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  • Writer's pictureMark Hunt

July 24th, 2023



Well it is another soggy July Monday morning, it is 13.8°C outside and pouring down with rain.


This run of Atlantic low pressure mid-summer is getting a bit wearying, no matter that it is doing us such a good job from a turf perspective and topping up our aquafers, it's all a bit dull and dreary. That said when you look to the dire situation in southern Europe with forest fires and drought, I think on balance I'd much rather have the current sunshine and showers scenario.


You wonder how long this will continue and whether when it ends we will just flip to the other side of the weather coin and enter into a period of extended drought for the back end of the summer. Wet summers and dry autumns maybe ?


I am getting asked what the outlook is for August and September given where we are currently. Well I do think they'll be a subtle change as we start August where the jet stream will edge a little northwards and allow some more drier and warmer interludes, but still with Atlantic low pressure systems able to influence our weather. More of a sunshine and showers scenario with some warmer, drier periods in-between. That's how it looks currently as far as the GFS outlook takes us.


General Weather Situation - w/c 24th July


So as you can see from the GIF above courtesy of tropicaltidbits.com, we start the week with a departing low pressure system that is pulling down north east winds (hence the chillier temps) and rainfall across the U.K. It graphically contrasts with the heat dome over southern Europe shown below us.


So Monday sees this band of slow-moving and persistently heavy rain moving across the U.K diagonally, stretching from The Severn estuary up to The Wash. North and south of this we have much nicer weather but of course this front is moving south eastwards so if you are currently dry in the south east, it won't be for much longer. On the flipside, for Scotland, the north of England, Wales and The South West, as this front moves south east, it'll clear these areas and provide sunshine and showers behind it, with warming temperatures. Being slow-moving, rainfall amounts may be high, we are on 7.3mm so far today and it falling with a rain rate of 2.8mm/hr. Temperature-wise, if you're under the rain, expect 13-15°C only, but 17-19°C away from it.


Better news for Tuesday as we have a brief hiatus before the next rain front pushes in. So Tuesday looks drier but not totally dry with some showers across eastern coasts, The North West and crossing Scotland. Away from these showers, it'll feel a little warmer as the wind swings round to the north west. So a sunshine and showers outlook for Tuesday with 17-19°C temperature range for Scotland and Ireland and nudging into the low twenties for Wales and England.


Come Wednesday and we see a new Atlantic rain front pushing into the west of Ireland during the morning. Initially light, this rain will intensify and become heavier through the course of the day as it crosses all of Ireland and pushes into West Wales late in the afternoon. Away from the arrival of this rain, it'll be a pleasantly warm day with some nice temperatures pushing into the low twenties in places. Now the good news is that this rain will pass across most of the U.K during the night, clearing the south east coast last during the course of Thursday morning / early afternoon (although some areas of The South East may hang onto rain into the evening). So an improving day on Thursday for Ireland, Wales, Scotland and most of England as that rain clears away. with temperatures nudging into the low twenties accompanied by a moderate south west wind.


Closing out the week on Friday and we have a new Atlantic low pressure pushing into the north west / west of Ireland bring showers. This low pressure will vector rain showers across Ireland during the course of the day accompanied by some strong winds. Away from this low pressure incursion, Friday looks like another pleasant day of warm sunshine, variable cloud (more in the west) with some showers crossing Scotland during the day. Later in the evening, showers will cross The Irish Sea and move into western coasts of England, Wales and Scotland.


The outlook for the weekend for the weekend is looking 'unsettled' and for some areas it can only be described as 'pants'. Saturday is a sunshine and showers type of day but for Scotland, Northern Ireland and particularly The North West / Lakes, it could be very wet. Away from this heavier rain we will see sunshine and the odd shower further south across Ireland, England and Wales with temperatures in the 18-20°C. It'll be pretty windy though from the west. Sunday looks a role reversal as a heavy rain front looks to arrive into the south west of Ireland and England during the early morning. This will then progress north east across Wales, the north of England and The North West. So the driest areas on Sunday will be across the east coast until the rain arrives later in the day. A pretty wet day under that rain I am afraid :(




Weather Outlook - w/c 31st July


So after a decidedly up and down weather week to close out July, we enter the first week of August in this outlook forecast. Any change ?


.Well yes a subtle one as the jet stream will just hike up a little and that means more high pressure incursions although presently I can't see a dominant high pressure formation on the cards yet. So this means more changeable weather but possibly less heavy rainfall.....possibly...


The start of the next week sees that very wet low pressure still affecting our weather with a pretty windy and wet start to the week, The affects of this low will clear the west during Monday leaving behind a sunshine and blustery showers scenario. It'll be a bit cooler as the wind will be from the north west. Tuesday looks drier but it'll still vector showers down the east coast of England and the wind will remain north westerly so still on the cool side though Ireland and the south west of England / Wales will pick up some nice warmth from an Atlantic high pressure.

This high pressure will push in during the course of Wednesday to give a pleasant day in the southern half of the UK and across Ireland with lighter north westerly winds, but rain is due to arrive into the west of Ireland during the afternoon. This rain will cross The Irish Sea into Scotland and northern England on Thursday night and provide a wetter, unsettled day though the far south and east may miss the worst of this. Towards the end of next week, high pressure continues to edge in so lighter winds, warmer and maybe a little drier, but still with the chance of showers across the north I think.


Is this the start of some warmer, more settled weather for August ? We will see........


Agronomic Notes


In this week's agronomic notes I thought I'd look at how this summer has panned out from a plant available light perspective. I was wondering if this run of unsettled weather had seriously impacted the level of plant available light or to give it its more scientific name, plant available radiation (PAR).


I know a lot of you are overseeding with Agrostis sp. be that Creeping or Browntop. The former has a known sufficiency level for light, that is to say, research has identified how much light is required for healthy growth. Now for Browntop bentgrass, there isn't the data available to the best of my knowledge. The DLI level for Creeping Bentgrass is held to be 30 mol per m2 and I suspect the figure for Browntop is around 15-20 mol per m2. The latter is a SWAG answer as the late, great Dr James Beard used to declare, a scientifically-wild-ass-guess at my best.


Before I look at how light levels have shaped up over this summer let's go back a step. for a refresher.


Plant available light is the portion of the light spectrum (400-700 nm) that plants

utilise for photosynthesis. Photosynthesis = energy production = growth, so that's why it is important. Is the grass plant getting sufficient light to enable it to grow ?


Plant available light is held to be between the wavelengths of 400 - 700 nm of the light spectrum as you can see from the image above. We measure it on the Davis weather station by using my fav sensor, an Apogee SQ-212, it is such a cutie.

The Apogee measures the amount of PAR light available and converts it into a measurement called the Daily Light Interval (DLI). This is the total amount of PAR light that is measured across a 24-hour period. Obviously light levels vary across a day according to the weather, there also isn't any PAR light at night, unless you are running a lighting rig that is....


Below is a graph of PAR light measured across a 24-hour period yesterday (23rd July) utilising an Apogee SQ212 hooked up to a Davis Vantage Pro weather station.


You can see the variability across the day, with PAR levels starting at 05:30am as the sun rose, peaking at 14:15pm and dropping back markedly during a period between 15:00 - 16:00pm when a passing storm brought rainfall and heavy cloud cover. PAR levels ceased as the sun set around 21:00pm.



So going back to my original thought process (I do this a lot you know), I have charted out the DLI levels as measured in Central Scotland and England since the beginning of May, 2023 up until yesterday. What we are looking for is DLI levels > 30 mols per m2 for Agrostis stolonifera and > 11 mols per m2 for Lolium perenne (Perennial ryegrass).


Here's how it looks with the sufficiency threshold levels marked out for the 2 species.


Now this data really interested me.


Firstly, if we look at a comparison between Scotland and England, we can see that the levels of light are actually higher in Scotland than in England. The chart below shows the average monthly DLI (mols per m2) measured this summer.


This is of course because the further north you go, the longer the days are in the summer as anyone who has done the NC500 in summer will testify. Back in the day I worked in seed production in agriculture and I used to have to crop inspect fields of Spring Barley for certification purposes. A lot of my seed production acreage was in Scotland and I remember walking crops in the summer at gone 10pm at night with beautiful visibility.


Now given that I have already stated the sufficiency level for Creeping Bentgrass is 30 mols per m2, you can see in May and July, the average PAR light level was below this, but in Scotland it was above.


You can also see the effect of our topsy-turvy summer on daily DLI levels with early May and mid-late July recording more days when the DLI < 30 mols per m2. This effect was more marked for Scotland than England and was caused by more rainfall and associated cloud cover for Scotland during these periods. So the Scottish data shows more variability, their DLI peaks are much higher than for England but they also have lower DLI days than England through the summer.


So has this summer weather provided less PAR light than Agrostis sp. need for growth ?


Well for Creeping Bentgrass yes it has, but there has also been plenty of days when the grass plant would have had more than sufficient light. If you count up the number of days the DLI <=30 mols per m2 for the two locations, it comes out as 30 for Scotland and 37 for England out of a total of 84 days. That's over a third of summer days when the light level was insufficient for growth of Creeping Bentgrass.


For Lolium perenne, it is much more clear cut, with a lower DLI requirement, there are only a handful of days since May 1st when DLI levels has been insufficient for growth. 3 as a matter of fact out of a possible 84.


Next week I will dig out the DLI data for 2022 just to see how it compares over the same period from the same two locations.


OK, that's me for another week, I have 2 weather stations to preconfigure and install so tempus fugit !


All the best.


Mark Hunt










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