What is the ventilation index (VI)?
The VI is the National Weather Service's indicator of the relative degree of air circulation for a specified area and time period. Basically it is a measurement of the air's ability to "clean" itself. The afternoon VI is predicted by analyzing information acquired from a weather balloon sent up early each morning from the National Weather Service in Medford (located by the airport). VI of 0-199 = poor ventilation, 200-400 = fair, 401-599 = good, 600+ = very good.