atmos.

atmos., atmosphere pressure The characteristic level of atmospheric pressure at Earth's surface, most particularly the standard atmosphere, now taken to be 101.325 kPa. The distinct technical atmosphere = 1 kgf·cm² = 98.0665 kPa, the figure being from the original standard gravity.

Atmospheric pressure declines with increasing altitude (and ever more rapidly) because of the thinning of the atmosphere and the declining gravitational acceleration. The decline is significant even over the altitude range of major human habitation, exceeding 10% at 1 000 m (3 200.∼ ft) – below the altitude of Denver, Mexico City, Nairobi, and many other cities. Weather maps and reports give values adjusted to implied sea level values (and aneroid barometers must be adjusted accordingly to the site).

While the specific pressure at any altitude depends on current atmospheric conditions, just as it does at ground level, the routine progressive change with altitude allows for measurement of altitude by measurement of pressure. Long advantageous in mountain climbing, this is of great relevance now in flying; the standard settings for aircraft altimeters are shown in Table 3.

Table 3

Altitude	Pressure
m	ft	kPa	mm of Hg	p.s.i.
The negative figure applies to the Red Sea zone, etc.
−305	−1 000	105.040	787.86	15.235
0	0	101.325	760.00	14.696
305	1 000	97.716	732.93	14.173
610	2 000	94.213	706.66	13.664
914	3 000	90.812	681.15	13.171
1 219	4 000	87.910	659.38	12.750
1 524	5 000	84.307	632.25	12.228
3 048	10 000	69.681	522.65	10.106
6 096	20 000	46.563	349.25	6.753
9 144	30 000	30.089	225.69	4.364
12 192	40 000	18.754	140.67	2.720
15 240	50 000	11.598	86.99	1.682

See also head of liquid.