The 3 main factors that affect barometric (air) pressure are:

  • Temperature of air.
  • Altitude or Elevation.
  • Moisture.

Temperature of air



Temperature of air has an inverse relationship to air pressure.

Warm air = lighter and rises = less density = lower barometric pressure.
Cold air = weighs more and sinks = higher density = higher barometric pressure.

However, the relationship between local temperature and barometric pressure is more complicated, whereby the change in air pressure in a given location may be affected by a moving weather system rather than the change in local temperature. For example:

City A
Air temperature  falls from 30C to 20C

Apparent Answer: Temperature falls and  barometer shows barometric pressure rise.
Actual answer: Air pressure rise was actually due to an atmospheric region of high pressure passing over the location that brought in cooler temperatures and not directly due to temperature change.

City B
Air temperature rises from 20C to 30C

Apparent Answer: Temperature rises and  barometer shows barometric pressure fall.
Actual answer: Air pressure fall was actually due to a storm with an atmospheric region of low pressure passing over the location and not directly due to temperature change.

Altitude or Elevation



Altitude has an inverse relationship to air pressure.

Higher altitude = less air pressing down = lower barometric pressure.
Lower altitude =  more air pressing down = higher barometric pressure.

Moisture



Air moisture has an inverse relationship to air pressure.

Even though it seems counterintuitive to many people, actually, dry air has a higher density than moist air. Therefore,

Moist air = reduced air density = lower barometric pressure.
Dry air = increased air density = higher barometric pressure.

How does this relate to real world examples?

Warm air and associated high moisture = less air density   = lower barometric pressure   (such as produced at the equator)
Cold air and associated low moisture =  higher air density = higher barometric pressure  (such as produced at the polar regions)