But there's no horizontal gravity force to counterbalance them. So the horizontal pressure gradient force generally forces the air to move, producing wind. Horizontal pressure gradients are typically the only force mentioned on the nightly weather report. Forecasters are always talking about the positions of high and low pressure systems. Between the highs and the lows, there are horizontal pressure gradients and winds.
Friction is one force that only exists if air is in motion.
Although there can be frictional forces within the upper atmosphere, they are usually very weak. The only place friction becomes a significant force for the atmosphere is near the ground. There, objects as small as blades of grass or waves in the ocean, or as large as trees or buildings, slow down the air. The coriolis force is the final atmospheric force of fundamental importance. Like the centrifugal force, it is caused by the rotation of the Earth.