Real time monitoring of environmental and social conditions is an important part of developing early warning of natural hazards like epidemics and floods. Rather than relying on dedicated infrastructure, such as sensor networks, it is possible to gather valuable information by monitoring public communications from people on the ground. A rich source of raw data is provided by social media-blogs, Twitter, or Facebook. Two experiments based on the use of Twitter content in the United Kingdom show it is possible to detect a flu epidemic and to assess rainfall levels by analysing text data. These measurements can in turn be used as inputs of more complex systems, for example, for the prediction of floods or disease propagation.