There are pros and cons to owning a convertible car model. You get to enjoy the beautiful sunshine as a pro, and as a con, you don’t get the same type of protection from the elements that you do in an ordinary car. The “Openometer” is a feature that some convertible models have, where the roof will open when there is a certain temperature outside (via Are You Selling).
The openometer isn’t a feature that has caught on with many consumers, which might be why you’ve never seen it. If you think about it, not every driver will want their convertible top to open automatically. In areas where the temperatures are humid, the roof could accidentally open during a summer rainstorm.