What causes the horizontal variation of atmospheric pressure?

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Atmospheric pressure is the pressure created by the combined weight of gasses in the atmosphere on the surface of this earth.
Air pressure=density*temperature*Constant
So,horizontal variation of atmospheric pressure is related to the density & temperature while the constant being an independent variable(i.e.,doesn’t get affected by any changes in other variables of the equation) is neglected.Since,within the Atmosphere the volume of air can change,then temperature variations can affect The air density.As air is compressible hence the pressure decreases rapidly with height decreases since the pull of gravity compresses the air & this is maximum near the Earth's surface.when air is heated it expands resulting in spreading in outward pressure of molecules over a large area due to heat energy provided by radiation.Thus,air pressure decreases & on the contrary ,cooling induces contraction of molecules of air resulting in density increase which in turn results in increased atmospheric pressure.This is because due to contraction the surface inspite of compression offered by gravity.Atmospheric pressure again varies over ocean & land surface.this variation is due to variation in Albedo difference between water & land,difference in specific heat capacity of water & land,difference in thermal conductivity between land & water,difference in transparency/evaporation between the two.the Specific heat capacity of water results in low pressure in winter & high pressure in summer over sea & oceans.This results in high pressure in higher Latitudes & vice versa.

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