Forecasting weather

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I bought this cheap weather station keyring the other day. It displays temperature and humidity, but doesn't display barometric pressure. And yet it has a forecast feature - sunny, cloudy and raining. On the day I bought it we had rain but the stupid thing kept showing partly cloudy. But the last few days it's been quite accurate. We've had rainy days and sunny days and the forecast has reflected this.

But what I want to know is if it's actually possible to forecast weather (as in raining, sunny etc) just from temperature and humidity. I don't think so from what I know of weather forecasting. If not then I suspect it has an inbuilt barometric pressure sensor for forecasting but just doesn't have the ability to display it. Do you think that's right?

Your weather station is probably not secretly monitoring barometric pressure. The chance of precipitation increases with relative humidity, so it's possible to roughly forecast the weather based solely on that metric.

Yes but we've had rainy days when the humidity has been low - or rather the indoor humidity was low, and also partly cloudy days when the humidity was high. And it still was giving a reasonable forecast. Might pull the back off and have a look.