The term "en plein air" means painting outside. In fact it directly translates to "in open air" as you might have guessed. We do this in the art world - we hang onto terms that make what we do seem elevated and esoteric.
Artists sketching in the White Mountains by Winslow Homer 1868
But it seems that we don't do that very well either. Plein air was a term originally used to differentiate the techniques that painting straight from nature requires against the purely studio approach. Almost straight away the rules were stretched to a point where factions argued about what percentage of the painting should be completed outside to qualify to be called "plein air".
A bewildering number of ways to look at what these three words mean is amazing. You may want to read more here
Persistence of Memory - Salvidor Dali 1931
To work en plein air you need daylight - and we stopped saving it on 7th April. Of course we are not really "saving it" , we are adjusting the label that we give to certain time so that we trick ourselves into being up when the sun is.
The system was first used in Canada (Thunder Bay) in 1908 and briefly caught on in Europe at the height of WWI. It was invented by one of us though. George Vernon Hudson presented a paper in 1895 suggesting a two hour shift every summer.
The idea didn't catch on straight away and, in fact, was ridiculed. In 1927 we begrudgingly moved the clocks 1 hour but that was later changed to 30 minutes.
During the WWII we set the clocks forward 30 mins so that we were exactly 12 hours ahead of GMT. That change was made permanent. We then put in place The Time Act (such a cool name) in 1974 and have been toying with the duration ever since. We seem to have settled on last Sunday in September until 1st Sunday in April.
So during that time you have one more hour available for plein air painting. Next year you should do it.