As tee cee says, it could be the different approach rain induces that has something to do with it, sitting under a brolly listening to the rain does sort of induce a hypnotic state of mind, where the whole world is shut out and it is a nice slowed down, calming state of mind; and you are less likely to be fiddling about, reeling in every five mins, changing the bait etc. Well, that's me more often than not. Rain does sort of make you tune in more to what your doing and looking at.
I remember well, sitting with Dad on uncomfortable stools, in his gaberdine raincoat and his cloth cap with the rain pouring of it all day. his pipe would just stick out enough to stay alight. Tougher breed however, he did end up with arthritis.
Respect Pete, standing, wading and trotting rain or sunshine, you learned your trade from a few masters I suspect and it pays off. We could all take a lesson or two from you.
Of course a lot depends on the type of rain, where and when and the time of year but, I will still say in general it has a good effect.
As to air pressure, I could produce similar comparisons with my rain figures. Low is best, below 1000m pressure is extremely good generally and 1030m+, a bit deadly but, again its a matter of degree on the species, and other factors might outweigh the high pressure and fish will still feed.
However, low usually denotes rain, storms, overcast skies; is it this or the actual low pressure that has the desired effect. I have never made up my mind on that one. But, reading that sea trout and salmon can shoal up and prepare to run up a river when a low pressure is on its way but, not actually there, makes me suspect that at least these fish can detect actual pressure, even when it is only on its way, could be sixth sense that we do not understand and ordinary coarse fish could have that as well. Who knows and some mysteries should remain so.