3) How does air pressure change?

Millibar values used in meteorology range from about 100 to 1050. At sea level, standard air pressure in millibars is 1013.2. Weather maps showing the pressure at the surface are drawn using millibars ... This change in pressure is caused by changes in air density, and air density is related to temperature.

Air pressure decreases as you move higher in the atmosphere. Think of a column of air directly over your body ... Just as air pressure decreases with altitude, so does the density of air.