Frictional Convergence


Low level convergence will lead to rising air. This is because the air piles together and has no place to go except up. The most dramatic examples of low level convergence occur along fronts and low pressure systems but more subtle convergence can occur within an air mass that can also lead to a burst point for convection.

Subtle convergence can occur when air has to flow into topographic barriers, over land with different friction characteristics, and at different velocities. When air flows into a barrier such as a large hill, some of the air will deflect around the barrier while some will go over it. This will lead to convergence as the air moves into the topographic barrier. Air just above the surface will flow faster since there is less friction but this air will interact and slow down as it interacts with higher elevations. Also, air will flow at different velocities over different surfaces. For example, air will flow faster over a water surface than over a land surface with trees. Air will flow faster over a treeless field than over a forest. When friction is weaker such as over water or a treeless field, the wind will flow faster. When this faster moving wind flows into a rougher surface it will cause convergence to occur. Convergence caused by topography and land cover can ignite a thunderstorm if only subtle convergence is needed to get a storm going.