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  • Writer's pictureMark Hunt

October 10th

Hi All,

You get some lovely colours at this time of year, especially as the leaves begin to turn and we pick up some autumn sunshine with the sun at a low angle to the horizon.

This picture also represents quite an accurate weather forecast for the week ahead with some good periods of sunshine, but rain as well and we finish up the week with some high winds, heavier rain and cooler temperatures.

October '22 is probably following a more 'normal' transition into autumn / winter than previous ones where we have picked up muggy and humid weather with tropical temperatures and an associated high disease risk. I was chatting to a course manager / friend last week about Microdochium control and how by this time last year he had already applied his first fungicide, whereas this year he hadn't seen the need to.

Onto this week's weather and as I type this early on Monday morning we have been the recipient of a nice drop of rain which is currently sitting over the south east of England and due to move off into the channel shortly. Sunshine follows behind it.

General Weather Situation - w/c 10th October, 2022

Well a bit of a calm before the storm job I'd say this week as we await the arrival of an intense and cool low pressure system from The Atlantic later in the week / weekend.

So we start the week with a departing low pressure exiting the U.K bottom right leaving behind a mostly sunny day on Monday for us all. Showers are expected across north western coasts but aside from that (and the earlier rain) Monday is a nice start to the week with temperatures pushing up into the low to mid-teens for most areas. Monday and Tuesday (for most) represent the driest days of the week with Tuesday looking sunny and dry but a much cooler start to the day as clear skies allow temperatures to drop into low single figures.

As we approach mid-week, things become more complicated with a weak weather front pushing rain across Ireland and into the Scotland and the north of England for Wednesday. This front will slip south through the day bringing rain further south across Wales and into the southern half of England clearing Ireland as it does so. Not for long though because another rain front is due to push across Ireland, Wales and the southern half of England on Thursday. Some debate about exactly where it will track but it currently looks more southerly-orientated. We close out the week with more rain for Scotland and the north and a sunshine and showers theme for Ireland and further south, but more sunshine than showers. It'll become noticeably windier on Friday and cooler.

Wind-wise, we start the week with northerlies and then adopt a south westerly theme until the end of the week when winds strengthen to gale force and push in from the west.

The end of the week and weekend is set to be cool, wet and very windy with a strong westerly wind in situ. You can see the low pressure arrive on the last few frames of the animated GIF above courtesy of Saturday looks the windiest day by far with gale force winds and showers pushing across the U.K & Ireland. Temperatures will drop back into the low teens, 12-13°C will be typical across the U.K & Ireland. Sunday looks to still be windy but all the time that wind be dropping back. It should be drier too but this may just be a temporary situation before we start next week.

Weather Outlook - w/c 17th October

Bearing in mind that the GFS outlook correctly predicted this weekend's weather last week, it does show that sometimes we are able to get a handle weather trends even 10-14 days ahead of the game.

That's because we have a more predictable weather scenario at present with a strong Atlantic jet stream pattern however it isn't the usual 'L' shape (see below's graphic from

Now this is the jet stream scenario we start next week can see the significant loop that is projected for next Monday. The consequence of this loop is that Atlantic low pressure systems will push into the south of England from the south west rather than following the more traditional north west - north east routing.

So next week looks to start very windy from the south (so it'll be very mild as well I think) and very wet on Monday for the southern half of the U.K, Wales and Ireland as that low pressure moves through. This rain my not reach Scotland till Tuesday. We may see a brief respite for England (though the west looks like it'll be wetter) on Tuesday before another low pressure pushes in and brings more wind and rain on Wednesday and Thursday before we see the winds swing round to the west, temperatures drop a little and things settle down a bit thereafter.

Agronomic Notes

So in the spring I did quite a bit of work looking at light levels and comparing them between central Scotland and England and showing that it took quite awhile for them to reach a level sufficient for Agrostis stolonifera and in some cases even Lolium perenne.

Well now we are well and truly on the backward slope towards shorter days I thought it would be good to plot the data again for the two sites where we have the lovely little Apogee PAR sensor (left) hooked up to a Davis Vantage Pro. As a quick recap, plant available radiation (PAR) is measured across a 24-hour period known as the Daily Light Interval (DLI) and there are some known minimum sufficiency levels for cool and warm season grass species.

So here's how the data looks measured from February 1st, 2022 up to and including yesterday ;

Looking at the data from Scotland, we can see that we didn't really reach good DLI levels (>30 m mols per m2) for Agrostis stolonifera till the beginning / Mid-May and even these levels weren't consistent. We then lost that level consistently from mid-August, finally dipping below that threshold in the middle of September.

For Lolium perenne (Perennial ryegrass to you and me), we hit good light levels around mid-March and maintained them but by the beginning of October we have started to dip below the minimum level required to sustain this grass species.

So if your rye up north is looking a bit sad, it could be down to the lack of plant-available light.

Let's see the same graph using data for Central England ;

So looking at the same data we see an earlier date (Mid-March) for > 30 m mols per m2 but like Scotland, it wasn't held consistently until early June. That said we can see that we hold higher DLI levels here more consistently and that's a major reason why Bentgrass of this species is more likely to succeed in the southern half of the U.K vs. the northern half. That said, we can see that once we got to early September, DLI levels begin to decrease dramatically and to a level below the sufficiency point for this species.

Now let's just think about that statement.

If we are renovating and overseeding as part of that process with A.stolonifera, the newly emerged seedlings will require good lights levels in order to establish. From this data we can see that those levels only really hold until the 3rd week of September latest and if you were judging the date vs. the point when we dipped below 30 m mols per m2, it would be the beginning of September. Now of course it won't work like that in practice, the grass plant will still be growing at 25 m mols per m2, but I think once you get to the third week of September and we are talking 15-20 m mols per m2, it will have a detrimental effect on growth and therefore establishment for this species.

For Lolium, the picture is a better one because it has a lower DLI requirement (held to be 11 m mols per m2). From the data above we can see we are still above the threshold sufficiency level although we did dip down briefly below it in early October.

It is worth pointing out that both of these sensors are in open situations and so reasonably unaffected by shade / lower sun angle. For those sites that are, I would say we are very much now approaching the point where Bengtrass and in some cases even Lolium, begin to drop back from a growth perspective because of low light levels.

Now light is only one part of the growth equation, temperature is the other very important factor and of course moisture.

In November I am talking at the Bigga conferences. (all of the above venues, except Stirling as I'm in for a small shoulder op the day before and I get dagger looks from my consultant if I discuss driving ) The above is a subject area I am going to cover because I think it relates directly to what we are trying to achieve in terms of overseeding and the bigger picture of species conversion, disease reduction and the like. You can find out more about the program of speakers, etc here

Why does it get colder as the sun comes up ?

I was asked this question at a talk I gave recently and I couldn't really answer it without having some data to refer to. Being in this profession, we are often in the situation of watching the sun come up and for a short while it often feels colder. Many times dew and frost only form at daybreak. Why is this, what is going on ?

Well above is a graph showing air temperature measured at 2m height and at ground level using a Davis vantage Pro 2 weather station fitted with an additional temperature / humidity sensor at ground height.

If we follow the trace of both graphs starting at midnight, we can see the air temperature decreasing but there is a difference between ground height and 2m. The ground height is cooler, often 1-1.5°C below the temperature of the air above it. What is happening here is that the earth is losing temperature to the air and this warm air is rising so the coolest point is at ground height. Now I picked a reasonably clear night to show the data, if it was cloudy, the rate of decrease in temperature would be less pronounced because clouds provide an insulation effect and prevent heat radiating upwards from the earth. Another factor that has an effect the type of soil because not all soils cool at the same rate. Sand tends to contain more air than a heavy soil like clay for example, it has a higher air-filled porosity, so as the air cools, so does the sand around it and at a much faster rate than a heavy soil like clay. So high content sand rootzones will cool faster than heavy soil ones and they'll heat up quicker too.

As we approach sunrise (at 7:15 am) we can see that the air temperature at 2m drops slightly before the rays of the sun warm the air and it begins to increase in temperature.

Look at the trace for ground height and you can see that even though the sun has risen, the temperature keeps dropping and this is because the earth continues to lose heat as radiation to the atmosphere for a period after sunrise and until the suns rays begin to warm up the earth. In the graph above, the sun rose at 7:15 am but the ground height temperature continued to drop for another hour. So it took an hour for the sun to begin to warm the earth directly beneath our temperature sensor.

It is ground height temperature (along with a whole bunch of other factors) that cause dew and frost formation. So if conditions suit this is why you'll see dew formation after sunrise because the air temperature continues to drops and may reach the point where water vapour condenses onto the plant leaf. This point is called the Dewpoint temperature. If it is colder, this moisture will then freeze.

Interestingly, if we look at the graph at midday, the opposite is true in that the temperature at ground height is higher than the temperature at 2m height. To be accurate, the temperature at ground height at midday was 3.1°C higher. This was likely due to the effect of the wind cooling the air more significantly at 2m height vs. ground height where you have more of a sheltering effect.

As the sunset, the temperatures began to drop, but the ground height temperature fell faster and so we go back to the same pattern again.

The dynamics of air temperature at different heights and across different soil types make for our lovely early morning mists (the above is early morning mist across the dam at Hollowell Reservoir) and often you'll see this same process giving rise to mist over a river valley or particularly cold area of land. Hopefully now I've explained the process to you. It is actually more complicated than my explanation in that there are lots of factors involved but the basic principles hold true.

OK that's all for this week.

All the best.

Mark Hunt

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