When you press or push something, the force you apply is
called pressure. Pressure is measured as the force you use divided by the area
over which you use it. If you use a bigger force, or if you use the same force
over a smaller area, you increase the pressure. We experience AIR PRESSURE all the time because of the weight of air pressing in on our
bodies. WATER PRESSURE increases as you go deeper in the
ocean.
The gases in Earth’s atmosphere are made up of
tiny molecules that are constantly crashing into your body and trying to press
it inwards. This pressing force is called air pressure. It is greatest at
ground level where there are most air molecules. At greater heights above
Earth, there are fewer air molecules and the air pressure is much less. It is
possible to compress (squeeze) air, and this is used to inflate vehicle tyres
and to power machines such as pneumatic drills.
Heavy construction machines have large tyres for two reasons. The
compressed air in the tyre helps to absorb bumps, so the ride is much smoother
than it would be with a solid wheel. Large tyres also help to spread the weight
of the machine over a much bigger area. This reduces the pressure on the ground
and stops the machine sinking into the mud.
Water behaves differently from air when it is under
pressure. It cannot be compressed (squeezed). This makes it useful for
transmitting force in machines, using a system called hydraulics. Water is also
heavier than air, and an increase in water pressure affects humans more than a
drop in air pressure. Even with a snorkel or other breathing apparatus, it
feels much harder to breathe underwater. The water above you presses down from
all sides on your body, so your lungs find it harder to expand to take in air.
The deeper you go, the more water there is above you and the greater the
pressure on your body.
CHANGING AIR AND WATER PRESSURE
The higher we go, the less air there is in the atmosphere above
us. The deeper in the sea we go, the more water there is pressing down on
us.
20,000 m (65,600 ft) HIGH
At this height, air pressure is less than one-tenth that at sea
level.
AIRLINERS 11,000 m (36,000 ft)
Aircraft cabins are pressurized to allow us to breathe as easily
as at sea level. Oxygen is also supplied in case of emergency, as there is less
air at this height.
MOUNTAIN TOPS 7,500 m (24,600 ft)
At this height, climbers often use breathing apparatus to give
them more oxygen.
The human body is ideally adapted to deal with the air pressure
at sea level.
Divers cannot go any deeper than this without special suits to
protect them from the pressure of the water.
SUBMERSIBLES 6,500 m (21,300 ft)
Underwater craft such as submarines have strong, double-skinned
hulls to withstand water pressure. The world’s deepest-diving crewed
submersible can dive to 6,500 m (21,300 ft).
10,000 m (32,800 ft) DEEP
At this depth, the pressure of water is 1,000 times greater than
it is at sea level.
