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Why is April so dry?

  • 24-04-2022 9:59am
    Registered Users Posts: 4,293 ✭✭✭

    I'm a keen Gardner and nearly every April we hit a dry spell after the seeds have been sown. This is when we really need the rain. This has been more pronounced in the last few years.

    Anyone know if there is an underlying meteorological reason for this?


  • Registered Users Posts: 8,913 ✭✭✭Danno

    April and June are the driest months of the year (at least in these parts). The polar vortex easing off after Winter encourages high pressure to form more easily in the northern latitudes, this has the effect of blocking out Atlantic lows and their associated fronts from crossing Ireland should these high pressure cells set up in the correct place.

  • Moderators, Science, Health & Environment Moderators Posts: 16,744 Mod ✭✭✭✭Gonzo

    The Atlantic shuts down for us most Aprils, as Danno said the Polar vortex usually collapses end of March or beginning of April and we regularly see easterlys or notherlies during April which is a much dryer wind direction than westerlies which keeps the conveyer belt of low pressures well away from Ireland. This doesn't happen every April but certaintly over the past few years this has been the case.

    Now if only we could get this regularity of shutting off the Atlantic during mid winter and mid summer, we would have proper seasons.

    As for the remainder of this April, it looks like most places will remain largely dry until the final day of the month. We may turn more unsettled as we begin May and it will remain on the cool side until then.

    Post edited by Gonzo on

  • Registered Users Posts: 14,370 ✭✭✭✭M.T. Cranium

    It has been a run of drier Aprils than average too, what is often a rather dry month has recently been even more so, thanks to more persistent blocking highs. There can be more mobile April patterns with greater amounts of rainfall. It may be the second week in May before we see a lot of change to this current pattern.

  • Registered Users Posts: 13,530 ✭✭✭✭sryanbruen

    April naturally is a dry month due to reasons previously stated with regards to the hibernation of the stratospheric polar vortex which keeps our winters relatively mild and stormy a lot of the time as a result of an intensified jet stream across the North Atlantic. The average date for the final stratospheric warming (where the zonal mean zonal wind at 60N 10hPa in the stratosphere reverses easterly until late August) is April 15th - sometimes this can be significantly earlier like mid-March which it was this year and other times it can be significantly later like early May. The latter tends to happen more in winters where a mid-winter major sudden stratospheric warming has occurred due to the fact that a sudden stratospheric cooling can occur following these warming events.

    The above doesn't apply only to April however, it applies to the whole period from February to July which is Ireland's driest portion of the average year. It can vary from place to place where has the driest month - like for example, Dublin's driest month is February but at Valentia in Kerry has its driest month in May. The latter is likely down to the fact that May is the most easterly month of the year on average and the month with the highest probability to have a blocked Atlantic which Valentia is very much exposed to Atlantic fronts. Each month has their unique characteristics with impacts from external drivers like for example July is very different to April in the contrast between the continent and the ocean increases in July so we tend to see a European monsoon pattern set up or as often touted as 'the return of the westerlies' where whilst the stratospheric polar vortex is still in hibernation or non-existent, we tend to get an increased chance of cloudy southwesterly winds which results in July and August being cloudier most of the time than May or June. Recent climate averages also show April now being a sunnier month than the latter two summer months for many.

    The last sentence brings me nicely into my main part of this post. April is traditionally a dry month but it has been especially dry recently with Aprils 2007, 2011, 2015, 2017, 2020 and 2021 all providing notably dry conditions. This is not without precedent as there was a fair run of dry Aprils in the 1970s/80s including 1974, 1976, 1980 and 1982. We can get these runs of a particular month of the year sharing a similar theme at times. A few examples are cold Decembers in the 1960s where only 2 of the Decembers in the decade were relatively mild (the opposite in the 1980s when almost all Decembers were mild except the severe cold 1981 and chilly 1989 in the north and winters that decade tended to be backloaded with regards to cold), warm Aprils in the 1940s where every single April bar 1941 was warmer than average, the frequency of warm to hot Augusts in the 1990s (1990, 1991, 1995, 1997, 1998), many exceptionally cold Novembers from 1909 to 1925 that remain unbeaten since etc. I feel it's another one of those quirky repeating trends.

    The blocked April trend shows up nicely in mean sea level pressure anomaly reanalysis of Aprils since 2007 with an anticyclone centre to north of Scotland and ridging over us.

  • Registered Users Posts: 162 ✭✭Whatdoesitmatter

    @Orion402 Prehaps you might favour us with your opinion

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  • Registered Users Posts: 8,913 ✭✭✭Danno

    The above doesn't apply only to April however, it applies to the whole period from February to July which is Ireland's driest portion of the average year. It can vary from place to place where has the driest month - like for example, Dublin's driest month is February but at Valentia in Kerry has its driest month in May. 

    Point well made, and one I forgot to expand on above.

    Throughout winter and into early Spring there is still a bit of heat in the ocean which creates showers out at sea. Warmth from the sea rising up into cold airflows (convection over sea) usually coming from the west drives showers of rain, hail, sleet and snow into the west coast of Ireland. As these showers travel inland they decay and die out as the heat source (the sea) is replaced by the colder landmass. When the airflow comes from the east, as it crosses the Irish sea, snow and sleet showers form if the air is cold enough - which what happened in the days before Storm Emma.

    As we move on into mid-Spring, the seas are at their coldest - so there isn't much heat rising into the cool air of the north Atlantic and we see very few showers hit the west coast as a result. However as this westerly airflow moves inland, the land has warmed up quite a bit as we reach early April (stronger sun) this heat rises into the still cool air above and thus showers form (convection over land) and move eastwards. As these showers move across the midlands they continue to grow and get bigger as they pick up more and more heat from the land. By the time they hit the east coast there can be some quite beefy showers pouring down - and hence the east coastal areas don't have a notable dry April on average.

    If the winds blow east at this time of year, you'll notice that the East coast stays dry - but very chilly owing to the Irish sea being at it's coldest around this time of year. Persistent easterly winds through April will come as a surprise to east coast folks as they note the absence of showers but note the cold air from the sea.

    Differences in temperature between the air and what's below it usually determines the weather that happens...

    cold air, cold sea - usually fine and sunny under high pressure, wintry conditions possible under low pressure

    cold air, warm sea - showers especially for windward coasts under low pressure, gloom and cloud under high pressure.

    Warm air, cold sea - fog, drizzle, rain especially near coasts under low pressure, cloudy with breaks or clear spells to the lee of mountains in high pressure.

    Warm air, warm sea - rather wet with low pressure conditions, fine and warm with sunny spells along with cloudier spells in high pressure

    Cold air, cold land - usually fine and sunny under high pressure but cold, wintry conditions likely under low pressure

    Cold air, warm land - showers especially away from windward coasts with low pressure, settled but prone to fog under high pressure

    Warm air, cold land - cloudy, damp and misty under low pressure, mostly fine though prone to hill fog under high pressure

    Warm air, warm land - thunderstorms in low pressure, hot and dry in high pressure.

    The above is not set in stone, but is a useful guide.

  • Registered Users Posts: 15,337 ✭✭✭✭Supercell

    Please don't!

    Nicely done Danno!, never thought about the nuances of land/air and sea temperatures like that, all reasonable conclusions.

    Have a weather station?, why not join the Ireland Weather Network -

  • Registered Users Posts: 408 ✭✭Orion402

    The polar vortex is linked to the position of the North pole and the expanding and contracting circles with the poles at their centre where solar radiation is constantly present or absent.

    Presently, the circle where solar radiation is constantly present, with the North pole at its centre, is expanding as the North pole (and by association, the entire surface) turns to a midpoint on the June Solstice when the circle reaches its maximum circumference also known as the Arctic circle. The North pole is furthest from the dark hemisphere of the Earth on the June Solstice so as it continues towards the September Equinox, the radius between the North pole and the dark hemisphere shortens and so does the circumference where the Sun remains constantly in view. The presence and disappearance of Arctic sea ice follows this expanding and contracting circle along with the atmosphere above the North pole.

    Understanding the seasons and events in the atmosphere, oceans and landmass by a more productive use of the Earth's daily and annual motion should not receive the resistance it suffers here, after all, this is information sharing and I don't have the sole right to explain why the planet turns in two separate ways to the Sun or the dark hemisphere of the Earth. It is a 100% observational certainty and meshes nicely with seasonal variations in weather patterns-

  • Registered Users Posts: 162 ✭✭Whatdoesitmatter

    @Orion402 thanks for your excellent response. I greatly enjoy your posts. Please don't let the cavemen who inhabit this forum drive you away

  • Registered Users Posts: 408 ✭✭Orion402

    To be fair, it would be far better to get commenters to engage in genuine modelling for interpretative reasons rather than the disruptive predictive conclusions which experimental theorists engage in.

    The expanding and contracting circles with the North/South poles at their centre are represented by a fluctuation in surface and atmospheric temperatures North and South of the Equator across an orbital cycle, once again, a property of two separate rotations working in combination-

    If the Earth had the inclination of Jupiter, the Arctic/Antarctic circles would be tiny, so there would be no appreciable fluctuations and temperatures across latitudes would be more or less homogenous.

    If the Earth had an inclination similar to Uranus, those expanding and contracting circles would be extreme so the hemispheres would shift rapidly from red to blue across an orbit where one hemisphere would be totally red while the other is totally blue around the Solstices.

    It is just a different approach to the seasons and ultimately a gateway into more productive and creative climate research.

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  • Registered Users Posts: 14,370 ✭✭✭✭M.T. Cranium

    Don't these meaningless insults and deliberately obscure posts violate the forum charter?

    We used to have much more intrusive moderators. I once got a stern note because I posted "I heard a bang" on a storm thread.

  • Registered Users Posts: 408 ✭✭Orion402

    The dynamics behind the polar vortex and its collapse mesh neatly with the motion of the North pole out of the dark hemisphere of the Earth on the March Equinox-

    Tomorrow is the beginning of summer (Bealtaine) as our ancestors divided the year into two halves with the dark half from November 1st (Samhain) to April 30th, at least in the modern calendar, and the light half beginning May 1st. This puts midsummer on June 21st when the daylight hours are the longest, once again, a product of the motions of the planet.

    A more productive way to approach the seasons is the expansion and contraction of those circles, with the North/South poles at their centre. The current expanding circle around the North pole occurs as the radius between the North pole and the dark hemisphere is currently increasing as a function of the orbital motion of the Earth. It replaces the less productive 'tilting Earth' perspective by incorporating two distinct surface rotations acting in combination.

    Nobody is being upstaged, information sharing makes research a much more democratic affair in this era and not all people have the necessary faculties to adjust to a less hierarchical structure where genuine large adjustments are necessary.

  • Registered Users Posts: 4,426 ✭✭✭maestroamado