The ability to predict incoming weather clearly intrigued civilization before modern technology arrived, with many myths relating to weather forecasting. One idea is that seaweed could be used to predict imminent rainfall.

The workings are basic to say the least. Hang a piece of dried seaweed outside and keep an eye on its moisture level. If it remains dry, the weather will be also be dry. If the seaweed becomes wet and more flexible it is a sign of rain.

Interestingly enough this can work!

Seaweed can re-hydrate itself repeatedly without damages. It also has the ability to absorb atmospheric humidity from the air. Moisture in the air is an indicator of rain, therefore if the dried seaweed appears wet it could be a sign of rain.

It is not perfect by any means since humidity also increases in clear weather. But as a simple way to predict rain, it can work!