When gas and liquid flow together up a vertical tube, several flow patterns are possible. An isolated bubble will rise, relative to the liquid, with its terminal velocity. As the bubble concentration increases, the rise velocity decreases due to interference between the bubbles, as shown by Nicklin. At higher bubble concentrations coalescence can produce larger bubbles with higher rise velocities and, more significantly, convection cells can be formed which enable some of the bubbles to rise extremely fast. On the small scale the situation is complicated, with some bubbles rising rapidly and others, possibly, being carried downward. On the large scale this can be averaged out to give a mean gas velocity, which is the superficial gas velocity divided by the mean volume fraction of gas, which can then be correlated with the superficial gas and liquid velocities.