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  • Writer's pictureMark Hunt

April 22nd, 2024

Hi All,

Well we are 'enjoying' a chilly 2nd part of April with temperatures struggling into double figures and a keen northerly wind direction. The events of early March when we saw a Sudden Stratospheric Warming and then of late, a reversal in the Polar Vortex have conspired to bring our normal west-east pattern of weather to a halt and in fact some weather systems will move east-west this week. This is the last hurrah of winter, for soon the Polar Vortex will disappear and reform again in the autumn.

I fly fished yesterday at Rutland Water from the bank and found a spot out of the keen north easterly wind. The air was full of midges (non-biting) and the first arrival House Martins and Swallows were making hay while the sun shone, their happy chatter just made me smile. My mum used to love hearing the Swallows chat to themselves in flight or sitting on a wire and I share that feeling. They are overdue a big calorie top up after flying the 6,000 odd miles from South Africa and Namibia across the Congo Rainforest, Sahara Desert, Morocco, Spain, France and then the short hop across The North Sea.

Just consider that, 6,000 miles.

So the next time you see and hear these happy migrants sitting on a telegraph pole wire, chattering away above your heads, tip your hat to them, for they are a hardy and brave bunch. We are seeing less and less Swallows and Martins nowadays due I think in part to issues along the way with the environment they feed at before crossing The Sahara.

No issues on the Hedgepiglet side with multiple visitors at night (plus the odd Fox)

to partake of the pile of Ark Wildlife Hedgehog food I put out. (I only mention the brand because people ask me what I feed them). And yes I know my lawn needs a cut, but since it sits in shade from the end of October to the end of February, more leaf is an asset. I will be touching on light levels in my blog later as I have a comparison from one winter (Sept '22 - Mar '23) with another (Sept'23 - Mar '24).

General Weather Situation - w/c 22nd April, 2024

So currently we have high pressure sitting over the U.K & Ireland with the west enjoying by far the better temperatures. Northern Ireland hit 19°C yesterday whilst we struggled with 7-8°C. The leading edge of the high pressure is sat over the east of England, so that means we have a north easterly wind in situ and furthermore, that wind is currently pushing showers in from The North Sea across the north of England, Midlands and east of Wales. During today those showers will move south and west into the southern half of the U.K. That very much sets the scene for today's weather, dry and warm (15-18°C) across the west (Ireland) and north, wet cool (8-9°C) and dull south of The Humber I'd say. Wales will be mid-way between the two at 13°C. Night temperatures will be bordering on ground frost territory in places with 2-4°C, should the skies clear locally.

Tuesday is a similar day just less in the way of rain pushing down from The Humber and Wash across the country from north east to south west. These showers will not form until later into the day, so a drier day for all areas, still cool and dull across the southern half of the U.K, warmer and drier away from that swathe of cloud across Ireland, the Scotland and Borders. Still that big temperature differential as well and a keen north easterly wind to blame.

Wednesday is a much drier day for everyone, a pretty clear weather picture and with the wind turning round to the north, that'll promote a brighter picture with more in the way of sunshine I think for all areas. A north wind is never going to mean much in the way of temperature, so the bulk of the U.K will sit in the 8-9°C range. For Ireland and Scotland, it'll be cooler with an easterly and north westerly wind respectfully, so more like 11-13°C, still better than the south. Wales holds on to those better temperatures.

By Thursday we see that high pressure shuffled south and west by weather patterns moving eastwards from the continent towards the U.K & Ireland. Weather patterns don't normally move eastwards and its all to do with the reversal of The Polar Vortex and weakening of the jet stream. With low pressure systems surrounding the U.K & Ireland, it isn't a surprise that we see rain re-enter our weather picture. So Thursday sees rain push into the west of Ireland and move across country during the day. For the U.K & Ireland we will see more in the way of cloud and showers pushing in from the north west across the south west of Scotland, The North West and later, northern England and Midlands. Still with that big temperature differential 13°C for Ireland and Wales, 11°C for Scotland, but only 8-9°C for England.

Closing out the week, that low pressure off to the south west of Ireland will push south of the U.K as it butts up against the easterly airflow. This means the wind direction will switch round to the east and lighten a little. With low pressure sitting south of the U.K, this means rain will push in across the southern half of the U.K and Ireland, probably if you drew a line from The Mersey to The Humber.

As intimated last week, my concern with the easterly-flowing jet stream was the potential for a trough pattern to form and a low pressure system to sit in that trough. This is the pattern for the coming weekend with a south easterly / southerly wind ushering slightly milder winds and rain across the southern half of the U.K. The main rain band associated with this low pressure will cross the southern half of the U.K during Saturday. Ireland will miss this first rain band on Saturday but later in the day will see rain, some of it heavy, tip into Leinster and at the same time, rain and wintry showers will move into Scotland. Sunday sees a drier day initially across the south of England but showers will soon push into the south and move northwards. Further north and west we will see the rain band cross Ireland and push into Scotland, with some of these showers wintry in nature. 11-13°C will be the temperature range with light south easterly winds.

Weather Outlook - w/c 29th April, 2024

After a two week pause in the normal west - east pattern of weather systems moving across the U.K & Ireland, the end of April promises a reversal of the err... reversal, back to the west-east direction we normally get. So the start of next week sees the wind shift from south east to south west and that'll immediately hike the temperatures back up into the mid-teens I think. In addition, night temperatures will also take a hike upwards to 6-8°C. These two factors will means grass growth rates will also increase, more on that later.

With low pressure sitting north west of the U.K and high pressure to the south, the temperature and rainfall differential will reverse next week, with more in the way of rain and cooler temperatures across the north and west and warmer across the south.

Next week then will see us switch back into a more sunshine and showers scenario with rain across all areas on Monday. We will also see a more typical north west rainfall trajectory. On Tuesday, a new low pressure pushes rain, some of it heavy across Ireland and then tracks north and east into The North West and Scotland. Showery on Wednesday but thereafter high pressure attempts to push up and that'll reduce but not remove rainfall with The Midlands and north of England caught between a northerly low and southerly high.

Spoiler alert, the 1st weekend of May could be a wet one before there's a hint of a warm high later into the 1st week of May. Now that's an age away time-wise, a lot can change, but I think the 'reversal of the reversal' looks like a strong weather signal for the start of next week.

Agronomic Notes

The 1st important news from an agronomic basis is the effect that the reversal of the reversal is going to have on grass growth. With the change from a north easterly / northerly airstream to a more atypical south westerly, we will see an increase in day and night temperatures.

As I have often commented, night temperatures in spring are the handbrake to grass growth. Above you can see how air temperature affects the potential of grass to grow as measured by Growth Potential.

Example 1 is typical of what we currently have, a cold night and nothing-to-shout-about day temperature. The resulting Growth Potential (G.P) calculation gives us 0.06, in other words, the grass plant is hardly growing.

Example 2 is more typical of what we can expect when we have the typical sunshine and showers pattern of weather in April / May. More cloud cover and a south west wind direction mean night and day temperatures take a hike upwards and so does the Growth Potential, pushing up to 0.5 (analogous to 50% of optimum growth from a temperature perspective).

Now unfortunately the weather from 29th April onwards also looks wet and so for the courses just drying out, maybe this upward hike in G.P won't be greeted with such a welcome. That said, we will have a brisk south westerly wind in situ next week which means it'll be a good drying wind with reasonable E.T. I'd be expecting 2.5 - 3.0mm per day E.T loss, so this will work in our favour to dry us down between the showers. In the April Prodata Reports summary from a golf course in Central England, I have highlighted how the daily E.T levels are now typically 2mm or higher (apart from today that is !). You can also see this location has had 57.8mm of rain so far this April BUT 40.8mm of E.T.

In other words 70% of the months rainfall has been evaporated. That is the beauty of measuring E.T. So don't despair at the prospect of more rainfall.

Might be worth applying a PGR + iron on faster-growing areas if you are indeed able ?

I mentioned earlier that I have been doing some data crunching when it comes to light levels this winter vs. last. So I took the period from September 1st to present day (well yesterday actually because I can't get a 24-hour DLI reading for today at 11:24 am !!!)

So I compared the Daily Light Interval, that's the total amount of plant-available light that we measured using the Apogee SQ212 light sensor, fitted to a EnviroMonitor node that is hooked up to a Davis VP2 Weather Station. The location is Central England.

Here are the 2 graphs ;

So I have drawn the usual lines at 11mols per m2 and 30mols per m2 for Lolium perenne and Agrostis stolonifera respectively. I have also equated a rough period when the DLI dropped below the sufficiency level for L. perenne in the autumn and picked up again the following spring.

Now I freely admit, it is open to visual interpretation but I am surprised how similar the date range is. It looks like once we get past the 3rd week of October, then we are down below where we need to be light level-wise for Lolium perenne and it pretty much stays that way till the 3rd week of March.

One thing to bear in mind is that the DLI level that is often quoted came from the U.S and there (to the best of my knowledge) the Lolium cultivars tend to be thicker-leaved, blue-green cultivars, whereas European modern-day, Lolium culitvars are much finer-leaved, lighter green in colour (to the point where I don't think a golfer could tell the difference between that and Fescue 😮). Now I am not a seedsman, so I am happy to be corrected on the above, but if my generalisation is correct, I'd expect European cultivars of Lolium to have a lower DLI requirement possibly ?

If we look at Agrostis stolonifera and the minimum sufficiency DLI level of 30 mols per m2, well we dipped below that threshold at the beginning of September 2023 and have only just started to register DLI levels above it this last week. Again in what is a caveat-laden bit of tech writing on my behalf, you could apply the same statement to the DLI level for this species (as I did for Lolium perenne) and say that DLI figure has been kicking around for donkey's years and modern-day cultivars can grow at lower DLI levels because they are finer-leaved than the original cultivars like Penncross.

When you compare the two seasons from two consecutive years, it is striking that the period of sufficient light is so similar from one year to the next. Indeed if I add up the DLI from September 1st to April 21st, I get the following totals ;

Sept '22 - April '23 (y.t.d) = 2480 mols per m2

Sept '23 - April '24 (y.t.d) = 2461 mols per m2

Almost identical.

Now we know that the period from September '23 - April '24 has been one of wettest on record. Wettest = Cloudy = Dull = Low PAR and yet the total PAR light levels aren't significantly different from a drier scenario the year before. So maybe the fact that day length is a constant over that period, in terms of declining up until the Winter Solstice (Dec 21 typically) and then increasing thereafter until the present day has the bigger effect. One to keep an eye on in the future perchance ?

This piece of work takes me to another wider subject in terms of weighting the parameters that impact on plant growth, be that temperature, light, moisture and of course, atmospheric CO2. If we are looking at plant growth, are they all equally weighted ? I don't think so.....

When you look at a top-specification football pitch running lighting rigs and undersoil heating, they are altering soil temperature, air temperature and DLI, but which one has the bigger effect on growth of the grass plant ?

All the best.

Mark Hunt

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