Generally air is considered as not having weight. Any substantial amount of air
is so light that the weight is normally not considered. A column of air measuring
one metre by one metre across (1 square metre of area), and extending from
the earth’s surface at sea level to the extreme of the atmosphere, would actually
have a significant weight. This weight, on an average day is approximately
10,000kg, as illustrated in Figure 16 (left) or 14.7 lbs/in2 in the imperial system
(right). Therefore the pressure that continuously exists at sea level due to
the weight of the air above is 100,000Pa. This is referred to as a standard
atmosphere, or the atmospheric pressure on a typical day at sea level which is
also known as 1 bar or 1000 millibars.