as you zoom in, you can see teh individual reporting stations.

The Met Office WOW site is here. Search on ‘Teddington’ if you would like to view the station data.

Ignorance: Eggs & Weather Forecasts

November 26, 2018

Every so I often I learn something so simple and shocking that I find myself asking:

How can I possibly not have known that already?“.

Eggs

Eggs

Eggs

While listening to Farming Today the other morning, learned that:

Large eggs come from old hens

In order to produce large eggs – the most popular size with consumers – farmers need to allow hens to reach three years old.

So during the first and second years of their lives they will first lay small eggs, then medium eggs, and finally large eggs.

On Farming Today a farmer was explaining that egg production naturally resulted a range of egg sizes, and it was a challenge to find a market for small eggs. Then came the second bomb’shell’.

The yolk is roughly same size in all eggs

What varies between small and large eggs is mainly the amount of egg white (albumen).

How could I have reached the age of 58 and not  known that? Or not have even been curious about it?

Since learning this I have become a fan of small eggs: more yolk, less calories, more taste!

But my deep ignorance extends beyond everyday life and into the professional realm. And even my status as ‘an expert’ cannot help me.

Weather Forecasts & Weather Stations

Professionally I have become interested in weather stations and their role in both Numerical Weather Prediction (NWP, or just weather forecasting) and in Climate Studies.

And as I went about my work I had imagined that data from weather stations were used as inputs to NWP algorithms that forecast the weather.

But in September I attended CIMO TECO-2018 (Technical Conference on Meteorological and Environmental Instruments and Methods of Observation) in Amsterdam.

And there I learned in passing from an actual expert, that I had completely misunderstood their role.

Weather station data is not considered in the best weather forecasts.

And, on a moment’s reflection, it was completely obvious why.

Weather forecasting work like this:

  • First one gathers as much data as possible about the state of the atmosphere ‘now’. The key inputs to this are atmospheric ‘soundings’:
    • Balloon-borne ‘sondes’ fly upwards through the atmosphere sending back data on temperature, humidity and wind (speed and direction) versus height.
    • Satellites using infrared and microwave sensors probe downwards to work out the temperature and humidity at all points in the atmosphere in a swathe below the satellite’s orbit.
  • The NWP algorithms accept this vast amount of data about the state of the atmosphere, and then use basic physics to predict how the state of the entire atmosphere will evolve over the coming hours and days

And then, after working out the state of the entire atmosphere, the expected weather at ground level is extracted.

Visualisation of the amount of moisture distributed across different heights in the atmosphere based on a single pass of a 'microwave sounding' satellite. Image credit: NASA/JPL-Caltech

Visualisation of the amount of moisture distributed across different heights in the atmosphere based on a single pass of a ‘microwave sounding’ satellite. The data gathered at ground level is just a tiny fraction of the data input to NWP models. Image credit: NASA/JPL-Caltech

Ground-based weather stations are still important:

  • They are used to check the outputs of the NWP algorithms.
  • But they are not used as inputs to the NWP algorithms.

So why did I not realise this ‘obvious’ fact earlier? I think it was because amongst the meteorologists and climate scientists with whom I spoke, it was so obvious as to not require any explanation.

Life goes on

So I have reached the age of 58 without knowing about hen’s eggs and the role of weather stations in weather forecasting?

I don’t know how it happened. But it did. And I suspect that many people have similar areas of ignorance, even regarding aspects of life with which we are totally familiar – such as eggs – or where one is nominally an expert.

And so life goes on. Anyway…

This pleasing Met Office video shows the importance of understanding the three-dimensional state of the atmosphere…

And here is a video of some hens

 

How hard did it rain last week?

August 22, 2014

Hardcore Protonistas – and that’s you if you are reading this – will know that I love measuring things.

I love the way that measurements allow an extra level of wonder at the intricate detail of the world around us.

The rate of rainfall at NPL Teddington on 14th August 2014. The rainfall rate initially exceed 100 mm per hour. The first event resulted in 28 mm of rain and the second 18 mm of rain.

The rate of rainfall at NPL Teddington on 14th August 2014. The rainfall rate initially exceed 100 mm per hour. The first event resulted in 29.3 mm of rain and the second 18.4 mm of rain.

Last Thursday 14 August 2014 there were two torrential rain events at NPL.

They were kind of event that makes you stop what you are doing, go to the window and just stare. Personally they evoked memories of my childhood home and the security of being indoors and protected.

All very nice. But how much rain fell? And was it exceptional? These questions can only be answered by looking at data.

So I downloaded data from a weather station on the roof of one of NPL’s buildings. The graph at the top of the page shows the key features of the data.

  • In both events the rate of rainfall was initially over 100 millimetres per hour
  • The first event deposited 29.3 mm of rain and the second 18.4 mm of rain.

These data show that these were indeed powerful weather events.

Over the main NPL site, which comprises approximately 400 m  x 100 m = 40,000 square metres, the first event result in the deposition of approximately 1160 tonnes of water in approximately 50 minutes. Wow!

But were the events exceptional? The Met Office keep a record of extreme weather events (Link) which states that the most extreme UK rainfall events have been:

  • Highest 5-minute total: 32 mm on 10 August 1893 Preston (Lancashire)
  • Highest 30-minute total: 80 mm on 26 June 1953 Eskdalemuir (Dumfries & Galloway)
  • Highest 60-minute total: 92 mm on 12 July 1901 Maidenhead (Berkshire)

Assuming these historic measurements are indeed reliable – which is not always the case – then the events in Teddington last week  were not technically ‘extreme’.

However they were astounding – and in the very best sense of the word – wonder-ful.


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