Parameter Tree:
EARTH SCIENCE > ATMOSPHERE > ATMOSPHERIC WINDS > WIND DYNAMICS > CONVERGENCE

Definition:
Pressure fluctuations at the Earth's surface are produced by movements of air which produce changes in the mass of air in a vertical column above the ground. A net gain in mass produces an increase in pressure leading to a high pressure system. A net loss of mass gives rise to a low. When there is a horizontal flow of air into a region (convergence) a vertical upward movement of air occurs otherwise there would be a continual increase in density of the air. Divergence occurs when there is a net horizontal outward flow of air from a region. This leads to a vertical downward movement of air.

Reference:
University of Sydney, School of Physics