The atmospheric pressure at any place is dependent upon the number of molecules present in the column of air above that place. The air acquires its temperature from the surface below so air over a warm surface starts to expand after getting heated up. As a result some air molecules move out of the column to some other column where air is sinking due to low surface temperatures. Hence density reduces in warm atmosphere and increases in cold atmosphere resulting into pressure decrease at one place and increase at another respectively. That is the reason that at Miami QNH (Mean Sea Level Pressure) reduces to as low as 980 mb in summers and as high as 1024 mb in winters. Another point to remember is that in troposphere, atmosphere often has layers of cold and warm air one above another. So a column with more cold air is likely to have more surface pressure as compared to a column having more warm air. So a changing surface pressure is a result of:-

(a) Air acquiring different temperatures from the surface below
(b) Air above being replaced by another air having different density