  Word clouds are a popular tool for visualizing documents, but they are not a
good tool for comparing documents, because identical words are not presented
consistently across different clouds. We introduce the concept of word storms,
a visualization tool for analysing corpora of documents. A word storm is a
group of word clouds, in which each cloud represents a single document,
juxtaposed to allow the viewer to compare and contrast the documents. We
present a novel algorithm that creates a coordinated word storm, in which words
that appear in multiple documents are placed in the same location, using the
same color and orientation, in all of the corresponding clouds. In this way,
similar documents are represented by similar-looking word clouds, making them
easier to compare and contrast visually. We evaluate the algorithm in two ways:
first, an automatic evaluation based on document classification; and second, a
user study. The results confirm that unlike standard word clouds, a coordinated
word storm better allows for visual comparison of documents.
