This next mini lecture is on a topic
that's
always been fascinating to me as someone
who's lived in the desert southwest for
a while
and this is-- the topic is monsoons.
And s,o we're going to talk a little bit
about what causes monsoons and start off
with a more general
explanation. And, we're going to look at
the moving tropical rain belt.
So, as we discussed in some previous mini
lectures,
we saw that the earth heats up along the
equator,
more than it does in the other parts of
of the globe because that's where the
sun's radiation is the most intense.
And so,
this causes evaporation to occur along
that
belt of the equator. And then,
precipitation
as a result as it spills out over the--
around the equator. So, we also discussed
the fact that the earth is on a tilt.
And so, sometimes when -- so when the earth
is tilted such that the northern
hemisphere is tilted towards the sun. So,
in our summer
then the heating is going to be more
intense
during the that summer season. And so,
it's going to not be right at the
equator but north of the equator if -- for
the northern hemisphere.
So, as the earth goes around the sun and
the seasons change, this causes the
tropical rain belt to shift.
So, in the summer for the northern
hemisphere it's going to shift
north. And so. May through June it's-- it's
that tropical rain belt is shifting up
to
the northern hemisphere then in November
and December that tropical rain belt
shifts
south of the equator into the southern
hemisphere.
So, as this rain belt moves around it
causes monsoon rains.
And, there's many different parts of the
world that receive
monsoon rains. In the northern hemisphere,
in the summertime, some places that have
strong monsoons are India, Southeast Asia,
Southeast China and then we also have
them
in this desert southwestern United
States.
In the southern hemisphere, in the-- in
their summer which is our winter,
areas such as Australia, South America,
and Africa
those -- those continents all and countries
all have
some kind of monsoon activity. Okay. So,
I'm going to try and shift
out of here to show you a really quick
movie
of this.
Hopefully.
Okay. So, in this movie, this is showing
the rain belts. You can watch this blue
band of rain as it moves
up in the summer for the northern
hemisphere. And then,
it'll move south of the equator for the
southern
hemisphere summer. So, you can see that's
the how the rain belt moves up and down.
Okay. So, back to our
PowerPoint. So, just a little bit of
background information about monsoons.
The word monsoon comes from the arabic
word
which means season. And so, it was first
used to describe trade winds
over Asia in the summer time. And, these
trade winds brought torrential rains
and they still do but now this term has
become more general.
And, it -- it covers all of the different
monsoons
around the world and all the different
areas that we've discussed.
So again, why do these -- why does this
happen? We talked about differential
heating. So,
the -- wherever the
the sun is the most intense, so if that's
in the summer in the northern hemisphere,
it's going to cause the earth to heat up,
the land surface to heat up more quickly
than water heats up.
That causes that hot air to rise and as
that hot air rises off the land surface
then moisture and some moist and cooler
air from the ocean nearby
is going to be pulled into that area to
fill in
as the -- the hot air that was over the
land surface rises up.
And so, it's going to bring in humid,
cooler air.
As these hot, dry and cool,
moist air masses collide, this leads to
thunderstorms, rainfall,
and humid conditions on land.
And, we see the biggest monsoon activity
in areas where you have large elevated
land masses that are near
ocean systems or oceans or large
seas.
Okay. So,
India is famous for its monsoons and
that's because India is
a large land mass with a very high
elevation
and the center of the country and then
they also are surrounded by ocean.
And so, it's perfect ingredients for a
monsoon.
So, in this map you can it's vaguely
outlined
this is the country of India, the -- the
bottom peninsula of it
and that's the whole country.
And, you can
see
in their dry season what's considered
winter,
the air is cool, the air
over the landmass is cool.
And so, it's kind of settles in. You get
high pressure building up
and so actually the air is going to
travel from landmass out into the ocean.
So, the air is traveling out to sea.
In the summer, the reverse happens.
The land mass heats up
much more quickly than the ocean
surrounding it.
And then, you have these high mountain
ranges up in the northern part. And so,
there's a huge elevation change as you
go on to the continent.
And so, the air masses heat up,
they rise as they travel up the
mountains, and so
that's going to pull in all of this
moisture from the ocean,
as this moisture comes in it rises up on
the land mass as well.
It cools very quickly and then you get
these torrential rains coming down.
Okay. We have a similar system in the
desert southwest. It's not nearly as
strong as the Indian monsoon
but we do have a monsoon in North
America. It occurs in mid to late summer. I's not
a continuous rain
but -- but it does account for a large
amount of the precipitation that the
desert southwest
receives. So, up to about half of the
annual precipitation
for the southwestern United States.
There's a huge variation in the amount
of precipitation
these areas get from year to year so
some years we'll have a strong monsoon
season
and we'll get maybe seven inches in the
desert southwest.
And then, other monsoon seasons that
won't be so great and there will be
three or four inches.
Or, even less, depending on where you live.
So, the same system happens here. These
land masses
there's a big elevation change over
Arizona and New Mexico.
This area that's kind of brownish
there's the higher mountains here.
And, these are lower lying plains.
So, this the -- the land mass heats up very
quickly, that hot air rises, and pulls in
this
moisture from the Gulf of Mexico and the
Gulf of California.
And, as that moist air rises up on
from the plains onto the -- the
higher mountain areas, it cools quickly
and you get a lot of precipitation
falling out of those
clouds.
Okay. That's the end of the monsoon mini
lecture.
