Atmospheric pressure
reflects the average density and thus the weight of the column of
air above a given level, thus the pressure at a point on the earth's
surface must be greater than the pressure at any height above it. An
increase in surface pressure denotes an increase in mass, not
thickness, of the column of air above the surface point. Similarly a
decrease in surface pressure denotes a decrease in the mass. The
**gradient** is the difference in pressure vertically, and
horizontally.

The air throughout the column is compressed by
the weight of the atmosphere above it and thus the density of a
column of air is greatest at the surface and decreases exponentially
with altitude as shown in the following graph which is a plot of the
rate of decrease in density with increase in altitude. The plot is
for dry air at mid-latitudes. *( Mid latitudes are usually
accepted to be the areas between the 30° and 60° lines while low
latitudes lie between the equator and 30° and high latitudes between
60° and the pole.)* The atmosphere at about 22 000 feet has only
50% of the sea level density, density decreases by about 3% per 1000
feet between sea level and 18 500 feet and thereafter the **density
lapse rate** slows.

The dry air density gradient in mid-latitudes, refer
table in 2.1

As the pressure
decreases with height so, in any parcel of air, the downwards
pressure over the top of the parcel must be less than the upwards
pressure under the bottom: thus within the parcel there is a
**vertical component of the pressure gradient force** acting
upward; generally this force is balanced by the gravitational force
so the net sum of forces is zero and the parcel floats in
equilibrium. This balance of forces is called the **hydrostatic
balance**. When the two do not quite balance the difference is the
**buoyancy force** which is the upward, or downward, force
exerted on a parcel of air arising from the density difference
between the parcel and the surrounding air.

Atmospheric
pressure also varies horizontally, due to air mass changes
associated with the regional thickness of the atmospheric layer. The
resultant **horizontal pressure gradient force**, not being
balanced by gravity, forces air to move from regions of higher
pressure towards regions of lower pressure but the movement is
modified by Coriolis effect. The horizontal force is about 1/15 000
of the vertical component.

*(***Advection** is the term used for the
transport of momentum, heat, moisture, vorticity or other
atmospheric properties, by the horizontal movement of
air)

The following graph
plots the average mid-latitude vertical pressure gradient and shows
how the overall vertical decrease in pressure – the **pressure
lapse rate** – slows exponentially as the air becomes less dense
with height. In a denser, or colder, air mass the pressure reduces
at a faster rate, conversely in less dense, or warmer, air the
pressure reduces at a slower rate. *(The ***hydrostatic
equation** states that the vertical change in pressure, between
two levels in any column of air, is equal to the weight, per unit
area, of the air in the column.) If two air columns have the
same pressure change from top to bottom the denser column will be
shorter, conversely if the two columns have the same height the
denser column will have a larger change in pressure from top to
bottom.

In the ICAO standard
atmosphere, details of which are shown in section 2.1, the rate of
altitude change for each 1 hPa change in pressure is
approximately:

zero to 5000
feet: |
30 feet/hPa or 34 hPa per
1000 feet |

5000 to 10 000
feet: |
34 feet/hPa or 29 hPa per
1000 feet |

10 000 to 20
000 feet: |
43 feet/hPa or 23 hPa per
1000 feet |

20 000 to 40
000 feet: |
72 feet/hPa or 14 hPa per
1000 feet |

The change in altitude
for one hectopascal change in pressure can be roughly calculated
from the absolute temperature and the pressure at the level using
the equation: = 96T/P
feet.