Rainy days can be great for drawing, if the rain is not too heavy. This little study of an apple tree, trunk and and a big branch, was drawn while sitting under an umbrella. The one I have has a little rope at the top which I use to hang it into low branches of a neighbour tree.
Blind contour drawing helped a lot to depict some of the many twigs, for the small openings to the sky in between I concentrated on the negative forms.
The drawing was done with two different pens, broader and thinner nib. The final layer of grey was done with a light grey marker pen.