Ascent processes of small volcanic clouds are analyzed using video records obtained for some eruptive events at Asama and Miyakejima volcanoes, Japan. In this analysis, each segment of the volcanic cloud is treated as an isolated thermal, i.e. a spherical body ascending independently by its buoyancy. Our kinematic analysis reveals simple relationships among the ascent velocity, radius and height of volcanic clouds. The applicability of the entrainment hypothesis and the relation that was obtained from a dimensional analysis by Scorer is examined for natural volcanic clouds. The empirical dimensionless constants characterizing the entrainment hypothesis and the Scorer's relation are more or less scattered and their mean values are about 0.36 and 0.6, respectively. These empirical relations can be used to evaluate kinematic features of volcanic clouds approximately without thermodynamic consideration.