Halliday & Matthiessen (1999: 354):
In weather forecasting, while the weather itself is not constituted of language, the activity of forecasting is; the entire situation is built up out of black marks on large white pieces of paper towards which is directed the attention of some human consciousness. Out of this is construed a particular context or situation–type; since the semantics is specific to that situation, the reader knows where he is, construes the situation in a particular instantial form (‘oh — so it’s going to rain this afternoon’), and perhaps varies his actions in accordance with this construction.