A Beginner's Guide To Reading Winter Charts

For the newer members on here, it might be a good idea to give a brief guide to what to look for on the charts as we head into wintertime - a time when many boardsies come back to the feeding ground after 8 months wandering the plains. As people jostle for position around the trough (pun intended!), tempers can flare up, and some can get chased way, back to other grounds in such places as Coleraine.....

Firstly, there are several computer forecast models run by various agencies throughout the world. These models take in observational data from surface stations, ships, buoys, weather balloons, satellite and other sources, and run complex physical equations to determine the state of the atmosphere at various intervals into the future. Some models only deal with a limited area of the globe for a couple of days into the future (eg. HiRLAM (European Hi-Res Limited Area Model, NAE, RUC, etc), while other global models deal with the whole globe, for periods of several days to over two weeks (eg. ECMWF (European), GFS (USA), UK Met, GEM (Canadian), BOM (Australian), JMA (Japanese), etc.

Different models have different amounts of freely-available data to the public (eg. ECMWF - widely regarded as the best overall perfomer, run twice a day, but few freely-available data), while others have more. The GFS has the best array of data available, on a wide number of atmospheric variables, and is hence one of the more popular ones used on places like boards. It also runs four times a day, out to 384 hours (16 days), and so it the one used in the Fantasy Island thread.....

Here is one good source of models most people here use.....
http://www.meteociel.com/modeles/ukmo2.php?carte=1021&ech=6&archive=0


When determining the potential for cold weather, there are some key variables to look at. The first is the "upper chart" (500 or 300 hPa), which gives an outline of the broad global pattern of upper waves in the jet stream (at around 5.5 km and 9 km, respectively). As pressure falls with height, each pressure will be found at a certain height (geopotential) as you go up in the atmosphere. In general, the main "pressure levels" are 1000 hPa (near gound level), 850 hPa (around 1450 m), 700 hPa (around 3 km), 500 hPa (around 5.5 km) and 300 hPa (around 9 km). The exact height of these pressure levels (geopotential) is a function of the temperature of the atmosphere at that time - the warmer the atmosphere, the higher the pressure level will be, as warm air is less dense than cold air.

An area of low geopotential corresponds to a cool airmass and is called a trough or depression, while and area of high geopotential corresponds to warmer airmass and is called a ridge or high. These areas of low and high geopotential form the familiar Rossby wave pattern in the atmospheric circulation around the globe. These upper features are a driving force for systems at lower levels, including surface storms, fronts, etc.

Linked to these pressure levels are corresponding temperatures. Cold upper temperatures makes the atmosphere unstable, meaning warmer, less dense, air from below finds it easier to rise to great heights, forming showers and thunderstorms. Warmer air at upper levels puts a "cap" on such convection, and usually leads to more stable weather, with layered clouds. For snow lovers, a favourite chart is the 850 hPa one, which gives geopotential (in decameters, dm) and temperature of that level. To get snow at sea level, we generally look for 850 hPa temperatures (T850) of at least -8 °C, but it can still snow at -4 °C if conditions are right in the lowest layer of the atmophere (boundary layer).



Another important chart is the 500 - 1000 hPa thickness chart, which shows the thickness of the layer of atmosphere between the 1000 and 500 hPa pressure levels. As described above, the colder the airmass, the denser the air, and hence the closer these two levels will be. For snow in Ireland, usually we need somewhere around 528 dm or lower, as this will ensure that the airmass is probably cold enough at all levels for snow to survive the descent to the surface.



Of course, we need to also factor in conditions at the surface. Snow can fall with surface temperatures of below around 4 °C, provided the wet-bulb temperature is at zero or below. We look at the dewpoint readings to figure this out, as the wet-bulb temperature is usually around one third of the way between the dewpoint and dry-bulb temperature. So if the dry-bulb temperature is +3 °C, and dewpoint is -2 °C, the wet-bulb temperature will be around zero or just below, and snow is pretty likely.

Dewpoint


All too many times, conditions in Ireland can be "marginal", with a very fine line between rain and snow. That's when the "lamppost watching" takes place, with people desperately watching for those sleety drops to turn fluffier and fall slower as snowflakes......then they come on here and proclaim to the world that they have found the white gold!