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How to read the weather charts?
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18-12-2009 5:04pm#1
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The charts we're most interested in here in Europe are the
-ECMWF (European Centre for Medium Range Weather Forecasting)
Two runs per day at 00Z and 12Z. Freely available charts for 72hrs out to 240hrs.
-UKMO (UK Met Office) - Two runs per day at 00Z and 12Z. Freely available charts from 6hrs to 144hrs.
-NAE (North Atlantic European Model) - A high resolution model version of the UKMO global model above. 00Z, 06Z, 12Z, 18Z runs in 3hr steps out to 48hrs.
-GFS (Global Forecasting System) - 00Z, 06Z, 12Z, 18Z). Freely available charts from 6hrs to 384hrs
-HIRLAM (High-Resolution limited Area Model) - The Met Éireann model runs every hour, giving precipitation accumulation and wind data for 3hrs in advance.
There are other models available, such as the Canadian GME, US Airforce, NOGAPS, Japanese JMA, NCEP, etc. These are available on a variety of websites, such as Weatheronline.co.uk (Expert Maps), Wetterzentrale, Netweather, Wetter3.de, etc.
The first thing to do is decide what time period you want the forecast for. The UKMO, ECMWF, GFS are good for long range forecasting, but just remember that, due to the butterfly effect, the charts they give can be wildely inaccurate at longer time periods (>5 days, depending on the atmopsheric setup).
The first chart to look at would be the H500 (or 500mBar) chart. This usually gives the height of the 500mBar pressure level, in decametres, dm, (eg. 542 = 5420m).
Isobars are only used on a surface chart, which shows the places which have the same sea level pressure. For upper air charts it is better to use heights, which show at what height the particular pressure is at. As pressure decreases with altitude (by around 1mBar for every 9m in the lower atmosphere), a weather balloon will be at a certain height when it gets up to where it's 500mBar (on average, about 5520m or 18100ft). This is the 500mBar height. Lines joining points of equal height are called isohypses and are drawn on the 500mBar chart.
What dictates the height of a pressure level is the average density of the airmass in which it is. Density is related to two things - temperature and moisture content. As the average temperature of the airmass increases, or more moisture is added to it, it's density will decrease, it will expand, and therefore the 500mBar level will rise. So warm tropical airmasses have relatively high heights (~576dm). Conversely, in the poles, where it's bloody freezing and dry, heights are very low (500dm or below).
A H500 chart will show a series of waves (Rossby waves), which show where the warm and cold airmasses are located. Where the waves point south, you have a trough, where they point north, it's a ridge. In general the jetstream follows these waves where the lines are packed closest together. Depending on where the maximum windspeeds are located, you can get divergence or convergence (spreading apart or bunching up of the air flow). When you get divergence aloft, it causes air to rise from below, which is what causes surface lows to deepen. With convergence aloft, the air subsides downwards from above, leading to High pressure. Rising air causes clouds, subsiding air causes air to dry and heat up, hence clouds are relatively uncommon in High pressure systems.
The H500 may also give other information such as temperature, vorticity, and 500-1000 thickness. This thickness is the thickness of the layer of atmosphere between the 500 and 1000mBar levels and is another good indicator of the type of airmass. The denser the air (colder and/or drier) the lower this thickness, and vice versa. This is one of the things to look for when forecasting snow - anything below about 522dm thicknesses can produce snow.
The other important charts are the H850 (temperature, wind), H700 (temperature, wind, relative humidity), H300, H200 (temperature, wind (ie. jetstream, convergence, divergence), and of course the surface (synoptic) chart.
Forecasters will look at lots of different data, from soundings (weather balloon data), surface and upper analyses, satellite, aircraft data, etc to get a picture of what has and is now happening in the atmosphere. From that they will figure out where the the different airmasses are, which direction they are moving in, whether there is upper divergence, which could lead to surface lows deepening, etc. With that picture in their mind they will estimate future development, movement, etc, using the forecast charts as a guide. They don't rely on them 100%, they only use them to refine their forecasts. As the different models have different strengths, weaknesses when it comes to forecasting different events, the forecaster will have an idea which model to trust more than another. If the models are in good agreement then the forecaster can be that bit more confident in his forecast. There may be local peculiarities of a region's climate, however, that could cause a different ourcome to what the charts suggest. The models won't know this but a forecaster will from experience, and will adjust his forecast accordingly. This is why you should always treat automated weather forecast sites with a pinch of salt.
Below are some charts for today, as an example. I have drawn on them areas of interest to us here and features that may hint at what to expect.
H500-H1000 thickness analysis Saturday 00Z
H500 Heights analysis Saturday 00Z
H850 analysis Saturday 00Z
Surface synoptic chart Saturday 00Z
Surface 24hr Forecast chart Sunday 00Z
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and ditto on the Liverpool gamenacho libre wrote: »excellent post. it almost took my mind off another depressing day for liverpool fc
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