How is the top of a mountain
different from sea level?
•Temperatures are
colder
•Pressure
decreases
Why is the vegetation so different
in California, compared to
Nevada?
What separates California
from Nevada?
The Rocky Mountains
Why is California green and
lush, while Nevada is dry and
arid?
•Coastal cities
receive moisture
from the ocean.
Why does the air have to come
from the Pacific and not from the
western plains?
air
molecules
at sea level
air molecules
on the
mountain top
At sea level the molecules are closer together so they bump
into each other more – this causes friction. Friction causes
HEAT
Can air go through the mountain?
Since air cannot go through the mountain it
is forced to go up and over it. Since there
is less friction the air becomes cooler.
Cooling due to expansion.
As the air rises it cools eventually
reaching the dew point and forming
CLOUDS
The overall result is temperature
will
Decrease
Humidity
Increases
And it will
rain
Windward
side of the
mountain.
The air will expand,
cool to the dew point
temperature and
condense as long as
condensation nuclei
are present.
The
moist air
rises
• As the air mass travels
down over the Rockies,
it is now dry, since all of
the rain fell on the west
coast. The air molecules
are pushed closer
together as the
pressure increases
with the decrease
in altitude.
As the air sinks pressure will
Increase.
This forces the molecules closer
together again increasing friction.
Temperature will
Increase.
Warming due to compression.
Deserts are on the leeward side.
• This is known as the
OROGRAPHIC EFFECT
• Windward side is Wet
• Leeward side is Dry
Windward vs. Leeward
WINDWARD
LEEWARD
Orographic Effect –
Rain Shadow
•orographic effect in
action
The air mass moves in
towards the mountain
carrying with it
evaporated water vapor.
The slope of the
mountain forces the air
mass to move up into
an area with less air so
the molecules can
expand. Due to the
decrease in friction it
cools. This is known as
adiabatic cooling.
Once the air reaches
the dew point the
water molecules
condense and clouds
begin to form.
As the air continues to
cool, but not as fast as
before since
condensation releases
energy, the drops
continue to grow and
when they are large
enough will fall as rain,
sleet or snow.
The air is dry since the moisture
was left on the windward side of
the mountain and also warmer.
The heat added to the air from
the condensation can make the
air up to 50 degrees warmer as it
sinks and compresses.
Once the air is over
the mountain it
descends and warms
as the molecules are
compressed back
together with the
increase in pressure.
This is known as
adiabatic warming.