Geography 140
Introduction to Physical Geography
Lecture: Dust and Water in the Atmosphere
II. The chemical composition of the lower atmosphere (continued)
B. Solid constituents of the homosphere:
1. Besides gases (air), the atmosphere also includes small amounts of
material in a solid state (solid state atmosphere -- now there's a
concept!)
2. Collectively, these materials are known by the impressive
scientific name, "dust." Sometimes, they're also called
"particulates."
3. They include:
a. Soils, kicked up in duststorms, plowing, animals running about
b. Salt from sea spray
c. Pollen and spores, as a lot of you are painfully aware
d. Ash, as from volcanic eruptions
e. Soot from combustion
4. They are a small and highly variable component of the atmosphere
a. Over the oceans, a cubic centimeter might hold about 500-2,000
of these particles; over a city, it might be more like 100,000!
b. Most of the atmospheric dust is concentrated in the bottom few
kilometers of the atmosphere, though volcanoes can lob lots of
it over 20 km up and meteors burning up in the atmosphere can
add dust even higher.
5. Even though dust is a relatively small constituent of the
atmosphere, it is very important to weather and climate: Without
dust particles, water vapor cannot condense or freeze to form fogs,
clouds, and precipitation from clouds. Water requires a surface on
which to condense or freeze, which is what dust provides:
"condensation nuclei."
6. Dust is of scientific concern, too, because one of the most
distinctive impacts of human beings on the atmosphere is increased
dustiness. This effect goes back perhaps 1.5 million years, when
Homo erectus began manipulating fire. Besides fire, we
raise dust in plowing and in allowing our animals to overgraze.
Overgrazing can set off desertification, or creation of desert in
once more heavily vegetated regions. In fact, dustiness emanating
from the Sahara and its borderlands (the Sahel) set off huge plumes
of dust visible in satellite imagery, which travel as far west as
the Caribbean!
7. Dustiness affects weather and climate by providing condensation
nuclei and by the cooling that its microshadows cast on the air and
ground below it.
a. More dust can permit more clouds, which reflect more of the
sun's radiation into space.
b. More dust also creates measurable cooling below due to its
shadow effect, as it reflects and scatters light into space.
c. Dust also absorbs energy, but that warming is done higher in the
atmosphere.
d. However you look at it, less radiation is available to be
absorbed at the ground and this can create more stable air less
prone to precipitation (more about instability and precipitation
in a later lecture)
8. In fact, some scientists worry more about the overall balance of
human activity being an ice age!
a. Scientific opinion was pretty much divided between those worried
about global warming due to carbon dioxide and those worried
about an ice age due to dust production, this up until about 20
years ago.
b. At present, majority opinion in the scientific community favors
greater concern over global warming, and even those worried
about dustiness fear that, in the short run, the net impact of
human activities will be atmospheric warming, while, perhaps, in
the long run (centuries to millenia), the net effect will be
dust-induced cooling.
c. Overshadowing the debate are the unknowns about secular changes
in Earth's climate due to orbital changes and other causes.
C. In addition to gaseous and solid constituents of the atmosphere, there
is also water, which is, like dust, a highly variable component.
1. Water vapor ranges from 0 to 4 percent of the atmosphere, depending
on where and when it's measured
2. Atmospheric water, however, exists in all three states of matter:
a. Gaseous, as water vapor
b. Liquid, as, well, water
c. Solid, as ice
3. The most important thing about atmospheric water is that it
constantly changes state.
a. Water is added to the atmosphere and leaves the hydrosphere
(oceans and other bodies of water) and land surfaces by
evaporating into the air.
b. Water vapor can condense or freeze into clouds and fogs.
c. Some of the water or ice in clouds will leave the atmosphere as
precipitation (rain, snow, sleet, or hail) over the land or the
ocean.
d. Water vapor can also be lost to the atmosphere when it condenses
or freezes onto surfaces on the ground (dew and fog drip or
frost).
4. Obviously, one reason this change of state in water is important is
that it governs cloudiness, fogginess, and precipitation (and the
distribution of fresh water).
5. Less obviously, a critical aspect of this change of state is its
effect on temperatures: It is a major regulator of air
temperatures.
a. Whenever ice melts into water or water evaporates into vapor or
ice directly sublimates into vapor, radiant energy is absorbed
and hidden in the water (without producing a change in the
water's temperature). In short, water stores heat whenever it
changes state in this direction and, because the effect on its
own temperature is hidden from our instruments at the point of
transition, this stored heat is called "the latent (hidden) heat
of evaporation." There's a nice elaboration of this in your
textbook on pp. 70-72.
i. It takes 330,000 Joules (or 78,820 calories) to heat ice
into one kilogram of water, that is, 330 J/g or 79 cal/g.
a. a calorie is the energy needed to heat one gram (about
the mass of a paperclip) of water 1° C, from 14.5 to
15.5° C. You may be familiar with the kilocalorie
-- that's the "big calorie" people use to keep track of
their food intake when they're dieting.
b. a Joule is 0.2388 of a calorie; there are 4.1868 Joules
per calorie.
ii. It takes a lot more energy for water to evaporate:
2,480,000 Joules (that is, 592,340 calories): 2,480 J/g or
592 cal/g.
iii. For ice to sublimate into vapor, then, it would take
2,810,000 J/kg (2,810 J/g) or 671,157 cal/kg (671 cal/g).
b. Whenever water changes state in the opposite direction, it
surrenders its latent heat, which then becomes sensible heat in
the air.
i. So, whenever vapor condenses into liquid water, it gives up
roughly 600 cal/g, which heats the air.
ii. Whenever water freezes into ice, then, it gives up roughly
80 cal/g to become sensible heat in the air.
iii. And, whenever vapor directly freezes, it surrenders not
quite 680 cal/g for sensible heating of the air.
6. This process of storing and releasing latent heat, then, slows down
the rate of temperature change, whenever there is enough water
present.
a. If temperatures are dropping, say, at night, and atmospheric
water begins to condense and/or freeze, it surrenders some of
its latent heat as sensible heat in the air. Mind you, the air
doesn't get warmer: It just doesn't get as cold as it would
have otherwise. The release of latent heat partially offsets
the drop in temperatures.
b. If temperatures are rising, say, in the morning when the sun
comes up, and water and/or ice evaporates, it will absorb some
of that increasing heat and so slow down the climb in
temperatures. It partially offsets the warming. So,
temperatures don't get as hot as they otherwise would when water
or ice are around.
7. This is why deserts get so awfully hot in the daytime and can get
life-threateningly cold at night. There is too little water to
interfere with the sun-driven temperature swings. This is also why
humid tropical areas, such as Puerto Rico or Hawai'i, never get
terribly hot, either (nor do they cool off much at night):
Atmospheric water interferes with the solar radiation forcing of
temperatures. Now, you can understand why places like the San
Fernando Valley are so much more extreme than Long Beach: hotter
in the days/summers and colder in the nights/winters. Long Beach
has beaucoup water available from its convenient location
against the Pacific, while the Valley is isolated from humid ocean
air by the Santa Monica Mountains. You can even see this contrast
(though not as strongly) from Downtown L.A. and Long Beach.
Come away from this lecture knowing the sources of atmospheric dust and its
impact on climate and really focus on the changes of state in water and the
concept of latent heat and its relation with sensible heat.
In the next lecture, I'll "talk" about the vertical pressure structure and the
vertical temperature structure of the atmosphere.
Document and © maintained by Dr.
Rodrigue
First placed on web: 10/08/00
Last revised: 02/17/01