There is no such thing as bad weather, only different kinds of good weather. - John Ruskin

An altimeter or barometer can be used to measure pressure in the atmosphere

Air pressure changes measured by your altimeter can be either the result of weather changes or altitude changes. When you are at a constant elevation is easy to realize that the changes in pressure are related to the changes in the weather conditions. When your altitude is changing it is a little more difficult to isolate the pressure changes due to weather conditions, but it can be done if you have known altitudes to work with. In any case, these pressure changes due to changing weather conditions can be used to forecast the weather for the coming hours and days.

In it's crudest sense, falling pressure typically leads bad weather, and rising pressure indicates improving weather. And remember, if you are looking at an altimeter, falling pressure is equal to an increase in altitude. So if your altimeter says your altitude is going up, and you know you're not, bad weather is approaching.