There’s smoke emanating from one of the reactors in Japan. The smoke will show which way the wind is blowing and which way the radioisotopes are heading, so they won’t need a weather vane to know which way the wind blows.

There’s fire too, probably, which does not augur well for getting things under control there.

It’s useful to bear in mind that there still can be a lot of radioisotopes coming out even if there’s no smoke. But smoke can be of particular concern because the many small particles that make up smoke can give the radioisotopes something to attach to, helping them ride up into the atmosphere, and later down onto land, water, or into people’s lungs.