The Novice's Chart Thread

[smich]

Hi All,

I've noticed how many appreciative comments appear when our resident forecasters (and others) post forecasts in plain English. Furthermore, after reading one of Steve Murr posts (which are more technical than most) I hit on the idea of creating a thread where those who DON'T understand synoptic charts can come and post their thought/opinions/questions.

One rule: No acronyms. "The CET will be lower because the GH is affected by the PFJ leading to a -NAO" won't help us.

I must stress - I'm a novice too (although I've learn't a lot these last few months). I'm hoping that some of the "seniors" will pop in every now and again and offer their guidance. Also, I recommend we all pop into the Net Weather Guides section as much as possible. There's plenty of help there too.

I'm going to start with a chart I DO recognise cos it's similar to ones we see on the beeb:

It's certainly topical: Midday Thursday! Exciting times... but now I'll add a chart that I find much harder to fathom, even though it looks similar (even if it is in colour!)

As I understand it, this chart is the forecast relating to the "upper level" of our atmosphere with air that is 500 millibars thick. That's at about 18,000 feet apparently. Should we expect to see a lot of differences? I find it really tough getting my head round the "depth" element to this chart.

Anybody got any info they've picked up that could help me here?

Smich

[Dazza]

Hi Smich, what a great idea. Maybe we could get this one pinned (a request if any mods are looking in).

Smich, I am sorry I cannot offer any help on your chart (as I am a novice too), but you or someone else may be able to answer a simple question I have about the 500hpa chart - what do the negative figures represent on the chart?

I would be grateful for any help.

[Andy H]

Smich

good idea mate lets hope we get a lot of help from the mods and the people all of us look for on here whos opinion we appreciate

Edited November 21, 2005 by andy2726

[Krasnoyarsk Yenesei]

I believe your numbers refer to the temperatures at your 18,000 feet mark. The temperatures vary at different levels in the atmosphere, as you may have noticed if you have ever tracked this on the screens of a plane during take off. In order to get snow, the temperatures at the 850mb need to be -6 minimum.

From what I have discovered on this site, I believe this to be correct information. If not, a visiting expert will surely put us right.

[Medway Snowball]

Hi Smich

Great Idea!

Coming from a non meterological background i have learnt to stick to the basics.

The UKMO chart is good at showing charts that are familiar to any bbc weather watcher.

It's weakness lies in that it only shows 2 charts per day, it's limited to T (Time) +132 hours and

does not show any temperature variations.

The Gfs 500 HPa charts do not show any "weather fronts" or temperature but does have the advantage?? of being

updated every 6 hours. It also runs out to T+384 hours although anthing after T +168 hours runs into

F.I. territory ( better known as Fantasy Island - Or unlikely to happen).

Since each run contains different sampling, there tends to be differing outcomes especially beyond

T +120. 06Z/12Z seem to be the colder "outers" and for cold lovers, like myself, always worth looking

at. Probably what everyone is looking for is over a number of days are there any trends developing.

Below T +120 all the charts are becoming within the accuracy range but when i hear the term "marginal"

you know the charts may not be accurate right up to Time Zero, in which case the best thing to do is look

outside of the window to see what the weather is like! Finally they superbly show the "DAM???" lines which

seem all important for snow lovers.

The final chart i normally view is the Gfs 850 Hpa charts which do show temperature information. When

i first saw these charts i got really excited and then i realised that as a rule of thumb you need to ADD

10 degrees Celcius to get a rough idea of temperature at ground level!

In summary my own predictions? are based on these three charts, particuarly the Gfs charts.

Firstly trying to spot trends and then when something special materializes in the Realistic timeframe

following the "Rollercoaster" down to Time Zero. I look for the "DAM" to be below say 524, accompanied

by 850 Hpa temperature down to say -5 to -8.Finally i look at UKMO to see if there are any weather front

or lines of organised showers up wind of me. With a bit of luck they will fall as SNOW!

Regards MS

[Thundery wintry showers]

There are also some pretty useful guides in here:

http://www.net-weather.co.uk/forum/index.php?showforum=5

[Dazza]

I believe your numbers refer to the temperatures at your 18,000 feet mark. The temperatures vary at different levels in the atmosphere, as you may have noticed if you have ever tracked this on the screens of a plane during take off. In order to get snow, the temperatures at the 850mb need to be -6 minimum.

From what I have discovered on this site, I believe this to be correct information. If not, a visiting expert will surely put us right.

Hi Africanprotea,

Thanks for that, that makes sense. I hope to be one of those members that are incredibly active in the future (steep learning curve first though) .

[tugmistress]

ok silliness time

to me that chart looks like the low north east of the uk is sat right over the coast of norway, going by the blue colours the air way up at 18,000ft is pretty dam cold...hang on, scrolling for a look, between -35c and -40c (the lightest blue i am looking at) that is going to hit the top of the uk in wind. ok i know it wont hit at that temp at ground level it will be considerably warmer.

am i right in thinking the air goes around a low in an anti-clockwise direction? if so, then is that why the air going back north around the same low is coloured green and warmer?

ok i'll shut up now as i have just totally confused myself let alone anyone else

[smich]

Well things are off to a splendid start here!

Lots of useful stuff for me already. Thanks MS for all that info. Especially useful for me was 850 hPa temps are about 10 degrees lower than ground level.

TM - that's right. Anti-clockwise for a low, clockwise for a high. (Although sometimes the GFS charts are blimmin littered with letters and I can't work out where the wind is going).

TWS cheers for the link to the section I mentioned at the top.

I'm gonna read up on DAM lines today and see if I can a) get my head round the subject and explain it in plain English

Smich

[BrickFielder]

Here is a rough outline of what the chart heights are :

1000hPa = 0 feet

900hPa = 3000 feet

850hPa = 4750 feet

800hPa = 6000 feet

700hPa = 10000 feet

500hPa = 18000 feet

300hPa = 30000 feet

200hPa = 39000 feet

At 200-300hPa which is near the top of the troposphere (The lowest level in the atmosphere where our weather is, above which is the Stratosphere) is where we find the jetstream. The jetstream is a ribbon of wind which seperates cold air from warm, which can arch and pulse as it circulates the globe. Actually there are a number of jet streams and the one which normally affects us in winter is the Polar Front Jet (PFJ) as opposed to the subtropical jet (STJ) or the Stratospheric Night Jet found in the artic circle.

The question which is usually asked is why look at higher levels when we live at low levels. The answer is that we are interested in the what happens when the air rises or sinks and where air is moving to and from. Cold air above us tends to make air rise and as it rises it is likely to form clouds. The colder the air the more unstable it will be. By unstable I mean that the air will have a tendency to rise. So when we look at charts at different levels we are trying to assess how far air will rise and in effect will there be clouds and will they reach great heights like thunderstorms (25000+ feet). We also look at how quickly temperatures drop so that we can judge how strong winds will be.We also try to gage what type of precipitation (rain or snow) will fall based on the temperatures.

We are also interested in dewpoints and humidity which are the temperature at which water starts to form when air is cooled and how much water is in the air. The moister the air the higher the dewpoint, the more cloud and lower cloud base we tend to have.

At the minute you should probably be interested in the fact that high pressure is sinking air and in the northern hemisphere means a clockwise circulation and low pressure means rising air and an anticlockwise circulation. Remember that low pressure does not always mean low temperatures at low levels. Low pressure usually has air rushing towards it from somewhere else and if that somewhere else is the artic then it brings cold air and if it is the tropics then it can bring humid warm air. Similarly high pressure does not always mean high temperature, it does usually mean lots of sunshine, but even here if it approaches from the south, then warm moist air will be cooling and form a thin layer of cloud.If high pressure approaches from the north you should get clear skies. High pressure also means sinking air so cold air from above sinks to the ground and this quite often means overnight frosts or low temperatures (In addition to no clouds holding the warmth in).

Troughs and Ridges are formed where the jetstream arches upwards or downwards. Where it arches up then a pool of warm air is swept north and this is known as a ridge. Troughs form when the jetstream arches south and a pool of cold air is swept south. The 500hPa chart that smich posted above shows a ridge up towards greenland and a trough down over the UK. The arching of the jetstream is often caused by pulses of stronger wind in the jetstream which are often caused by low pressure spining winds fastly around them.In turn low pressure systems tend to form in troughs and at the entrance or exit regions of these pulses(jetstreaks). Weather fronts also tend to form at boundaries between cold and warm air, which is also where the jetstream can be found. On the 500hPa chart above the boundary would normally be where green changes to yellow, so you might expect a band of cloud there.

[JohnAcc]

ok silliness time :blush:

to me that chart looks like the low north east of the uk is sat right over the coast of norway, going by the blue colours the air way up at 18,000ft is pretty dam cold...hang on, scrolling for a look, between -35c and -40c (the lightest blue i am looking at) that is going to hit the top of the uk in wind. ok i know it wont hit at that temp at ground level it will be considerably warmer.

am i right in thinking the air goes around a low in an anti-clockwise direction? if so, then is that why the air going back north around the same low is coloured green and warmer?

ok i'll shut up now as i have just totally confused myself let alone anyone else :blush:

Hi all,

I thought it would be useful if we add an on-line tutorial that people can refer to from time to time. Every so often I will post a basic guide to enhance peoples knowledge of the various influences on our climate.

OK, following on from TM's question I will give an overview of what happens in the upper air.

Overview summary 1 - The upper air

We can split the upper air into 3 layers. The troposphere which is between 8-17km, the stratosphere which is between 17-45km and the mesosphere which is greater than 80km.

We can then further split the troposhere into 2 sections which are the upper layer (also known as the free atmosphere) and the lower, or planetary boundary layer

Within the boundary layer air movements and temperatures are affected major weather patterns but also by localised effects (How the air reacts to the planets surface)

Above the Boundary Layer, winds are directed by the Coriolis Effect working in conjunction with pressure-gradients. The Coriolis Effect is the force exterted by the Earth's rotation, and in weather terms its importance is the effect it has on the atmosphere. In the Northern Hemisphere, it causes airmasses to be deflected to the right - the opposite happens in the Southern Hemisphere. Thus, in the Northern Hemisphere, warm and cool airmasses around a developing low-pressure centre start to circulate in an anticlockwise (cyclonic) direction. So the Coriolis Effect is what makes depressions rotate; on a larger scale it helps to maintain the prevailing west-to-east airflow around our hemisphere.

In upper-air meteorology, pressure-patterns are as important as they are down here at the surface. Atmospheric pressure is simply an expression of the force applied by a column of air upon a fixed point of known area:

p=F/A where p = pressure, F = force and A = area.

The greater the altitude, the lower the atmospheric pressure, hence the rarified air encountered in high mountain ranges. In upper-air meteorology, goings-on aloft are observed with satellites and directly sampled by weather-balloons carrying measuring instruments. The results of the balloon ascents, called soundings, are plotted on charts at several pressure-levels (925hpa (900m) 850hpa (1500m), 700hpa (3000m), 500hpa (5750m) and 300hpa (9500m))

Pressure at any given height can change quite drastically as weather-systems move through. This goes on at the surface too. In the case of the UK, as an Atlantic low-pressure system is replaced by a large high-pressure area, the pressure over a few days at sea-level can change from maybe 970 hPa to 1030 hPa. The same applies aloft, but unlike surface charts, where the data are plotted in terms of pressure, the upper air data are plotted in terms of geopotential. Geopotential is the height above sea-level where the pressure is, say, 850, 500 or 300 hPa, and is measured in Geopotential Metres (gpm or gpdm). In an area of high pressure (an anticlyclone) the 850hPa level will be at a higher altitude than in an area of low pressure, so that although a different method of measurement is being used in the upper air, the resulting charts will look just like the sea-level pressure charts in terms of distribution of high and low pressure systems. So they make it possible to examine forecast data for the upper troposphere as well as close to the surface.

Finally, just as surface air has various physical properties (warm or cool, moist or dry etc), the properties of the upper air are important too. While convection - the vertical transport of heat and moisture - is obviously important to the storm-chaser working at the surface, the horizontal transport of upper air with certain proprties into an area is also of great importance. For example, storm formation in an unstable lower troposphere is markedly encouraged if cold dry air is present aloft. The process by which this cold, dry air moves horizontally into an area is known as cold air advection. Advection is simply the horizontal transport of air - and with that air comes a set of physical properties including temperature, moisture, stability and so on.

[Me & angel fiance]

Excellent thread... Very informative. :blush:

[johnholmes]

A great idea, just remember there are Guides on most topics at

http://www.net-weather.co.uk/forum/index.php?showforum=5

any that have my name on then please pm me if you are unsure of anything,

dewpoints

air masses

how weather systems form

and when I finish it the connection between deepening lows and the jets stream that I ran a couple of weeks ago

oh, nearly forgot, very relevant for this week,

will it snow, with some fairly simple guides on what to look for in the various charts.

regards

John

[tugmistress]

wow, thanks everyone :blush: this is really a steep learning curve, but some great information in language i can understand :blush:

i for one and i am sure the others too really appreciate your patience with us and understanding

[johnholmes]

just ask Paula, or anyone else, that is how most of the learning is done.

regards

John

Edited November 22, 2005 by johnholmes

[Stoxs]

top stuff chaps and chapesses i will be watching with intent to learn more about the weather from this thread

10/10 to SMICH for thinking this one up

well done!!!!

[smich]

Thanks everyone for their help here, this is really excellent.

From what I've been reading here and elsewhere, I think I've got a handle on DAM lines. Let's see if I'm right.

DAM stands for decametre (or 10 metres). So 552dam is equivalent to 5520metres in height.

So on a 500hPa chart the 552 dam line is the height at where the pressure is 500hPa. The 556 dam line indicates where the 500hPa pressure is to be found a little higher up. And a 600 dam line would show where that pressure would be found a lot higher still. It's like hills and valleys in the upper atmosphere.

With me so far? (God I hope so!)

An area of high pressure would mean that you'd have to go higher to reach that 500hPa pressure level. In other words the atmosphere is THICKER. More DAM means warmer, thicker atmosphere and less DAM means thinner cooler atmosphere.

If the dam is low enough, that means it may be cool enough to snow!

Here's a chart from tonight's run. What I see now is that the colours represent where the air is cold enough to snow AT 500HPA (or 18,000feet)

Apparently, 528dam(or less) means snow is possible. Look at the bar on the right. Turquoise, Blue Pink and Purple are 528 or less. So any area one of those colours has a chance of snow AT THAT ALTITUDE:

Oh look, that area is almost directly over the UK Friday lunchtime...

Have I got it cracked? (Pleeeeaase say yes! )

Smich

[BrickFielder]

Yes thats right.

It's like hills and valleys in the upper atmosphere.

Thats why we call the high pressure areas ridges and the low pressure areas troughs (i.e between wave crests).

Now how does this chart help you, other than knowing where it will snow at 18,000 feet.

[smich]

Er..................

Hi BF, well, the fact that these charts of the upper atmosphere exist atall implies to me that what's going on "upstairs" has a lot of bearing on whats going on at sea level. The next stage for me, I feel, is to combine what I interpret from, say, the 500hPA chart with other charts. And it's going to be difficult!

I know that the jet is at a very high altitude, so the steering effect of that will need to be taken into account. Also, the 850hPa chart and then possibly the 2m temperature chart will tell me whether or not the air is the same temperature all the way down to the surface. If it isn't then this would mean sideways motion of the air (advection?) which is wind...?

If I get the time today, I'm hoping to compare some these charts for the same timeframe, and see how I get on.

Problem is, I get so busy at work plus I'm addicted to the latest run discussions!

Cheers for your help

Smich

[BrickFielder]

Kinks in Isobars can often show where fronts are likely. Look at the north of Spain and up through Norway on the 500hPa chart smich posted.

The difference in temperature between the 500hPa and the 850hPa (lapse rate) or any layer below is a good indicator of the stability of air. Unstable air will tend to rise and form clouds. So from smich's chart we see the -35 Degrees Air is to the south and east of the Uk and this is where cloud is likely to be thickest and so give heaviest precipitation ( you would need to check lower height charts to confirm this though).

We could look at how the temperature at the center of the ridge is slightly lower at -20 Degrees C indicating cool air is descending into the ridge, so that it is unlikely to remain a ridge for a long period. Possibly hinting the high pressure could move north.

[tugmistress]

ok looking at hese two charts here, i am guessing that personally we are going to get a pasting on friday with both wind and white fluffy stuff... not white wet stuff?

and i would assume temp (without windchill) of 1c or thereabouts?

[johnholmes]

ok looking at hese two charts here, i am guessing that personally we are going to get a pasting on friday with both wind and white fluffy stuff... not white wet stuff?

and i would assume temp (without windchill) of 1c or thereabouts?

correct re wind prob not so correct re snow

you will prob get some fairly wet, then for a short time it will get colder but then less cold air will come in from the ne.

hope that helps

regards

John

[tugmistress]

thanks john

wasn't too bad a go though

Edited November 23, 2005 by tugmistress

[johnholmes]

thanks john

wasn't too bad a go though

we will soon have you on Scottish TV!

[BrickFielder]

Same chart actually on the day.