The principal of a hot wire anemometer is based on a heated element from which heat is extracted by the colder impact airflow. The temperature of the hot wire is kept constant via a regulating switch, and the current (amp draw measured internally) is directly proportional to the air velocity.
When using a hot wire in turbulent air streams the measured results can be impacted by turbulent airflow striking the measurement sensor from multiple directions. This could indicate a higher measured value than a vane probe. This characteristic is typically prevalent in ducts where turbulent airflow can occur even at very low velocities.