Question

Weather forecasters use a barometer to measure

Answer 1

Answer:  air pressure

Whenever you hear things like "high pressure" or "low pressure" on weather reports, they measured the pressure using a barometer. Knowing the air pressure helps determine if there is a higher chance of rain or storms for instance.

Typically but not always, high pressure correlates to sunny days and dry weather. Low pressure often brings about rain. This is of course a very oversimplified viewpoint.