Honolulu, Hawaii has the least amount of fluctuation in barometric pressure changes in the United States. Although places like Denver, CO may consistently have low pressure, they may also have a greater range of pressure changes. Honolulu’s pressure changes range from 29.34 to 30.32 inches in mercury (993.56 to 1026.75 millibars). Barometric pressure, also known as air pressure, measures the weight of the air and is affected by many factors. The planet’s gravity consistently weighs down the air, but the temperature, altitude, weather and air density all contribute to pressure changes. Air pressure decreases as altitude increases, due to less dense air and less mass pushing down from above.