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The main driving force of motions in the atmosphere is the horizontal pressure gradient force. Horizontal variations of air pressure are much weaker than vertical pressure variations, only about one ten-thousandth as large. 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 work just like vertical pressure gradients: a stronger pressure on one side of an air parcel than the other causes a net force (push) in a particular direction.
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.
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