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02-01-2021, 22:38   #1
Danno
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Micro-Climates and Local Geography

So, it's 9.30pm under the eastern flank of an anti-cyclone cell positioned off the NW coast generating a subtle northerly airflow down the country.

The south midlands region is an interesting area to observe during such conditions.

Lets set out the stall:

* Long land fetch to the northern coasts
* 1025hPa pressure in a slack airflow
* Clear sky
* Nocturnal cooling underway several hours

Here in south Laois, just on the eastern side of the small town and on the western banks of the river Nore as it flows south. The map below shows current temperatures and I've highlighted upland areas in brown and enhanced the rivers in blue lines. I will demonstrate subtle airflows and cold pooling and how they are affected both by the stall above and the geography to be explained in tandem with observations of other stations nearby:



My own station at Durrow has yet to drop below 0c (currently 0.3c) and under a WSW airflow coming down off the hills to my SW. The airflow at 10m is running a subtle 4mph to 5mph. I'm at just under 90m elevation.

To my SW is the Johnstown station in north Kilkenny County. At 9.30pm they're at -0.9c ( a 1.2c difference over 17km - they're at almost 140m elevation). The wind is calm.

Further north from Johnstown is the tiny village of Galmoy on the border with Laois and Kilkenny, not as high up as Johnstown at almost 130m and they're recording -1.4c. The wind is calm.

Just east of Castlecomer is another station high up on the pleatau at almost 240m elevation. Again, at 9.30pm they're recording -0.9c. Wind direction is NNE at 6mph.

Finally, Kilkenny city is also below freezing at -0.3c with a persistent N breeze blowing from the north. Elevation 63m.

Conclusion - geography (topography) is playing an important role in determining wind direction and temperature at this stage of the evening. All elevated ground is gone sub-zero, areas to my west are calm, to my east and southeast there is a northerly element to the wind and both are sub-zero. But why is Durrow the odd one out with a positive temperature reading?

Here is why: The hills to my SW and slightly higher grounds to my W have "bled out" their daytime heat and will now cool slowly. As heat rises, cold air sinks to take it's place and this process has largely completed at these locations and while further cooling will take place, it will be at a much slower process.

Castlecomer is still loosing daytime heat and cold air is sinking down off the higher slopes neatby into the river valley that flows down to the Nore just north of where the N77 and N78 roads meet on the map.

In Kilkenny, that cold air coming down the valley is pooling, but still coming down the valley and generating a breeze. This will continue for another while yet.

Similarly, in Durrow the breeze should start to die out in a couple of hours and then the natural cold pooling will commence with probably a sharp drop in temperatures, for the moment, I'm stuck with alot of cool, but not cold air using the Nore valley gap to "get out".

Let's predict this further: Johnstown and Galmoy will probably get close to or just below -3c tonight. Both are on semi-dry to dry soil types and should naturally loose heat over the coming hours, but there may be weak northerly breezes keeping a little airmass mixing going on owing to elevation.

Over Castlecomer side - again there should be a close to a -3c recorded there, but I would expect more air mixing from the northerly breeze than at any of the other locations, so they might end up with the highest minima out of the four stations tonight.

In Kilkenny City, by the banks of the Nore, only the small UHI effect might stop them reaching below -4c.

Here in Durrow, I expect a sharp drop in temperatures, probably a few fluctuations throughout the night - yo-yo effect as cooler air follows the river south, but by morning all that should have stopped and I wouldn't be surprised to see a low value of almost -4.5c and likely to end up with the lowest minima out of the four stations.

All the above predictions could put egg on my face in the morning, but I've seen this happen before many times.
Let's see!
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02-01-2021, 23:44   #2
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So, 2hrs later and here is where we are at.

Durrow has hit -1.7c earlier, but risen to -0.9c currently. Kilkenny has went from -0.3c earlier to -0.9c currently - so we are both tied.

Johnstown dropped to -1.5c but has since risen to -1.3c currently.

Galmoy rose to -0.9c after 930pm but has now dropped to -2.5c (cooling continues) while Castlecomer has dropped to -2.0c in the last two hours.

Plenty of cool air still to drop down from the higher altitudes yet.

The yo-yo effect will continue here for some time more as winds have backed W and decreased to 1.6mph.
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03-01-2021, 00:00   #3
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Thanks for sharing. Very interesting. My temp has been varying a lot as well.
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03-01-2021, 00:06   #4
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Fascinating
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03-01-2021, 01:00   #5
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Quote:
Originally Posted by mcburns07 View Post
Fascinating
It certainly is, high pressure situations certainly afford us opportunities to study, observe and evaluate the effects our regional topography has on our local weather. We don't really have much of a continental climate in this island as the landmass is too small, so these weather conditions give us an insight into that relationship outlined on a smaller and rarer occasion.
In contrast, the summer time warmth will throw up equally distinct observations, cold pools are often hot spots once the right weather conditions prevail, especially inland where the seas gradually loose influence.
It keeps us hooked!
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03-01-2021, 03:25   #6
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Katabatic winds at play here
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03-01-2021, 08:45   #7
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Danno, you were bang on with your low prediction of -4.5! You got -4.2 °C at 04.26.

It's like what I posted two days ago, showing the large difference between the 4 eastern Met Éireann stations.
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03-01-2021, 10:39   #8
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Originally Posted by Gaoth Laidir View Post
Danno, you were bang on with your low prediction of -4.5! You got -4.2 °C at 04.26.

It's like what I posted two days ago, showing the large difference between the 4 eastern Met Éireann stations.
At Durrow the Stevenson Screen recorded -4.6c
Galmoy recorded -4.5c
Johnstown recorded -4.1c
Castlecomer -2.5c
Kilkenny -2.3c.

Galmoy and Johnstown colder than I thought, Kilkenny a bit milder than I thought. Otherwise not far off what I felt would happen.

I'm sure that there are lots of other stations around the country that have similar influences.
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03-01-2021, 12:56   #9
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We've had the old debate over Dublin Airport's sometimes cold readings compared to other stations (though in effect there's not much difference in overnight minimum temperatures), so here are the aerial photos of each of the four stations Dublin Airport, Casement, Dunsany and Phoenix Park.

Just taking yesterday's daily data from Met.ie, the overnight minima were as follows:

Dublin Airport: -2.4
Casement: -3.7
Dunsany: -2.8
Phoenix Park: -1.8

Dublin Airport and Dunsany are the most openly sited station enclosures, with the Phoenix Park very bad (by their own admission; they don't have any anemometer there because it's so enclosed). It's just 15 metres from a building and surrounded by dense, high trees. Of course, this station would be representative of some sheltered parts of the Phoenix Park, though it is in the most undulating landscape of all four stations (±40 metres in small distances), so some parts of the park (e.g. down near the Chapelizod gate) will have been much colder.

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07-01-2021, 17:55   #10
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Another little quirk this afternoon when temperatures took on a mad notion:

http://www.laoisweather.com/images/temp.png



At 3.00pm with still air over snow covered ground and recent fog lifted, temperatures nose-dived from +0.9c to -0.7c in little over half an hour. However, within 10 minutes they had recovered and rebounded to +1.1c!

So, why the change?

At 3.40pm a westerly breeze kicked in, bringing in milder air coinciding with a shower trough moving south through the mid-west:

http://www.laoisweather.com/images/wind.png



So, an interesting night ahead - if we loose the breeze, how sharp will temperatures drop if skies remain relatively clear?

Follow it on: http://www.laoisweather.com/wxgraphs.php
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07-01-2021, 19:14   #11
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Temperature anomaly is in effect here too again. Down to 2c.

https://www.wunderground.com/dashboard/pws/ILIMAVAD4

https://www.wunderground.com/dashboard/pws/ILIMAVAD8

We are supposed to be in a warm sector. I am not sure if it’s a Katabatic wind of sorts.
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08-01-2021, 23:00   #12
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Danno, I know very little about it but I live in one of those high areas you’ve highlighted and we do have a microclimate for sure. In cold weather, as we drive upland towards home you can watch the temperature reading on the car drop at specific points in our journey and we know that we’ll see about a negative 2 degree difference between town and home.
When it’s hot, it’s scorching, so we never venture near the cold beach areas, we stay home to enjoy the high temperatures.

Last edited by screamer; 08-01-2021 at 23:04.
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09-01-2021, 13:58   #13
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First 8 days of January and the AWS throws up a mean of -0.3c!

Gonna enter the manual thermometer data into an Excel sheet to see if thats any different.
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09-01-2021, 22:29   #14
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https://twitter.com/LaoisWeather/sta...323816969?s=20

Snow on the ground, frost in the air, but also - DRIZZLE!
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17-01-2021, 22:37   #15
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Took a ramble along a local walkway today, never been on this part of the walkway despite living here pretty much all my life!

A recent felling of trees allowed me to climb about 30m above the walkway and around 80m or so above the small town below. I was surprised to find out how elevated this land is and that there was further heights to be gained by venturing onto the farmland further south!

It has prompted me to take note of temperatures in a southerly in future to see if there is any fohn effect caused by this slice of higher land to my south that I have underestimated!

The mast in the right hand side of the picture would be south of the weather station, but obscured from view in this take.
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