Halliday (2008: 80-1):
Like all analogies, that between climate and language is oblique and partial. It is an abstract tool for thinking with, not in any sense a strict proportion. Semiotic systems are not like physical systems. For one thing, an instance of a semiotic system carries value. Instances of physical systems do not. You may prefer one kind of weather to another, but that has no relevance whatsoever to the significance of an instance of that kind of weather to another in relation to the theory of climate. Such an instance is simply to be observed, measured and taken account of like any other. But a text has its own differential value. The value is not something fixed and determinate, in most cases; if it is a recorded text, its value may vary at different times, in different places, or under different conditions.