The wind-measuring instrument Leonardo da Vinci improved was an anemometer.
An anemometer measures wind speed or, in some designs, wind direction. Leonardo’s version used a plate that moved in response to wind, with a scale indicating the strength of the airflow. He developed the instrument in connection with his investigations of flight and the behavior of air.
Leonardo did not invent every form of anemometer. Earlier wind-measuring devices existed, and later cup and vane designs became more practical for meteorology. His contribution was an important attempt to make airflow measurable rather than merely observable.
The instrument is sometimes confused with a barometer. A barometer measures atmospheric pressure, while an anemometer measures wind. Leonardo’s broader studies of air included observations of birds, currents, resistance, and the mechanics needed for flying machines.