It seems my AD wife is somewhat morose and disagreeable on gloomy, dark and cloudy days, and more upbeat and happy on bright and sunny days!

There are full spectrum lights that can help
Changing regular light bulbs to LED bulbs can help
A lot of people experience "winter doldrums" due to the decrease in sunshine.

As are we ALL apparently and statistically.

It's not just folks with dementia that are affected by the weather OldArkie. Lots of people, especially those that suffer from depression are in the same boat.
You could try light therapy boxes that are used for those who suffer from seasonal affective disorder(SAD)as they have been proven to be quite effective.