I found that conte' soft pastel worked well for getting down a quick impression, especially when a subtractive technique ( placing a dark ground and erasing away for lighter tones) was used. Oil pastel was very difficult to use as it wasn't as easy to rapidly sketch with in situ because it drags on the paper and relies on being built up in layers.
For research I looked at the work of Constable, Kurt Jackson and Alexis Rockman and i put some examples in their work in my sketchbook. In particular the drippy gouache rainy sky is inspired by Alexis Rockman's work.
My husband has alway been keen on photographing cloud formations and weather effects so I was lucky to have some interesting reference photos on my computer. Using these for reference I was able to have more of an experimental approach (because I wasn't sketching a moving target).
I tried spraying ink onto the page and then working over it with oil pastel. I was tempted to stick cotton wool onto the page like a childhood collage. Instead I chose some handmade paper. I was able to tear this to leave fibres trailing at the edges. I thought this would be good for representing the wispiness off the ages of the clouds. I painted on this with gouache and then used oil pastel on the undersides for the shadowing. This was quite effective (below)
I chose some darker tissue paper for a sunset and worked into this with marker pen, gouache, soft pastel and oil pastel giving quite a dramatic result (above). Clearly this way of working would be difficult to achieve when working in plain air though.
I thoroughly enjoyed this exercise because I was able to be free and experiment without any pressure to produce a finished piece of work.