Large amounts of information are posted daily on the Web, such as articles published online by traditional news agencies or blog posts referring to and commenting on various events. Although the users sometimes rely on a small set of trusted sources from which to get their information, they often also want to get a wider overview and glimpse of what is being reported and discussed in the news and the blogosphere. In this paper, we present an approach for supporting this discovery and exploration process by exploiting term clouds. In particular, we provide an efficient method for dynamically computing the most frequently appearing terms in the posts of monitored online sources, for time intervals specified at query time, without the need to archive the actual published content. An experimental evaluation on a large-scale real-world set of blogs demonstrates the accuracy and the efficiency of the proposed method in terms of computational time and memory requirements.