Learning Weather - Part One - Understanding Models.

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stephenprudence
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Learning Weather - Part One - Understanding Models.

Post by stephenprudence » Tue Nov 15, 2011 10:40 pm

I know a few don't understand models when they'd like to, so I will start topics about the weather, in order to help out people who want to understand what it all means and this can then help towards better being prepared in the garden when a frost is next forecast.

First of all, the models are basically tools of prediction, they use data sources to make their predictions, either from Satellites, or some from Buoys in the ocean. The fact models are able to predict weather at all is fascinating and unlikely.. but they can and do.

You may notice you see a lot of talk about 0z, 6z, 12z and 18z.. these are just the times the model run is released (0 being midnight, 6am, 12pm and so on). The 12z run collects the most data points and therefore is considered to be the most accurate run, whilst the 6z run collects the least data and is referred to as the least accurate run.

Below is an annotated chart showing how the chart works and to some extent, the basic principle workings of the atmosphere.

The first thing you'll notice on the first chart (500hPa/heptapascals) is that there are lots of colours and lines.. the lines represent pressure, and these are measured in 'isobars'. The best way you can imagine pressure is like a wedding cake, with pressure piled ontop of each other in layers. In low pressure (marked by the letter 'T' on the map below), airflow is anti-clockwise around the low pressure system, whilst on high pressure (marked by the letter 'H') the air circulates clockwise. so the best way to tell the air flow is to follow the lines around depending on whether it is low pressure or high pressure, and you will find your airflow direction. The colours are not temperature indicators and many people first think, the colours on the map below are pressure indicators. The higher the pressure is, the more oranges and reds will exist while conversely, the lower the pressure is the more greens and blues there are. If we look at the chart below we can see that Britain is sitting in High pressure, and the winds are from the south (using the anti clockwise, or clockwise method - in fact follow the instructions for the high pressure to the east of Britain and the low pressure to the West of Britain - you'll notice they both draw up southerly winds over Britain.

Image

The next chart often used is called the 850hPa temperature chart - this measures the temperature at 850 metres altitude (under normal conditions). The temperature can usually give you an idea of whether it'll be warmer or cooler on the ground. In general the warmer 850 metre temperature will correspond with warmer ground temperatures, but this isn't always the case, in an inversion situation for example (this will be covered in another guide). Typical 850hPa temperatures you may expect in high summer are roughly 8-10C, and in winter, typical -2C to -3C. This might sound cold, but remember it's higher in altitude and therefore naturally colder than average sea level temperatures. The accepted 850hPa temperature for snowfall, is circa -5C, this happens fairly frequently during winter, but doesn't always result in snowfall. In a similar way +10C at 850hPa doesn't always guarantee a heatwave especially in Autumn/Winter.

I have attached two charts below, one from 2003 showing an exceptionally hot 850hPa profile which led to the hottest temperature on record in 2003, and also a cold 850hPa profile, from a winter cold spell in 1981.

Image

Image

The next chart to study is the 'thickness' chart. This is a chart which shows airmasses, an airmass has a certain thickness depending on the temperature of the air. In general colder airmasses tend to be less 'thick' than warmer airmasses. In general the thickness lines are there to show what type of airmass there will be over any given area and these are measured are by the ZDL (or the zero degree level)

There are three main thickness zones that occur in an average year over the British Isles; they are 528DAM, 546DAM and 564DAM. There is also 492DAM, 510DAM and 582DAM which occur much less frequently, or not at all. DAM stands for Decameters (10's of metres) so the higher than decameter value, the higher the freezing level is, and so the warmer the ground level will be. So in the Arctic circle in January you might see a zone of 510 decametres, this is a cold, dry airmass, subsequently you may see 582DAM over North Africa in July.. signal of a very hot airmass. On the chart which shows the decameter thickness measurement, you will also see some white misty shading, this is the cloud amounts, it shows the position and intensity of cloud (fairly straightforward). On the chart below you'll notice that the 546DAM line is north of Britain, this indicates a fairly mild airmass sits over the UK.. not as cold as the 528DAM (which is often a marker for snow along with the -5C at 850hPa as explained above), and not as warm as the 564DAM which often bring warm, humid weather with it in summer.

The map below shows the Thicknesses and clouds.

Image

Next chart is the rainfall chart, it's fairly straightforward - the darker blue/pinks represent the heaviest rainfall, the lightest blues and greens represent the lightest rainfall - there is a intensity measurement key on the right of the chart. You'll notice other things on these charts, namely convective potential. Convective potential is represented by dots, and where these dots occur, it represents rainfall that will be convective in nature (in other words showers creates by the thermal input of solar radiation). Rainfall that is not over-layed with these dots is either none-convective frontal rainfall or drizzle.

Chart below showing rainfall and convective potential.

Image

These are the only charts you'll need to know to have a basic understanding of the charts we post/use, we can post others but it's very rare we do. If you have any questions or still don't understand, let me know.
Last edited by stephenprudence on Tue Nov 15, 2011 10:59 pm, edited 2 times in total.
Heswall, Wirral, UK
USDA equivalent average temperature zone: 9a/RHS zone 3
AHS Heat Zone: 1
Last 5 winter minimums:
2007: -0.1C, 2008: -4.2C, 2009: -5.7C, 2010: -10.5 (record), 2011: -4.9C, 2012: -5.3, 2013: -4.5C (so far)


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Re: Learning Weather - Part One - Understanding Models.

Post by flounder » Tue Nov 15, 2011 10:49 pm

I don't often swear, but this is effin' brilliant!! many thanks for putting in the time to explain icon_thumright icon_thumright icon_thumright
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Re: Learning Weather - Part One - Understanding Models.

Post by Conifers » Tue Nov 15, 2011 11:02 pm

In the first map, what's the difference between the white lines of pressure gradient, and the different colours of pressure gradient? They don't always match, e.g. over Nova Scotia, the white lines show pressure rising from NE (low) to SW (high), while the colours show pressure rising from NW (low) to SE (high).


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Re: Learning Weather - Part One - Understanding Models.

Post by stephenprudence » Tue Nov 15, 2011 11:28 pm

The colours show the 'geopotential' of the high pressure (in other words to what extent high or low pressure may influence, geographically speaking), whilst the white lines is pressure defined by altitude and area. For example high pressure over Greenland maybe influenced by its topography as Greenland is a high plateau, therefore very tight isobars may show this.

Below is a chart to try to explain how the isobars (white lines) work.
Attachments
pressure.jpg
Heswall, Wirral, UK
USDA equivalent average temperature zone: 9a/RHS zone 3
AHS Heat Zone: 1
Last 5 winter minimums:
2007: -0.1C, 2008: -4.2C, 2009: -5.7C, 2010: -10.5 (record), 2011: -4.9C, 2012: -5.3, 2013: -4.5C (so far)


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Re: Learning Weather - Part One - Understanding Models.

Post by jacko » Wed Nov 16, 2011 11:37 am

in the first graph you've got white boxes saying LOW PRESSURE CLOCKWISE and HIGH PRESSURE ANTI- CLOCKWISE. is'nt this the wrong way round?
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Re: Learning Weather - Part One - Understanding Models.

Post by stephenprudence » Wed Nov 16, 2011 12:12 pm

jacko wrote:in the first graph you've got white boxes saying LOW PRESSURE CLOCKWISE and HIGH PRESSURE ANTI- CLOCKWISE. is'nt this the wrong way round?
Yes apologies, I hadn't noticed that, the arrows are correct though. Thanks for spotting that, I missed it icon_thumleft
Heswall, Wirral, UK
USDA equivalent average temperature zone: 9a/RHS zone 3
AHS Heat Zone: 1
Last 5 winter minimums:
2007: -0.1C, 2008: -4.2C, 2009: -5.7C, 2010: -10.5 (record), 2011: -4.9C, 2012: -5.3, 2013: -4.5C (so far)


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Re: Learning Weather - Part One - Understanding Models.

Post by Dave Brown » Wed Nov 16, 2011 3:30 pm

Very interesting and informative icon_thumright

I would never have got the colour being pressure as this suggests pressure is always higher nearer the equator, and our highest pressures occur in mid winter, so I assumed the polar winter highs were much higher. Cold very dense air weighs more increasing pressure at ground level icon_scratch

Also, I can understand using Kilo meaning 1000 in kilometer, but what is the point in reducing meters by one digit, then having to add two extra characters to identify it ???

100m = 10DAM :lol:
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Re: Learning Weather - Part One - Understanding Models.

Post by stephenprudence » Wed Nov 16, 2011 3:45 pm

Dave Brown wrote:Very interesting and informative icon_thumright

I would never have got the colour being pressure as this suggests pressure is always higher nearer the equator, and our highest pressures occur in mid winter, so I assumed the polar winter highs were much higher. Cold very dense air weighs more increasing pressure at ground level icon_scratch

Also, I can understand using Kilo meaning 1000 in kilometer, but what is the point in reducing meters by one digit, then having to add two extra characters to identify it ???

100m = 10DAM :lol:

I'm not sure why, although you do see higher pressure at times when there is a strong Greenland High. The issue is, that Greenland Highs are sometimes known as plateau highs, and is a high pressure caused by the topography of Greenland. True High Pressures are orange, so you know it's high pressure. When I learnt I always assumed the colours were the temperatures at that altitude, but now I know not, although I don't fully understand why pressure is always orange in Southern Europe/Africa.

As again who knows, the must go for 3 significant figures however they work it out.. I'm not really knowledgeable on the mathematics side of modelling so I can't explain why they use Decametres.
Heswall, Wirral, UK
USDA equivalent average temperature zone: 9a/RHS zone 3
AHS Heat Zone: 1
Last 5 winter minimums:
2007: -0.1C, 2008: -4.2C, 2009: -5.7C, 2010: -10.5 (record), 2011: -4.9C, 2012: -5.3, 2013: -4.5C (so far)


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Re: Learning Weather - Part One - Understanding Models.

Post by kata » Wed Nov 16, 2011 5:31 pm

I can't understand them Stephen

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Re: Learning Weather - Part One - Understanding Models.

Post by Nigel » Wed Nov 16, 2011 6:40 pm

stephenprudence wrote:
jacko wrote:in the first graph you've got white boxes saying LOW PRESSURE CLOCKWISE and HIGH PRESSURE ANTI- CLOCKWISE. is'nt this the wrong way round?
Yes apologies, I hadn't noticed that, the arrows are correct though. Thanks for spotting that, I missed it icon_thumleft
Its really strange that because earlier I made exactly the same freudian slip, strange we both did same thing Stephen.
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Re: Learning Weather - Part One - Understanding Models.

Post by stephenprudence » Wed Nov 16, 2011 8:15 pm

Yes Nigel, I was convinced I'd done it properly last night, like you say, strange.
Heswall, Wirral, UK
USDA equivalent average temperature zone: 9a/RHS zone 3
AHS Heat Zone: 1
Last 5 winter minimums:
2007: -0.1C, 2008: -4.2C, 2009: -5.7C, 2010: -10.5 (record), 2011: -4.9C, 2012: -5.3, 2013: -4.5C (so far)


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Re: Learning Weather - Part One - Understanding Models.

Post by Andy P » Wed Nov 16, 2011 9:34 pm

I Presume when they refer to 0z , 6z, 10z etc the `Z` refers to Zulu time? or UTC
“I do not fear death. I had been dead for billions and billions of years before I was born, and had not suffered the slightest inconvenience from it.” Mark Twain.


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Re: Learning Weather - Part One - Understanding Models.

Post by stephenprudence » Wed Nov 16, 2011 9:41 pm

Andy P wrote:I Presume when they refer to 0z , 6z, 10z etc the `Z` refers to Zulu time? or UTC
It will be zulu time, however the charts are release 5 hours after the charts inception in the UK, for example at this time of year;

0z shows at 5am, 6z shows at 11am, 12z shows at 5pm, and 18z shows at 11pm.

I think it takes a fair while for the computer to process the data, hence the 4-5 hour delay before it shows.
Heswall, Wirral, UK
USDA equivalent average temperature zone: 9a/RHS zone 3
AHS Heat Zone: 1
Last 5 winter minimums:
2007: -0.1C, 2008: -4.2C, 2009: -5.7C, 2010: -10.5 (record), 2011: -4.9C, 2012: -5.3, 2013: -4.5C (so far)


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Re: Learning Weather - Part One - Understanding Models.

Post by Andy P » Sat Nov 19, 2011 2:57 am

Something i would like to ask on this subject -- And bear with me cos i`m a bit dumb when it comes to meteorology and it`s a difficult question for me to get across.
From what i understand, all the earth`s weather systems ( in the Northern hemisphere) move from West to East. The Gulf stream for example moves warm waters from the Caribbean -West to East. Whenever i`ve travelled East to Asia in an aeroplane- the flight there is a lot shorter than when you travel back. Due to the very fast and strong winds of the Jet stream that flow from West to East.And likewise, if i flew to the USA, the flight there takes longer than the flight back, Fighting against these winds. So why is it ( if the winds and tidal flows move so strongly from West to East in the Northern Hemisphere) do we sometimes get weeks of freezing weather - apparently from Siberia.
Siberia is thousands of miles East from the UK ,so how can their weather ever reach here considering the strong West to East jet streams,tidal flows and the huge distance between us?
“I do not fear death. I had been dead for billions and billions of years before I was born, and had not suffered the slightest inconvenience from it.” Mark Twain.


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Re: Learning Weather - Part One - Understanding Models.

Post by redsquirrel » Sat Nov 19, 2011 7:08 am

also Steve,could you possibly mark a chart showing where highs and lows would be in an ideal winter to compare against please? if the ones shown are that,just say.it would me help look to see and maybe understand what we dont want
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