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    Big drop in temp in Cheshire, UK 25 August 2019.

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    RayProudfoot
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    Big drop in temp in Cheshire, UK 25 August 2019.

    Post by RayProudfoot » Sun 25 Aug 2019 9:30 pm

    Today was a rare day meteorologically. My station recorded a maximum temperature of 31.2C at 14:34 and at 17:44 it was still 30.8C. But in the next hour it dropped 7C to 23.2C with a rise in pressure and change of wind. Full details here. https://www.wunderground.com/dashboard/ ... 8-25/daily

    All that points to a cold front passing through but there was no cloud or precipitation. I'm baffled. Any weather experts out there and anyone else notice the dramatic drop in temperature?
    Cheers,
    Ray, Cheshire.

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    freddie
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    Re: Big drop in temp in Cheshire, UK 25 August 2019.

    Post by freddie » Mon 26 Aug 2019 10:08 am

    RayProudfoot wrote:
    Sun 25 Aug 2019 9:30 pm
    Today was a rare day meteorologically. My station recorded a maximum temperature of 31.2C at 14:34 and at 17:44 it was still 30.8C. But in the next hour it dropped 7C to 23.2C with a rise in pressure and change of wind. Full details here. https://www.wunderground.com/dashboard/ ... 8-25/daily

    All that points to a cold front passing through but there was no cloud or precipitation. I'm baffled. Any weather experts out there and anyone else notice the dramatic drop in temperature?
    Yes, we had a similar sequence down here in Shropshire.

    What you experienced wasn't due to a "classic" cold front - although the observable changes were similar. Here's what happened (I'll try and keep it simple - but the atmosphere is far from simple):
    • Upper tropospheric changes (relating to the position of the jet stream) induced a small rise in pressure over Ireland and the Irish Sea.
    • The same upper tropospheric changes induced a small pressure fall over the Midlands and eastern England.
    • At the same time, intense heating of the warm air over most of England and eastern Wales induced a further small fall in pressure.
    • The upper tropospheric changes gradually moved northeastwards during Sunday - but the resulting pressure rise was initially masked by the heat-induced pressure fall over east Wales and England.
    • Once the heating from the sun started to fade the resulting pressure fall stopped or reversed, which led to a (relatively) quick rise in pressure over east Wales and the west Midands (as well as northwest England).
    • All the above created a pressure gradient over western parts that caused a westerly flow at low levels to develop, which veered northwesterly as the pressure effects from daytime heating were removed.
    • This resulting flow caused air that had been in residence over the Irish Sea throughout the day to move inland on the flow. This air had been cooled by contact with the sea surface, and had become moister as well.
    • The result of this (marine) air being forced inland by changes in the pressure pattern caused your changes in temperature, wind direction and humidity. It has also caused a pretty grey morning in those areas where the moisture in the marine air has condensed into cloud by contact with the nocturnally cooling land surface underneath it.
    It should eventually become sunny by early afternoon, though, as the cool and moist marine air is relatively shallow (about 1500 feet thick). The air will be dried out from the bottom upwards as solar radiation penetrates the cloud and warms the land surface underneath, which will evaporate the cloud. This process will be helped by the fact that areas to the east and west of the marine air are cloudless so will warm up rapidly and evaporate the cloud from its edges too.

    If you get a minute today, have a look on a website that offers animations of visible satellite imagery - and you can watch this process happening :-)

    So - to summarise - it wasn't what I would call a cold front, but it had all the appearances of one (apart from lack of cloud and weather).

    Hope that helps!
    Freddie
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    RayProudfoot
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    Re: Big drop in temp in Cheshire, UK 25 August 2019.

    Post by RayProudfoot » Mon 26 Aug 2019 11:01 am

    Hi Freddie,

    What a brilliant explanation of yesterday's weather. Thank you very much for taking the time to explain it. It's all very complicated and I'm guessing that combination of air is not that common. I can only recall a couple of other occasions when the temp has dropped so quickly and those have been when a cold front passed through.

    I'm surprised the BBC Weather people didn't remark on it but I suppose they don't get the time for interesting stuff like this and in any case a lot of people may not have even noticed it. Days like yesterday are what makes owning a weather station so interesting.

    And you're right too about the sunshine. After a very murky morning the sun has broken through in the last 20 minutes. I have a link to this satellite imagery on my website. https://en.sat24.com/en/gb
    Cheers,
    Ray, Cheshire.

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    freddie
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    Re: Big drop in temp in Cheshire, UK 25 August 2019.

    Post by freddie » Mon 26 Aug 2019 11:26 am

    RayProudfoot wrote:
    Mon 26 Aug 2019 11:01 am
    Thank you very much for taking the time to explain it.
    No problem. The weather is my passion, so I can explain it all day long :-)
    RayProudfoot wrote:
    Mon 26 Aug 2019 11:01 am
    It's all very complicated
    I agree, and that's why we have supercomputers to attempt to model it. There's too much going on for forecasters to keep on top of it without the help of computers.
    RayProudfoot wrote:
    Mon 26 Aug 2019 11:01 am
    I'm guessing that combination of air is not that common. I can only recall a couple of other occasions when the temp has dropped so quickly and those have been when a cold front passed through.
    The pattern of events I described is more common than you think - although yesterday was quite a marked example. It helps if you live a bit closer to the sea than I do, as by the time the marine air had arrived in Shropshire it had been warmed up a bit so the temperature drop wasn't as dramatic.
    RayProudfoot wrote:
    Mon 26 Aug 2019 11:01 am
    I'm surprised the BBC Weather people didn't remark on it but I suppose they don't get the time for interesting stuff like this and in any case a lot of people may not have even noticed it.
    The BBC probably covered it with the phrase "cooler air pushing in from the west". It's not really that visible a process so not much to show on the TV/website in terms of viewer photos, etc. Also it happened when the temperature was naturally falling anyway. It happened earlier in places further west - for example, Hawarden in Cheshire had a 5 degree temperature drop between 14:00 and 15:00 - but still nothing to show for it in terms of photos/videos.
    RayProudfoot wrote:
    Mon 26 Aug 2019 11:01 am
    Days like yesterday are what makes owning a weather station so interesting.
    Definitely. That's why I would never be without one now!
    RayProudfoot wrote:
    Mon 26 Aug 2019 11:01 am
    And you're right too about the sunshine. After a very murky morning the sun has broken through in the last 20 minutes.
    It's started to come through the clouds here too now. It is fortunate that the marine air layer was so shallow. If it had been 2500 feet deep then I doubt whether the sun would've been strong enough to clear it until late afternoon. Looks like being another hot one, but hopefully not quite as warm as yesterday.
    Freddie
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    RayProudfoot
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    Re: Big drop in temp in Cheshire, UK 25 August 2019.

    Post by RayProudfoot » Mon 26 Aug 2019 1:06 pm

    Hi Freddie, lovely here now and in Shropshire too I see according to Sat24. No repeat performance today I think as it’s much cooler and as we both know no two days in the U.K. are ever the same.

    Thanks once again. :clap:
    Cheers,
    Ray, Cheshire.

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    Mapantz
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    Re: Big drop in temp in Cheshire, UK 25 August 2019.

    Post by Mapantz » Mon 26 Aug 2019 2:39 pm

    I usually get those sorts of drops 3 or 4 times in Autumn alone - quite a common occurrence.
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    RayProudfoot
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    Re: Big drop in temp in Cheshire, UK 25 August 2019.

    Post by RayProudfoot » Mon 26 Aug 2019 3:45 pm

    Mapantz, are you close to the coast? Might that be a factor? I’m 35 miles from the Irish Sea.
    Cheers,
    Ray, Cheshire.

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    Mapantz
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    Re: Big drop in temp in Cheshire, UK 25 August 2019.

    Post by Mapantz » Mon 26 Aug 2019 8:52 pm

    As the crow flies, Poole harbour is 4 miles directly to my East. The English channel is about 8 miles to my South. It's usually a cold front from the West or Northwest that has the best effect, particularly when it is fast moving.
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    RayProudfoot
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    Re: Big drop in temp in Cheshire, UK 25 August 2019.

    Post by RayProudfoot » Mon 26 Aug 2019 9:20 pm

    There are plenty of cold fronts in the north too and temperature changes can be noticeable. But yesterday’s event was not the result of a cold front. It was something altogether different. I misunderstood you when you said you experienced several of those each year.

    Perhaps Freddie can say whether yesterday’s event could also occur on the south coast or is it limited by geography?
    Cheers,
    Ray, Cheshire.

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    Mapantz
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    Re: Big drop in temp in Cheshire, UK 25 August 2019.

    Post by Mapantz » Mon 26 Aug 2019 9:38 pm

    It was shown as a decaying cold front on the FAX charts. The UKV model was showing a sharp contrast in temperatures between the North and South, but like the other models, it struggled with the positioning and timing of the cold front.

    FAX chart and GFS 2M temperature chart for yesterday. Initialised on the 22nd and 23rd respectively.

    https://postimg.cc/ykRkY4RW

    https://postimg.cc/SjJn3zN6

    It won't let me upload images here, comes up with an error.
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    RayProudfoot
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    Re: Big drop in temp in Cheshire, UK 25 August 2019.

    Post by RayProudfoot » Mon 26 Aug 2019 10:09 pm

    Interesting charts. Do you have one that shows the visible satellite image? The chart suggests a front but the skies remained clear throughout the transition.

    Looking at the data uploaded to WU the wind direction at 17:39 was SSE. 5 minutes later it was SW and 5 minutes later was WNW. That’s 157° to 293° in 10 minutes and yet no cloud. Not a front in the conventional sense.
    Cheers,
    Ray, Cheshire.

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    freddie
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    Re: Big drop in temp in Cheshire, UK 25 August 2019.

    Post by freddie » Tue 27 Aug 2019 2:58 pm

    RayProudfoot wrote:
    Mon 26 Aug 2019 10:09 pm
    Not a front in the conventional sense.
    Precisely what I said in my explanation. The fax charts would've needed a marker for the front edge of the air that originated over the sea in order to explain to the punter (one of) the reasons behind the west-east gradient in maximum temperature anticipated on that day - and what better marker to use than a cold front. The temperature contrast was limited to the lowest few thousand feet of the troposphere. It will take input of solar enery to "warm out" the resulting layer of cold air next to the surface (not to mention evaporating the water droplets contained in the fog and cloud, which were a result of the process I outlined), which will result in a reduced maximum temperature compared to areas further east - as the amount of solar energy available to warm the air is (relatively) constant.

    It was essentially the same airmass both sides of the "front" - the difference being that the sea cooled the lowest layers of the air to the west of the "front". Fronts (in the conventional sense) demarcate boundaries between air masses, and are found in a position relative to the jetstream (which, of course, is driven by the contrast between the air masses). Not the case this time round.

    At the end of the day, a front is just a conceptual model of atmospheric processes. In reality the situation is far more complex than a line dividing air masses. This particular scenario was a long way from (what is referred to as) "Norwegian" frontal theory. I just highlighted the processes that caused the (low level) contrast in temperatures and pressures, and identified why the two variables evolved as they did.
    Freddie
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    RayProudfoot
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    Re: Big drop in temp in Cheshire, UK 25 August 2019.

    Post by RayProudfoot » Tue 27 Aug 2019 5:25 pm

    Thank you Freddie. I can't add any more to the discussion.
    Cheers,
    Ray, Cheshire.

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