One of the easier tools to use in Photoshop is the Polygonal Lasso, as it allows you to create a region with straight sides. This is what happens, though, when a billowing ash cloud is realized in a graphic via the Polygonal tool. Oh and angularity of the ash cloud! (from http://news.bbc.co.uk/2/hi/uk_news/8628605.stm)
UPDATE: This article out on ash-cloud datagraphics.