Here's a reminder of where we were
working last Thursday. We finished class by looking at the
formation of and differences between dew, frozen dew, and fog.
When the
relative humidity in air above the ground (and away from objects on the
ground) reaches 100%, water vapor will condense onto small particles
called condensation nuclei. It would be much harder for the water
vapor to just condense and form small droplets of pure water (you can
learn why that is so by reading the top
of p. 92 in the
photocopied class notes).
Water vapor will condense onto
certain kinds of condensation
nuclei
even when the relative humidity is below 100% (again you will find some
explanation of this on the bottom of p. 92).
These
are
called hygroscopic
nuclei.
A short video showed how water vapor would, over time,
preferentially
condense onto small grains of salt rather than small spheres of
glass. The
figure
below
wasn't
shown
in class.
The start of the video at left
showed the small grains
of
salt were
placed on a platform in a petri dish
containing water. Some small spheres of glass were placed in the
same
dish. After about 1 hour small drops of water had formed around
each
of the grains of salt but not the glass grains (shown above at
right).
In
humid parts of the US, water will condense onto the grains of
salt
in a salt shaker causing them to stick together. Grains of rice
apparently absorb moisture which keeps this from happening and allows
the salt to flow
freely out of the shaker when needed.
This figure shows
how
cloud
condensation nuclei and increasing relative humidity can affect the
appearance of the sky and the visibility.
The air in the left most figure is relatively dry. Even
though
the condensation nuclei particles are too small to be seen with the
human eye you can tell they are there because they scatter
sunlight. When you look at the sky you see the deep blue color
caused by scattering of sunlight by air molecules mixed together with
some white
sunlight scattered by the condensation nuclei. This changes
the color of the sky from a deep blue to a bluish white
color. The more particles there are the whiter the sky
becomes. This is called "dry haze."
The middle picture shows what happens when you drive from the dry
southwestern part of the US into the humid
southeastern US. One of the first things you would notice is the
hazier
appearance of the air and a decrease in visibility. Because the
relative humidity is high,
water vapor begins to condense onto some of the condensation nuclei
particles (the hygroscopic nuclei) in the air and forms small water
droplets. The water droplets scatter more sunlight than just
small particles alone. The increase in the amount of scattered
light is what gives the air its hazier appearance. This is called "wet
haze."
Finally when the relative humidity increases to 100% fog
forms.
Fog can cause a severe drop in the visibility. The thickest fog
forms in dirty air that contains lots of condensation nuclei. We
will see this effect in the cloud-in-a-bottle demonstration coming up
later in the class.
With cold
and possibly wet weather being forecast, you might have a chance to see
some fog in Tucson. To produce fog you first need to
increase the relative humidity (RH) to
100%
You can do this either by cooling
the air (radiation fog) or
adding
moisture to
and saturating the air (evaporation or steam fog). Both will
increase the ratio in the RH formula
above.
Probably the most common type of fog in Tucson is radiation
fog.
The ground cools during the night by emitting IR radiation (left figure
below). The ground cools most rapidly and gets coldest when the
skies are free of
clouds and the air is dry (except for a thin layer next to the
ground.
Air in contact with the ground
cools and radiation fog can form
(right
figure above). Because the fog cloud is colder than the air right
above, this is a stable situation. The fog clouds "hugs" the
ground.
Radiation fog is sometimes called valley fog.
The cold dense foggy air will move downhill and fill low lying
areas. Because the fog reflects sunlight, it is often
difficult for the sun to warm the air
and dissipate thick clouds of valley fog.
Steam fog or evaporation fog (also sometimes known as mixing fog) is
commonly observed on
cold mornings over the relatively warm water in a swimming pool.
Water evaporating from the pool
saturates the cold air above. Because the fog cloud is warmer
than the cold surrounding air, the fog clouds float upward.
When you "see your breath" on a cold day (the figure below
wasn't shown
in class)
you're seeing mixing fog.
Warm moist air from your mouth mixes
with the colder air outside. The mixture is saturated and a fog
cloud forms.
Next it was time for a
demonstration that puts together many of the
concepts we have been covering. Cooling
air and
changing relative humidity, condensation nuclei, and scattering of
light are all involved in this demonstration.
We used a strong, thick-walled, 4
liter flask (vaccum flasks
like this are designed to not implode when all of the air is pumped out
of them, they aren't designed to not explode when pressurized).
There
was a little
water in the bottom of the flask to moisten the air in the flask.
Next we pressurized the air in the flask with a bicycle pump. At
some point the
pressure blows the cork out of the top of the flask.
The air in
the flask expands outward and cools. This sudden cooling
increases the
relative humidity of the moist air in the flask to 100% ( probably more
than 100% momentarily ) and water vapor condenses onto cloud
condensation nuclei in
the air. A faint cloud became visible at this point. The
cloud droplets are too small to be seen with the human eye. You
can see the cloud because the water droplets scatter light.
The demonstration was repeated an
additional time with one
small
change. A burning match was dropped into the
bottle. The smoke from the match added lots of very small
particles, condensation nuclei, to the air in the flask. The
cloud that formed
this time was quite a bit "thicker" and much easier to see.
Clouds are one of the best ways of cleaning the
atmosphere
(cloud
droplets form on particles, the droplets "clump" together to form a
raindrop, and the raindrop carries the particles to the ground).
A raindrop can contain 1 million cloud droplets so a single raindrop
can remove a lot of particles from the air. You may have noticed
how clear the air seems the day after a rainstorm; distant mountains
are crystal clear and the sky has a deep blue color. Gaseous
pollutants can dissolve in the water droplets and be carried to
the ground by rainfall also.
A cloud that forms in dirty air is
composed of a large
number of small droplets (right figure above). This cloud is more
reflective
than a cloud that forms in clean air, that is composed of a smaller
number of larger
droplets (left figure).
Just like in the cloud-in-a-bottle demonstration, the cloud that was
created when the air was full of smoke particles was much more visible
than the cloud made with cleaner air.
This is has implications for
climate change.
Combustion of fossil fuels adds carbon dioxide to the atmosphere.
There is concern that increasing carbon dioxide concentrations will
enhance the greenhouse effect and cause global warming.
Combustion also adds condensation nuclei to the atmosphere (just like
the burning match added smoke to the air in the flask). More
condensation nuclei might make it easier for clouds to form, might make
the clouds more reflective, and might cause cooling. There is
still quite a bit of uncertainty about how clouds might change and how
this
might affect climate (remember too that clouds are good absorbers of IR
radiation).
We had
a little time to start the next topic:
identifying
and naming clouds.
The ten main cloud types are listed
below (you'll find this list on p.
95 in
the photocopied class notes).
You should try to learn these 10 cloud
names. Not just because
they might
be on a quiz (they will) but because you will be able to impress your
friends
with your knowledge. There is a smart and a not-so-smart way of
learning
these names. The not-so-smart way is to just memorize them.
You
will inevitably get them mixed up. A better way is to recognize
that all
the cloud names are made up of key words. The 5 key words, we
will
find,
mostly tell you something about the cloud altitude and appearance.
Drawing a figure like this on a blank sheet of
paper is a good way to
review
cloud identification and classification.
Each of the clouds above has a box reserved for
it in the figure.
Clouds are classified according to the altitude at which they form and
the
appearance of the cloud. There are two key words for altitude and
two key
words for appearance.
Clouds are grouped into one of three altitude
categories: high, middle
level,
and low.
Cirrus or cirro
identifies a high altitude
cloud. There are three types of clouds found in the high altitude
category..
Alto in a cloud name means the cloud is found at middle altitude.
The
arrow connecting altostratus and nimbostratus indicates that they are
very
similar. When an altostratus cloud begins to produce rain or snow
its
name is changed to nimbostratus. A nimbostratus cloud is also
often somewhat
thicker and lower than an altostratus cloud. Sometimes it might
sneak into the low altitude category.
It is very hard to just look up in the sky and determine a cloud's
altitude. You will need to look for other clues to distinquish
between high and middle altitude clouds. We'll learn about some
of the
clues when we look at cloud pictures later in the class.
There is no key word for low altitude clouds. Low altitude clouds
have
bases that form 2 km or less above the ground. The summit of Mt. Lemmon in the Santa Catalina mountains
north of Tucson is about 2 km above the valley floor. Low altitude clouds will have bases that form at or
below the
summit of Mt. Lemmon.
Clouds can have a patchy of puffy (or lumpy,
wavy, or ripply) appearance.
These
are cumuliform clouds and will have cumulo
or cumulus
in their name. In an unstable atmosphere cumuliform clouds will
grow vertically.
Strong thunderstorms can produce dangerous severe weather.
Stratiform clouds grow horizontally and
form
layers. They form when the atmosphere is stable.
Cirrus
clouds
are
sometimes considered to be a third type of cloud appearance.
The last key word, nimbo
or nimbus, means
precipitation. Only two of the 10 cloud types are able to produce
(significant
amounts of) precipitation. It's not as easy as you might think to
make precipitation.
Nimbostratus clouds tend to produce fairly light precipitation over a
large
area. Cumulonimbus clouds produce heavy showers over localized
areas. Thunderstorm clouds can also produce hail, lightning, and
tornadoes. Hail would never fall from a
Ns
cloud.
While you are still learning the cloud names you might put the correct
key
words together in the wrong order (stratonimbus
instead of nimbostratus or nimbocumulus
instead of
cumulonimbus). You won't be penalized for those kinds of errors
in this
class because you are putting together the right two key words.
Here's
the cloud chart from earlier. We've added the three altitude
categories
along the vertical side of the figure and the two appearance categories
along
the top. By the end of the class we will add a picture to each of
the
boxes.
We'll look at slides and learn about some of the key characteristics of
each of the 10 cloud types in class on Thursday.