Rather than using the sky as a focal point, isn't Sisley using the sky as a foil to the surrounding images in his paintings?:
http://www.google.com/images?hl=en&source=imghp&q=alfred+sisley&gbv=2&aq=f&aqi=g10&aql=f&oq=&gs_rfai = http://www.timesrepublican.com/page/content.detail/id/526044.html
