Swarms of particles, produced by direct dumping of particlesinto a quiescent body of water, are investigated experimentally. A series of laboratory experiments is conducted, varying size as well as amount of dumping particles. It is found that such main flow characteristics of particle clouds as half-width, mean buoyancy force and falling velocity can be adequately described by a single-phase Boussinesq thermal theory as long as particle clouds remain in thermal-like phase. A transitional condition from the thermal-like phase to the free settling-like phase of particle clouds is roughly estimated semi-theoretically. It is also shown that turbulent intensity along the main axis of the particle thermal is proportional to its falling velocity, and its ratio to falling velocity of a particle is almost independent of its size.