Air Pressure• The weight of air.

• Measured with a barometer in millibars or inches of mercury.

• Decreases with altitude (less air).

• CAN CHANGE IF DENSITY CHANGES!

Air’s density changes two ways

heat

HOT IS LESS DENSE

REMEMBER…

COOL atmosphereWARMER atmosphere

What happens to the air

pressure on my head as

the temperature increases?

HOT atmosphere

1. Heat makes molecules expand. Hot air is less dense.

Heat = Air Pressure

Which coffee can

barometer is in the cold

air?

21o C - 4o C

Air pressure is usually lower when air

(1) gets colder(2) gets warmer(3) increases in density(4) weighs more

REMEMBER…different molecules have different densities.

2.7 g/cm3 8.9 g/cm3

Different air molecules ALSO have different densities.

Atomic Mass tells usHow dense the atom is.

14 amu + 14 amu =28 amu

16 amu + 16 amu =32 amu

1 amu + 1 amu + 16 amu =

18 amu

Water vapor is less dense than Oxygen

or Nitrogen.

2. Water vapor (H2O) decreases the atmosphere’s density.

Moisture = Air Pressure

Dry Air (mostly N2 &

O2)

Moist Air (more H2O less N2 or

O2)

Which

weighs

more?

Which is less dense?

• Air pressure increases as air becomes colder and drier.

• Air pressure decreases as air becomes warmer and moister.

Air pressure is usually lowest when the air is

(1) warm and dry(2) warm and humid(3) cool and dry(4) cool and humid

Air pressure is usually highest when the air is

(1) cool and humid(2) cool and dry(3) warm and humid(4) warm and dry