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03jtrickey

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Posts posted by 03jtrickey

    Ex Katia - Guess The Gust

    in Storms & Severe Weather

    Posted · Edited by 03jtrickey

    Hi all,

    I noticed yesterday that there was no 'guess the gust' thread for the upcoming storm, as there have been for past events.

    So just for fun I thought I'd create one. laugh.png

    Simply state which official weather station (ie ones on Met Office or xcweather.co.uk, but no high altitude sites such as Cairngorm etc) you think the highest gust will be recorded at and the maximum wind speed you expect.

    The 'winner' is the person who guesses the right location AND is closest to the highest recorded wind speed (no prize as such, just the satisfaction of winning unless Netweather is prepared to provide some sort of reward).

    How it will be judged:

    • If the actual max wind speed lies between two guesses, the person with the higher guess wins.
    • If no-one guesses the right location, then the guess closest to the highest recorded wind speed wins and if multiple people have guessed the same value then the first person to guess this wind speed wins.

    I will start off with:

    104mph in Tiree cool.png

    Great British Weather

    in Spring Weather Discussion

    Posted

    Just watching it now...a slightly embarrassing opening...I don't know why they needed to dumb down the warm and cold fronts and jet streams to angry rugby players... it seems a little bit condescending!

    Anyhow, I will keep watching and hopefully it will improve!

    El Gordo! - Spanish Plume Thunderstorm Discussion 3

    in Storms & Severe Weather

    Posted

    It's been cloudy in Oxford since late morning, although it's very humid.

    The Netweather Extra radar showed a band of rain passing over, but only a few specs have fallen, not even enough to wet the ground. There is fairly thick cloud but whatever is showing on the radar isn't reaching ground level.

    I'm wondering whether the more intense stuff off the coast of Wales will continue eastwards across the country and intensify as it does so? Any thoughts on this?

    Convective / Storm Chat And Discussion

    in Storms & Severe Weather

    Posted

    Lots of cumulus bubbling up in Oxford at the moment and I'm liking the forecast for London & SE England from the Met Office:

    "A mostly sunny start before rather more cloud develops. Some areas will stay dry throughout. However scattered showers are expected to develop, with a few of these heavy and perhaps thundery over inland parts of the west, especially Oxfordshire. Maximum temperature 21 °C."

    Convective Weather Discussion UK

    in Storms & Severe Weather

    Posted

    Regular thunder and have now seen a few decent flashes of lightning!

    According to my girlfriend there was heavy rain, thunder and lightning in Sevenoaks earlier... she is now in Penshurst and looking at the radar could get rained on again fairly soon.

    More interesting than here in Oxford, at least at the moment. I'm hoping to see some more convection later! Not a whole lot at the moment where I am.

    Conditional Instability Vs Convective Instability

    in Spring Weather Discussion

    Posted · Edited by 03jtrickey

    Hi,

    I have just started the Climatology section of my Geography course.

    I am struggling to understand the distinction between conditional instability and convective instability and I was wondering if anyone would be able to help me out.

    As I understand it, convective instability occurs when a layer of air which is more humid at the bottom than at the top, is lifted(by something - perhaps a cold front or a mountain range?). The bottom of the layer cools more slowly (briefly at DALR, then at the slower SALR) than the top of the layer which simply cools at the DALR. The temperature gradient increases and the layer becomes unstable and may overturn.

    I understand conditional instability to be when the lapse rate of an unsaturated layer of air lies between the saturated and dry adiabatic lapse rates. Therefore if a parcel of air from the layer is somehow lifted, when it reaches the lifting condensation level and becomes saturated, it will begin to cool more slowly at the SALR, and may rise above the environmental temperature (at the level of free convection) and will continue rising because it is less dense than surrounding air.

    However, I am finding it hard to see the difference between the two. Both types of instability seem to require the air to be lifted in some way (it is not very clear how).

    Is the difference about scale? (i.e. does convective instability occur only when a whole layer is forced up, whereas conditional instability is just a parcel being forced up?)

    Or does conditional instability only occur when the environmental temperature profile is right, whereas convective instability will always occur is a layer of moist to dry air is forced up?

    Another question that I had is - does convection occur with both types of instability? - it is a little confusing that one type is called convective but surely convective scenarios occur under conditional instability as well?

    Sorry for all the questions... I would really appreciate it if someone could help me understand these concepts better and I hope it is possible to make sense of what I have written!

    Tight Pressure Gradient Around Greenland - So Why Aren't The Winds Very Strong?

    in Spring Weather Discussion

    Posted

    I’m not sure I can explain it simply but I’ll try.

    Assume a particle of air moving, at say 20 degrees north and another one at 60 degrees north. The angle that both subtend to the centre of the earth are obviously different, that at 60 is greater than that at 20 north.

    Due to the earths’ rotation any air is given an acceleration that depends on its distance from the equator. Wind strength is dependent on a number of factors, the initial speed of the air, the effect of Coriolis Force, the force imparted due to the earths spin and where on its surface that air is.

    Air tries to flow from high to low pressure and as soon as it does the above begins to act on it.

    Another force called the Pressure Gradient Force also comes into play. As the Fax chart shows we can, without going into the complex mathematics and it shows clearly that a for similar isobar spacing that the wind strength is greater the further south one is.

    Greenland is, or most of it is, well above seal level, much around 500ft with thick ice, so trying to work out what wind in any synoptic set up is even more complex along with katabatic winds also occurring as air stream off the ice.

    Not a very good explanation but its very complex. Someone who has recently, well a lot more recent than 1971 when I did, a meteorology degree course may do it better.

    If you can get your local library to send off for

    Essential of Meteorology by DH McIntosh and AS Thom, it was in the Wykeham Science series, I got a copy, 2nd hand, off the web, it is an excellent book with diagrams and good explanations.

    Again hope the above helps.

    Anyone who spots errors please correct.

    Thanks

    jh

    ps

    a quick search showed this

    http://www.amazon.co.uk/0851090400-Books/s?ie=UTF8&keywords=0851090400&rh=n%3A266239%2Ck%3A0851090400&page=1

    Yes thanks for your detailed feedback, John. It certainly seems a very difficult subject, but now I have at least a reasonable explanation, without going into the detailed science and maths! With all those parameters consider it's incredible that computers can model and predict the weather. I'll see if can get a hold of that book sometime.

    Tight Pressure Gradient Around Greenland - So Why Aren't The Winds Very Strong?

    in Spring Weather Discussion

    Posted · Edited by 03jtrickey

    Nick gave some reasons, but I think the a main issue is latitude.

    ...

    Just always so much going on, when the wind blows.

    Thanks for your really helpful reply! That makes a lot of sense. So, in general, you need tigher isobars to produce the same wind speeds at higher latitudes.

    I found another site here with some details: http://www.tpub.com/weather2/7-9.htm

    And now I know what the Geostrophic wind scale is for on the synoptic charts! http://gliding.eusu.ed.ac.uk/met/syn.jpg

    What I'm still not quite sure about now is why latitude affects the wind speed. I've been trying to find out online and in "Atmosphere, Weather and Climate" but it seems extremely complex and I can't find a definitive answer! What is it that makes winds less strong at higher latitudes for a given pressure gradient?

    Tight Pressure Gradient Around Greenland - So Why Aren't The Winds Very Strong?

    in Spring Weather Discussion

    Posted · Edited by 03jtrickey

    I was browsing through this morning's 06Z GFS run and noticed that at +168 (well into FI I know), there is a really strong pressure gradient showing around Greenland.

    h850t850eu.png

    I've spotted this phenomenon on many occasions before but it only just occurred to me that - if the isobars are so tight - why aren't the winds really strong?

    The NA Wind map for the same time shows winds of "just" 40-44mph over Greenland, while simultaneously over Ireland, similarly-spaced (perhaps even less tight) isobars produce wind speeds into the high 60s mph.

    windvector.png

    Is there a reason for this? Perhaps because in Greenland the isobars are surrounding an area of HP whereas the winds over Ireland are from LP? Don't isobars of the same tightness always produce the same wind speed? Or does the Coriolis effect make the winds stronger to the RHS of an Atlantic low pressure system?

    Perhaps it is just because near Ireland the winds are over open sea? But even so the wind does not look as strong as I would have expected given how packed the isobars are near Greenland.

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