Mark Hunt weather corner; 121 - Understanding jet streams
Spring 2025 vs. 2024 couldn’t have been more different from a turfgrass maintenance perspective. The former, wet, cold and dull, the latter, bright, warm and very dry.
Why the big difference?
Well, it is all down to the position, speed and shape of the jet stream.
Last year, the jet stream flowed strongly and sat south of the U.K. That combination allowed low pressure system after low pressure system to push in from The Atlantic and provide one of our wettest, coolest, dullest springs ever.
This year, we have seen a slow-moving jet stream, which split into two and formed into a so-called ‘Diffluent blocking event’, pushing rain north and south of the U.K & Ireland and providing us with a very dry scenario from late March.
With the wind funnelling between the ‘stuck’ high and low pressure systems, the dominant direction was easterly/north easterly. Winds from the east tend to be drier and this year we also received much higher light levels to boot. That makes sense because more unsettled weather means more cloud and therefore less direct sunlight. The flipside of this coin is that brighter, clearer days means clearer night skies and consequently lower air temperatures and more frosts.
Data from our Davis weather stations showed 25% more plant-available radiation in Spring 2025 vs. 2024, but with clearer skies, average overnight temperatures were 2°C lower. Although it was cooler at night, the main issue with growth this spring was lack of moisture, particularly on un-irrigated outfields.
Let’s look at the two spring seasons from a rainfall and evapotranspiration perspective using data from our default Thame, Oxfordshire location.
Just look at the difference between the two years when it comes to the soil moisture deficit/surplus. In 2024, at the end of May, we were +30mm vs. -189mm at the same point in 2025.
Agronomically, we couldn’t have two more different spring seasons for us to manage turfgrass growth, yet the expectations are the same.
It graphically highlights the need for both efficient drainage (2024) and efficient irrigation (2025) as we strive to meet the challenge of our variable climate. This spring has also put water and its use in our industry in the spotlight, with a significant requirement for irrigation before we have even started meteorological summer. Quite where we will be in 3 months’ time is anyone's guess and entirely down to the behaviour of our unpredictable jet stream.
www.weatherstations.co.uk
Send your questions for Mark to: editor@pitchcare.com