I tried my hand at plein air painting at an afternoon workshop a few weekends ago.
En plein air is a French expression which means “in the open air”, and is particularly used to describe the act of painting outdoors. — Wikipedia
The challenge for me in painting plein air is that I must paint quickly. Because the sun is always moving, the light and shadow shapes are always changing. Unless you’re actually documenting it, you don’t even realize how fast it really moves. In just an hour, the whole perspective can change. On this one, I had to lay down shadows quickly before they moved.
It was fun working outside, and I’ll have to try more! Though winter is arriving (we had flurries today with more coming this weekend), so I may choose to remain warm inside. But neither rain nor snow nor cold front can stop the Colorado Rockies in the National League baseball playoffs! It’s Rocktober!!!! Go Rockies!!!
Front Steps on Forest Parkway
10″ x 8″
oil on panel
ArtStrokes -- Tracy Wall's Art Blog 
You have to make painting when the sun is at its top, like in afternoon. at that time sun is quite stable.