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Paul

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Posts posted by Paul

    Guide to...........DAM lines

    in The netweather guides...

    Posted

    A dam line is basically the height that you would have to go up to before the pressure falls to 500hPa.

    From a typical 500hPa chart, the best way to work out the 1000-500 thickness is:

    (1000-SLP) + 500hPa height

    ...where:

    SLP - Sea Level Pressure

    And the 500hPa height can be found by using the scale on the right hand side of the chart.

    --------------------------------------------------------------------------------

    Basically, all we need to worry about are the actual figures: the main figures to look for in the UK are:

    528DAM and below for an increasing chance of seeing snowfall, preferable values of 522 and below.

    546DAM tends to be associated with the mild and wet conditions in winter - so isn't very popular! In the summer this line is usually close to the northwest of the country.

    564DAM and above is usually found with a very warm airmass in summer, and most often in a Spanish plume affair. Very warm or hot conditions are likely.

    Usually we don't see much above 570DAM or much below 515DAM in the UK.

    Originally posted on UKww forum by Paul B (copied here with their permission)

    http://www.ukweatherworld.co.uk

    Guide to........forecasting snow

    in The netweather guides...

    Posted

    We keep hearing a lot about these different meteorological terms, but which ones are the most reliable? I will attempt to shed a little light. (without going into a great technical explanation of which each is).

    Thickness (TT) (typically 1000-500mb) Is widely used in forecasting snow, the 528DAM is banded about so frequently, but i have always found it never to be as clear cut as that, TT below 522DAM is really the figure i would look for to use as a benchmark, however sometimes even below this it will rain or sleet, and in some cases thickness well in the 530's will give snow. But why ?, it all relates to the lowest third of the atmosphere, that below 850mbs, (this area is crucial in snow/rain forecasting. It is often a better guide to use 1000-850mb thickness as an indicator for snow, simply because more often that not it is this area which is more important than above.

    850mb Temps the temp of the 850mb layer is widely used -5, -10 etc, this is more simple, as it is what is says it is :- the temperature at the 850mb level. For snow usually at least -5 must be present, but again this doesn't always work.

    The crucial thing we are missing up to now is the temps of the lowest layers of the atmosphere, near the surface, the boundary layer. It is these lowest layers that are important. This is where WBulb & 850mb WBPT comes in, (Wet Buld potential temperature). Dave & Sam have already spoken of the Wet Bulb Temp, the 850mb WBPT is simply the potential temperature of the wet bulb calculated at the 850mb level (this is more for fronts) Forecasters will use a mixture of all three to gauge the risk of snow, but typically it will be the Wet Bulb which will have greatest weight.

    Today for example we have thickness below 528mb / 850temps are at or below -5C in most places, yet the snow is above 300-400m. Why? Well wet bulb temps are above freezing (between 1-4C), so snow is only likely where the wet bulb falls to below freezing. (the wet bulb freezing level), however in the heaviest showers this is dragged down with the constant precipitation (the atmosphere aloft is cold as i have said above, so with heavy enough rain/hail this is dragged down, the snow level is also dragged down so eventually it snows at the surface temporarily) Over the next 24-30 hrs the wet bulb temps drop steadily, and more and more showers turn to snow. The frontal system, despite moving in with very cold air aloft will have an atlantic maritime warm sector, it will reintroduce wet bulbs above freezing for most of western britain overnight into thurs, thus rain is more likely, despite the low 850mb temps and TT. Only inland away from the sea will the wet bulb remain sub zero, so here on the eastern edge will it fall as snow, and poss more generally as it moves away south and lower wet bulbs feed south.

    Another situation is when you have high thicknesses and higher 850mb temps, but a cold undercut (continental typically), here wet bulb will be several degrees below zero, so snow is more likely than rain as the cold air at the boundary layer undercuts the warm air aloft. (this was responsible for the blizzrds in the westcountry in feb 1978, thicknesses here were well above 528 and 850mb temps in the SW nearer to 0 to +1C.

    Taken from the UKww forum - with their permission, originally posted by 'PJB'

    http://www.ukweatherworld.co.uk

    FIRS FROST

    in Weather reports

    Posted

    Hello Rade, welcome to net-weather! Glad you have managed to find us!

    Look forward to discussing the weather with you, its always great to chat to overseas visitors about the climate in their country. Croatia sounds amazing, 40ºc one week then 20ºc the next!

    If you have any questions about net-weather please feel free to ask me or any of the other mods/admins/hosts.

    Thanks

    Paul

    Guide to......lightning photography

    in The netweather guides...

    Posted

    BY STEVESTORMS

    Hello all, taking photos of Lightning is not as hard as you think, composing a photo is harder and requires practice and a little thought :p

    Let me make this very importaint point: YOU CAN ONLY SHOOT LIGHTNING IN THE DARK! If you have seen a daylight shot of lightning which I have its eigher a lucky shot! or lightning has fired several times down the same path, I dont know of any electronic gagets that are quick enough to activate your camera to get the shot.

    Taking a great lightning shot requires your chosen T-storm to be photogenic if it wont smile for you theres not much you can do, if all you have is CCs it wont make a very dramatic shot, However some spider lightning and forks which show themselfs can look very nice. :D

    Unless you have a high end digital camera it will be hard to catch lightning and even then I remain unconvinced that digital will catch lightning as well as film, the real magic of lightning photograthy is that its the lightning its self which makes the picture all you do is point your camera in the right place! as lightning is a fantastic light sorce it will paint its self on your film.

    Heres what you need! A 35mm SLR MANUAL camera with a "B" or "BULB" SPEED SETTING this will as long as the release is pressed will keep the shutter open, my recommended 35mm SLR is the OLYMPUS OM1/OM1N/OM1MD these cameras are top quality and do not require a battery to make them work as they have a mechanical shutter, one of these little gems will cost you between £80-£175 depending on condition and are widely avalable in most second hand camera shops, dont forget these cameras, I was selling myself 25 years ago! lol.

    Ok so you have a camera, (FILM) I use fuji print film 100asa, you need a slow film speed not a fast one! if you use 400asa the nightime background will soon become light :D

    TRIPOD AND CABLE RELEASE You must have these, thet will help frame the shot and stop any unwanted movement when you are "OPEN"

    TRANSPORT Its no good waiting for a decent Tstorm to come to you! get out there and go after it! perhaps thats why some of us call ourself STORM CHASERS?

    A THUNDERSTORM!!!! Oh yessssss you need one of these, heres what I do -------- Ok I position myself so that my T-storm is passing right to left or left to right not coming towards me (although sometimes you have no choice) and certainly not going away from you! Once i'm in position I set up, you should check that your film is correctly loaded before leaving home "TIP": gently rewind WITHOUT PRESSING THE FILM RELEASE BUTTON the slack on the spool when you feel resistance stop, when you wind the film on you will see the rewind handel turn as the film advances so you know its ok!! make sure your lens is wide open ie the small F STOP number ie F1:8 F2 F2.8 F3.5 depending on your lens, big zooms are no good for lightning unless you have one that cost as much as a house! Sorry "fastlane"!! I myself use a small 28-90mm lens which has captured all my best stuff!. I make sure my camera speed is set to "B" OR "BULB" place it on tripod and attach cable release.

    NOW LETS TAKE A PIC Watch the T-storm and the focal point of he lightning aim your camera in direction of the most frequent strikes and try to have something in the foreground like a church a building or a tree, now count the seconds between strikes for instance 20 secs as a ball park figure so I know roughly that my next strike is due around then Tstorms are like FLASHGUNS they charge up then fire off! its not exact but will give you an idea, ok so FLASH! Lightning has fired im counting 1234 ect when I get to 10 I open my shutter--thats 10secs of exposure I have safed and hopefully FLASH! yep THANKYOU VERY MUCH I GOT THE SHOT! You can place a black card in front of your lens as well to reduce exposure between shots equally as well, dont worry about waisting shots if you get a big CC then close and wind on and open again expect to waist 90% of your shots it dont matter because its great to capture lightning for the first time.

    OK hope this all make sense if you dont understand it email me, DONT FORGET LIGHTNING SAFTEY, DONT STAND UNDER TREES DONT STAND NEAR FENCES ETC ETC ETC ----- READ THEM!!! If its really active out there get in your car and move futher away one rule that WARREN FADLEY uses is "IF YOU'RE IN THE RAIN YOU'RE TOO CLOSE" it has served me well and its bloody difficult taking pictures in the rain as well!! Take care, chase safe and lets see some pics! STEVESTORMS

    The net-weather guides....

    in The netweather guides...

    Posted

    Welcome to the new net-weather guides forum area. Within here we will endeavour to add as many guides to the weather as we can. Our first guide is, the net-weather guide to clouds, hope you enjoy reading through it and find it informative.

    Coming soon will be a guide to reading the synoptic charts among others. If you would like to see a guide added or feel that you would like to contribute by helping to write or writing a guide, please private message me.

    Please note that this forum is locked for non 'guide' posts, so if you have any questions, please either send them via private message or post them within the welcome area.

    Many thanks

    Paul

    STORMS USERGOUP LOCATION MAP

    in Weather reports

    Posted

    This is the first run of our location map, it will be updated regularly as new users join the group, or current users move.

    Each number on the map corresponds to a different member of the usergroup:

    1. Andy Bown

    2. Baldie

    3. ChaserUK

    4. Dazmaster75

    5. Demon

    6. Gaz Smither

    7. GlennWrench

    8. Harveydean

    9. Highcliffe2

    10. Hurricane

    11. Markabuckley

    12. Markh

    13. Miss Snow Lover

    14. Paul Sherman

    15. Peter Tattum

    16. Polar Low

    17. RB_506

    18. Rixxgolf

    19. Robert H

    20. Rob_weiss

    21. Sparkle

    22. Stephen Prudence

    23. Stevestorms

    24. Stormguy

    25. Sunflower

    26. Supercell

    27. Surfer Dude

    28. Thundery Wintry Showers

    29. Tim

    30. Tony W.Baker

    31. Waverider uk

    32. Weather forecaster

    33. Wind turkey maz

    34. Yatesy

    stormgroupmap.jpg

    Obivously we will probably have to tinker with this to make it more user friendly, but I hope it proves useful.

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