The vertical component of the velocity of the volatilized component in the observation space (in mm s-1). It depends on the flame temperature, the solute nebulized, the observation height and the gas flow rate.
Source:
PAC, 1986, 58, 1737. 'Quantities and units in clinical chemistry: Nebulizer and flame properties in flame emission and absorption spectrometry (Recommendations 1986)' on page 1742 (https://doi.org/10.1351/pac198658121737)