There have been many recommendations for angling pre-emergence sprays in recent years, but I am not sure that the value of angling sprays is particularly obvious in this situation. A quick google showed quite a few UK sources recommending this and we still get a lot of questions.
This is the way I see it:
At midday, when the sun is overhead, shadows are short and the sun covers the most ground area.
At 7 pm when the sun is low, shadows are long, and there will be big areas of ground that are not covered.
The same is (sort of) true with angled nozzles. Anything that sticks up above the ground – stubble, clods – will cast a shadow and collect some spray, and the greater the spray angle, the more it will collect.
If you believe that putting more spray on a clod will give you better pre-emergence weed control, and some people do promote this idea, then feel free to angle the spray. But if you want to cover the ground around the clod, then a vertical spray will be best.
An AHDB-funded study in 2012 showed an increase in the shadow round a clod from angling nozzles. I found a couple of articles online – one from BASF from a few years ago and one current one from Syngenta relating to maize – suggesting exactly the opposite of this, but I am sure they are wrong.
The most vertical spray you can have is from a nozzle with a slight backward angle that compensates for the forward speed – this is what the Guardian Air nozzles were designed for, but the difference between these and ordinary vertical air-induction nozzles is not huge.
Remember that not all the spray from a nozzle is vertical anyway – probably around a third of the spray is at plus or minus 30 degrees and more, but sideways rather than forwards or backwards.
The only time when angling sprays really gives you a benefit, is when you want to put more on small vertical targets, such as blackgrass. So if you are targeting emerged grassweeds, there is good evidence that angling sprays can be helpful.
An interesting article from Syngenta Australia supports the use of vertical sprays particularly for no- and min-till situations where there is stubble. This is something where I am sure more research is needed in the UK – most of our available data is probably based on conventional cultivations, rather than min-till.