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The annual late March / early April cold snap ("Laethanta na Bo-Riabhach") - underlying cause?

  • 30-03-2022 10:51am
    Registered Users Posts: 17,797 ✭✭✭✭

    As we head into the well-forecasted few days of rain, wind, and colder temperatures, it occurs to me that once again, an old Irish legend which my dad likes to cite at this time of year is coming true, as it seems to do every year more or less. Laethanta na Bo-Riabhach, "The Days of the Brindles Cow", tells the story of a cow which survived a particularly harsh Winter, and led Winter to "borrow" a few days from Spring in order to finish off the cow as a matter of pride. This legend refers to the fact that virtually every year with few exceptions (as evidenced by a trawl of news articles going back years), we tend to have a period of relatively decent weather in mid-March with sunny spells and warmer temperatures, followed by a brief but bitter return to late-Winter conditions around the final week of March or first week of April, with much lower temperatures, wind, rain, sleet, maybe even a scattering of snow here and there.

    Given that this is a rather ancient Irish legend, it's clear that this phenomenon is not a recent development. It would seem therefore that it can't be put down to any long-term, large scale oscillations (AMO, ENSO, IOD etc) considering that it seems to strike independently of any of those background signals. I'm curious though, is there an actual underlying reason for this being such a predictable and frequent phenomenon? What exactly is the mechanism for a brief return to much more wintry conditions at this very, very specific time of year, that is late March and/or early April? It must be the result of a mechanism which recurs annually, which makes me wonder if perhaps it's related to the final warming of the stratosphere, the beginning of the warming of the oceans or melting ice, something to do with the days becoming longer and the increase in sunlight, radiation, etc - or something else entirely?

    Before anyone points to the NAO, yes, this feature is indeed caused by the NAO turning sharply negative, as you can see if you have a look at the NAO charts. What I'm wondering is, what's the mechanism which causes the NAO to turn negative at more or less exactly the same time of year, annually? It just seems far more regular than most related weather phenomena. It does seem to me that there must be a specific mechanism behind this rather than just the standard "Irish weather is unpredictable because of kinks in the jet stream" explanation, this seems to be something extremely regular, predictable, and recurring in nature so can't simply be put down to the usual random luck-of-the-draw chance that gives rise to Irish weather's changeability. 

    Anyone have an insight into this?


  • Registered Users Posts: 1,011 ✭✭✭Neddyusa

    Great question.

    The Sean Bhó Riabhach days (as we call them here) seem to happen every April.

    Also called "skinning of the old cow"

    For those not familiar with the saying/legend, basically it describes the temporary return of winter to kill off the old cow, who everyone had thought had survived to see one more summer at grass.

  • Registered Users Posts: 8,219 ✭✭✭Gaoth Laidir

    Why don't we all ask Orion402 for his take on it? 🤣

    It's probably linked to the transition from wintertime regimes (normally higher pressure forming over continents, including Europe) and summertime (low pressure tends to replace the continental highs, which shift more to the oceans for the summer (akin to the monsoons). We're between two large continental landmasses (North America and Europe). In late winter, a semi-permanent low sets up downstream of North America and other continents due to the low heights from the cold airmasses in place over land. This can pump heights downstream over Europe, which is what occurred a few weeks ago. With cold air already in place, these heights set up that solid high, which retrogressed a bit westwards over us.

    As the North American continent warms up over the next couple of months we should see a more zonal regime set up as the polar front starts its retreat northwards, with pressure over Europe generally falling to more average levels.

  • Registered Users Posts: 3,875 ✭✭✭Dickie10

    when your asking that question, could someone please find out what the reason is for August being such a wet month considering its summer. when was the last drought in august? im actually thinkin of reseeding a field this year in late july for august rains.

  • Registered Users Posts: 408 ✭✭Orion402

    The Earth's North pole is presently turning parallel to the orbital plane and into the light hemisphere of the Earth along with the rest of the planet as a function of the orbital motion of the Earth. The circumference where the Sun remains constantly in view, with the North pole at its centre, is currently expanding and beginning to melt the surface area of Arctic sea ice which gathered over the months where the area lacked any solar radiation.

    The time lapse, about 50 seconds in, affirms two distinct rotations referenced against the light and dark hemispheres of Uranus (as the dark hemisphere always remains out of sight).

    After the June Solstice when the Earth's North pole is at its furthest distance from the dark hemisphere of the Earth and the maximum circumference is reached (Arctic circle), the surface area will begin to contract as the distance between North pole and the dark hemisphere of the Earth begins to close.

    So many interesting topics to consider once genuine researchers come to appreciate that our planet has two surface rotations that interact in such a way that their combination makes life possible, even for those who are ungrateful regarding this new approach to the changing seasons and climate.

  • Registered Users Posts: 8,219 ✭✭✭Gaoth Laidir

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  • Registered Users Posts: 408 ✭✭Orion402

    Seasonal changes are a property of the Earth's daily and annual motions in a Sun-centred system so any other factors are secondary or tertiary across latitudes depending on proximity to the ocean, landmass and so on.

    Seasonal changes are a result of two surface rotations acting in combination so it is a matter of isolating one rotation using the North pole as a beacon (zero daily rotational velocity at that latitude) along with the expanding and contracting circumferences where the Sun remains in view or out of sight across the annual circuit of the Sun. Daily rotation swamps the slower annual rotation (roughly 1 degree per 24 hours) as a function of the orbital motion of the Earth, however, this should not detract from the inviolate fact that when daily rotation and all its effects such as the day/night cycle are subtracted, the entire surface of the planet still turns once to the Sun each circuit, parallel to the orbital plane and therefore at a right angle to the planet's divisor (line separating light and dark hemisphere).

    If others want to explain the alternating expanding and contracting circumferences with the North or South poles at their centre differently then be my guest. This is information sharing and goes into an entirely new approach to explaining seasonal changes using 21st century perspectives-

    Galileo — 'You cannot teach a man anything, you can only help him find it within himself.'

  • Registered Users Posts: 2,339 ✭✭✭The One Doctor

    I love the way you're responding and sounding credible without actually saying anything sensible.

    Are you a middle manager by any chance? Civil service perhaps?

  • Registered Users Posts: 9,658 ✭✭✭Birdnuts

    In my experience cold spells around the end of April/ start of May are even more common and embedded in our climate!!

  • Registered Users Posts: 43 Cornerstonelad

    Known traditionally as doineann na cuaiche. A spell of poor weather that coincided with the arrival of the cuckoo.

  • Registered Users Posts: 6,884 ✭✭✭timmyntc

    Fake spring - varies between March/April every year

    Last frost is usually end of April, and then it gets reliably mild

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  • Registered Users Posts: 408 ✭✭Orion402

    Since I wrote the last response, the radius between the North pole and the planet's dark hemisphere has grown and with it the circumference where the Sun remains constantly in view. At our latitude where the two surface rotations become more dominant, we experience longer daylight periods while in the Southern hemisphere the opposite is happening.

    As the South polar latitude turns into the dark hemisphere of the Earth, twilight is setting in and will remain for a number of weeks before polar night sets in fully.

    It is not possible to discuss planetary climate in any meaningful way without explaining the seasons properly. Despite the news presently on tv where academics are going all out with dire predictions using simulated modelling conclusions that ignore planetary dynamics at the most basic level there is as the planet turns daily and turns seperately as a function of the Earth's orbital motion.

    I could understand the facile sniping at me if it were the case that I couldn't supply an observational demonstration but that has been done using observations of Uranus-

    Post edited by Orion402 on

  • Registered Users Posts: 14,373 ✭✭✭✭M.T. Cranium

    Does the legend specify a time in April when this colder interval was supposed to happen, or is it more of a description for any April cold spell? The thread title suggests it would be around now but then the discussion doesn't provide much further evidence of this.

    I wasn't clear from the discussion whether anyone thought this always happened in a certain time frame, or just could be counted upon to happen at some time in April. I don't find it remarkable that you can count on one cold spell to happen some time in April (most years). Nor is it strictly speaking true, April 2011 for example had nothing but warm weather.

    Some of the more celebrated April cold spells seem to be more or less randomly scattered through the month of April. There was quite a wintry turn in early April of 1917, in mid-April of 1966, and late April of 1908 to name three that were record-breaking. More recently you'll recall that it was very cold in the last third of March 2013 and the first few days of April. Also it was anomalously cold last April around mid-month in particular.

    There is one more specific cold weather folklore item that I remember from reading Hubert Lamb's work, called the "Ice Saints Days" it referred to the 11th to 13th of May and was reputed to be a time when it would often turn unseasonably cold with late frosts and perhaps snow. I looked through the data for the CET temperatures and found only traces of it there, which is not to say it was invalid in the time when it was formulated (probably around the worst part of the LIA).

    Once again looking at CET normals over long intervals, there are minor oscillations against the general seasonal rise in temperature with increased solar angle. In recent years the days around 21st to 25th April have been warmer than the trend line and then from 26th to 30th April it has turned a bit colder. The trends in early to mid-April seem fairly ordinary in terms of averages creeping up at about the overall trend for this warming month.

  • Registered Users Posts: 14,373 ✭✭✭✭M.T. Cranium

    Since I had the data available, this is the temperature trend curve for the 250 years (1772 to 2021) of the CET data set, for March, April and May.

    _____ GRAPH of the MEAN DAILY TEMPS (March to May) in the Central England Temperature series 1772 to 2021 ______ (deg C)

    I would say that over that long period of time, the only obvious "singularity" where the seasonal warming reverses is the dip around the end of April (data point 61 would be 30th of April). Looking at the actual data, after a peak of 9.00 on 24th April, the values stop rising and fall back slightly to reach 8.87 on 27th of April. There is only a very faint signature of the "Ice Saints Days" but all the small jogs in this data set otherwise look entirely random "noise" that you expect in data dominated by the one driver of increasing solar energy.

    When I get some time I will break this into segments to see if there are trends over time.

  • Registered Users Posts: 1,649 ✭✭✭mr.stonewall

    @M.T. Cranium In farming folklore here, the way it was passed along the generations was that it would be the 1st 10 days of April. The last kick of winter, with cold winds and hail stones.

    A bit of a warning to stock that would be going back to grass and young stock. The importance of shelter, be a it a hedge or a dry bank cannot be understated. Calving kicked off here yesterday and it always amazes you when a cow can take her calf an hour or 2 old towards shelter from the cold wind

  • Registered Users Posts: 1,046 ✭✭✭Baybay

    My father always called the first few days of April the Borrowing Days as winter borrowed them from April to finish the job it started & kill off Wee Branny, the brindle cow.

    If older people were ill, he’d say if they made it to the end of March & saw off the Borrowing Days, they’d make through the summer. He himself didn’t make it through those Borrowing Days.

  • Registered Users Posts: 14,373 ✭✭✭✭M.T. Cranium

    So I recalibrated my data search for more recent years (1932 to present) and the same trends show up, so this has been going on throughout the series:

    There is a bit more support here for the first ten days of April being a bit colder or at least not warming up very rapidly (those would be data points 32 to 41). But the cooler reversal around 24-27 April shows up quite well here also. Another reversal shows up in mid-May that was not so evident in the longer-term data meaning that in the years before 1932 it probably wasn't a feature at all.

    What causes these? You might rule out anything to do with the Sun unless it has some connection to interactions between the Sun and fixed sources of gravitational energy in the galaxy (by fixed I am talking relatively short-term in astronomical time scales, but longer than this data series). Or it could be some process that involves events in a distant region having an impact on teleconnections. Just speculating here, but things like a sharp fall in pressures in the subarctic due to rapid increases in heat flux, or some monsoonal process starting up and taking a while to "kick in."

    The only thing I see here related to "climate change" would be the faster warming trend in early March (a statistical result probably from less frequent snow cover).

  • Registered Users Posts: 1,649 ✭✭✭mr.stonewall

    Just wondering is there any dataset with wind direction. This could help with seeing a possible link.

  • Registered Users Posts: 1,363 ✭✭✭Popoutman

    What's the confidence on the variations being real changes and not just statistical noise? (genuine question and not a criticism in the slightest - I do love seeing useful stats, but well-reduced stats are a thing of absolute beauty when done accurately)

    As an aside, I've got Orionxxx or whatever account they're using these days actively set on "ignore" on here, it's wonderful and I'd highly recommend it. Let them scream into the void and have no interactions, and full knowledge that they are being passively ignored. Takes the wind out of their trolling. My sanity levels improve when I don't read that blather..

  • Registered Users Posts: 14,373 ✭✭✭✭M.T. Cranium

    For the 250 years in the first graph, I think only the dip around 24-27 April is "significant" but as you say, for shorter data sets like the second graph (which covers 90 years) you need a larger variation to assume significance.

    There is always going to be some noise in these data sets, probably a thresh-hold would be 0.5 C deg, variations of 0.1-0.2 are probably noise for sure. Now these variations are against a rising trend line, so you don't need to see an absolute fall of 0.5, just the absence of a rise of perhaps 0.4 and the fall of 0.1 equals an unexplained variation of 0.5.

    Since we got into this, I might show the other four seasons at the same time scales and see what shows up.

    As to whether the UK "CET" is directly applicable to a discussion of Irish weather trends, I think it probably would be. It always seems close nowadays to the IMT that we track here for our contests. And the monthly anomalies seem to vary in phase.

    Sadly I don't think there would be wind direction stats going back as far as these temperatures, you might be able to link those up for the past 30-50 years, not sure how much further back that would be available.

  • Posts: 0 [Deleted User]

    Might be interesting to see the data ranges for each day. The first week of April theory may indicate that some years we get large reversals, but also significantly warmer days in other years. I would expect to a relatively tight distribution in early march. Widening and spring sets in and subsequently tightening in April.

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  • Registered Users Posts: 408 ✭✭Orion402

    There is a perceptive blindness or deafness attached to all this, after all, what is information sharing is really a symphony of motions acting in concert to create the seasonal variations we all love so well. I despise nobody and thereby is something of bewilderment for those who go out of their way to comment on participation in a forum meant to discuss weather changes while being deaf of blind to that symphony.

    The circle with the North pole is expanding where the Sun remains constantly in view reflecting the planet turns annually as a function of the orbital motion of the Earth.

  • Registered Users Posts: 1,011 ✭✭✭Neddyusa

    I think you're onto something here.

    This tends to be the period of the year with the greatest day-to-day ranges in weather. And the warmer sunshine and longer days further emphasise the contrasts when we get a wintry April day like today.

  • Registered Users Posts: 162 ✭✭Whatdoesitmatter

    I find your perspective refreshing and your posts deeply insightful. Please don't let the haters stop you from posting.


  • Registered Users Posts: 14,373 ✭✭✭✭M.T. Cranium

    This graph shows the variance of the spring temperature data for the CET series. The units are arbitrary and all that matters about the units would be their comparative values.

    This analysis is for the more recent period 1932 to 2021. It shows the expected overall reduction in variance from March to May, but clearly there are intervals with considerably higher variance than others. One such appears to be late March and early April. (data point 32 is 1st April). This validates the conjecture of the poster who mentioned that the phenomenon in question may be related to higher variability. Relatively low variance accompanies the colder turn I identified around 24-27 April which suggests that it quite often happens that way rather than relying on a few large anomalies.

    Variance is calculated from squared values of deviations from average. The value of the average does not affect the outcome.

    It would be interesting to speculate about what causes these apparent modulations of variance in the temperature trend. They appear to slow down through March and April to the point where the last two waves are on almost a monthly period by May. This is interesting enough that I will do the same analysis for the whole calendar year and post that shortly.

  • Registered Users Posts: 14,373 ✭✭✭✭M.T. Cranium

    So this is the annual variance on a daily basis:

    This analysis also covers the more recent data from 1932 to 2021 (CET data set).

    The narrowest range of temperatures appears to be in August and early September. (day 245 is Sept 1) ... winter variability is higher especially around mid-December and mid to late January into early February. An interval around the new year is less variable.

    The spring period that we already discussed runs from day 61 to 152. Shortly after that first graph, there would be a higher peak again in early June just around the day 153 on the scale provided (inspection of data reveals that it is day 158, 6th of June, with this peak). July in general has lower variability which is even more established in August through much of September. The high peak of variance in late October (305 = 31 Oct) sees a surge of higher variability in the period 28-31 October. One last "predictable" interval follows in early November (probably a mild trend there) and then the higher variance winter signal sets in around 20th of November.

    All of this of course is empirical, entirely related to the timing of the data set. There is no expectation that this would repeat in all details for other time periods. The annual trend curve probably would do so. I will take some time then post the longer-term version of this annual variance graph to cover the entire data series (1772 to 2021) to find out if there are significant differences.

  • Registered Users Posts: 408 ✭✭Orion402

    Thank you for the comment, after all, it is information sharing.

    I am sure many experience those days in March when we encounter the first warm days after the long and dark winter followed by a dip in temperatures to remind us we are not fully out of dark half of the year. It can't really be represented by a graph insofar as creation bursts back into life along with longer daylight hours, we get a buzz however short the sunny or warm period lasts.

  • Registered Users Posts: 5,565 ✭✭✭Pangea

    There is another Irish legend that is associated with bad weather in early May called 'Scread na Bealtaine' literally the May Scream also called 'Garbhshíon na gcuach' Rough weather of the cuckoo.

  • Registered Users Posts: 9,658 ✭✭✭Birdnuts

    This for me is an even more distinctive feature of our climate than what the OP mentioned - the reason it sticks in my head is that it overlaps 2 of my fav sporting events ie. World Snooker Championship and the Punchestown Festival, the former I seem to always watch on the telly while listening to hail or rain on my kitchen roof for much of it, while the later usually involves winter coats or rain gear if attending!!