A new measure of mixedness, based on entropy considerations, and a rel
ated mixing rate are introduced. It is argued that the time rate of ch
ange of the mixedness is proportional to the mixing rate. The new meas
ures are applied to experimental observations on an axisymmetric jet a
nd an array of jets in crossflow. The concentration field of an axisym
metric jet is measured at 10 downstream locations by optical imaging o
f Rayleigh scattering from a laser sheet. Mixedness and mixing rate ar
e calculated for each of the locations. In agreement with theory, the
mixedness of the self-similar jet is constant along the length of the
jet. The measures allow us to locate the instantaneous realization tha
t is most typical in terms of mixedness and mixing rate. Also analyzed
is the mixing of a row of jets injecting fluid into a crossflow, The
jets are sufficiently close that there is significant interaction betw
een neighboring jets. The mixedness and mixing rate were calculated. W
ell-mixed regions have a low mixing rate, whereas poorly mixed regions
tend to mix most rapidly. The mixedness and mixing rate allow the ass
essment of the effectiveness of different orifice shapes in promoting
rapid mixing.