Skying?

A noun become a verb? What is "skying"? Been reading about and looking at lots of John Constable recently.  From John Constable by John Walker published by Abrams, New York in 1978, comes this information:

In 1821 Constable began to work seriously at what he called "skying," making studies of cloud effects. To inform himself of the appearance of the sky under various weather conditions he determined to put together his own observations of atmospheric phenomena and to record these in quick sketches.

This charcoal study from last night was facing east with a fairly uniform cloud cover. The variation is shade was strongly influenced by ambient light from street lights beyond the range of the image. So much fun to get my fingers dirty with vine and hard charcoal!