Ice, Water, and Vapor on Earth
Water (H2O) is extraordinary in that it coexists on Earth in 3 distinct forms or states, 1: dry
solid (ice), 2: wet liquid (water) and, 3: dry, invisible gas (vapor). Nitrogen, N2 and Oxygen,
O2 are gases that also exist in liquid and solid forms but on Earth only in laboratories. The
photo shows water and ice while the steam plumes suggest water vapor (but are actually
condensed water droplets) at Yellowstone Park’s Morning Glory Pool in Winter.
Gas – Liquid – Vapor
In gases (which are dry and usually invisible), molecules are far apart (so that they have
low density) and move freely. In liquids (which feel wet and are visible but often colorless
and largely transparent), molecules are clustered together but slide around. In solids
(which are visible and may be transparent, but are often colored), molecules are packed
together and locked in place, often in crystals. Liquids and solids have much higher
density than gases because the molecules are packed closely together. Most solids
have slightly higher density than liquids because the molecules are packed more tightly
and more efficiently.
Temperature and Moving Molecules
All gas molecules move rapidly and
collide frequently. Collisions ensure that
lighter molecules move faster. Also,
molecules move faster as T increases.
Increasing T increases pressure, but
only in a closed container and not in the
open atmosphere.
Run Program COLLIDE
As T rises, atoms and molecules vibrate
faster and can become so rapid that
different molecules locked together in
solids separate, (melting). Increasing T
more frees the molecules, which
evaporate to gases. Further heating can
split individual molecules. The weaker
the attractive chemical bonds, the lower
the T at which these changes occur.
The triangular shape of the H2O
molecule gives it extraordinary
powers and properties. They shape
ice crystals into hexagons that are
packed so poorly that freezing
expands H2O. The concentration of
positively charged H+ ions on one
side and negatively charged
electrons of the O-2 ion on the
opposite
side
polarizes
H2O
molecules and makes them act like
magnets that can stick (bond) to
other H2O molecules. The strong
attractions (bonds) of polarized H2O
molecules raise the melting and
boiling points much higher than for
nitrogen, N2 or oxygen, O2 but when
they occur deep in the Earth they
pull apart the molecules of solid
rocks and melt them to magma at
much lower T than without H2O.
Molecular Structure of Ice = Hexagonal
Click to see up-down
pattern of each hexagon
Top View
Side View
Adjacent H2O molecules in each hexagon are on different levels
or planes. (as if they were in different floors of a split-level house.)
Snow and Ice Crystals
Each branch is rotated by
60 from the next branch.
Spikes on each branch are
parallel to the next branch.
The Hydrologic Cycle
The Hydrologic Cycle is the ceaseless flow of water through the Earth, Oceans, and
atmosphere. Fresh water evaporates from the surface of the land and the oceans. The
vapor is transported and lifted into clouds in the atmosphere and falls as precipitation.
About 2/3 of the water that falls on land evaporates. The rest collects in rivers and flows
back to the sea as Runoff, although some water flows underground as ground water.
The hydrologic cycle sometimes leads to flood or to drought, but essentially the same
water has been flowing through the system’s reservoirs for the past three billion years.
Evaporation, the invisible part of the hydrologic cycle, is the power behind the throne
of all storms. To see evaporation, fill a wide pan with water on a hot sunny day and
watch the water disappear before your eyes. (This happens slowly enough so that you
will need to be patient.) The evaporation rate depends on many factors including the
1. Moisture content of the surface
4. Wind Speed
2. Temperature
5. Plant Cover
3. Relative Humidity
Precipitation patterns depend on temperature and wind. Since T increases from Pole
to Equator, so does precipitation because vapor capacity does. Just as air at the
Equator contains about 70 times more vapor than air at the South Pole in January, it
also yields 70 times more precipitation. But to get precipitation, air must rise. Air rises
where it converges, as in the ITCZ or in low pressure areas, or where the winds are
forced to blow upslope against mountains. Air sinks to produce deserts in the
subtropical high pressure areas, such as dominate the Sahara.
The Hydrologic Cycle: Global Water Fluxes and Supplies
Floods and droughts are part of the hydrologic cycle
Delaware River Flood 04 April 2005, Milford, NJ http://www.flickr.com/photos/frenchtown-nj/8559476/
Hydrographs are used to predict floods. For
a simple quick rainstorm each stream has a
standard response. Water rises to a peak
some set time after the rain. Double the rain
and the flood is twice as large but the time
delay is the same. Thus we can time floods
for each stream very accurately.
Run Storm Hydrograph
Current Hydrographs on the Web http://waterwatch.usgs.gov/?m=real&r=us
Rural vs Urban
Hydrographs
Paved areas in
cities speed the
arrival of rain water
to streams. This
leads to
1: Quicker rise
2: Higher peak flow
3. Faster recession
Thus, urbanization
can increase the
severity of floods.
Rivers and River Basins
Egypt’s Nile River Valley
and Delta and Sahara
Desert (MODIS Image).
Amazon River
Basin
All rain that falls in
the light pink area
and
does
not
evaporate, flows into
the Amazon River.
The Amazon has the
world’s
greatest
water volume flow of
any
river.
Its
average discharge
is 180,000 m3s-1
(cubic meters per
second), enough to
fill about 2 million
bathtubs
each
second.
Determining a River Basin
Any River Basin can be determined because it resembles the branches and
trunk of a tree. The dashed line or DIVIDE separates different river basins. It is
located along the highest elevations (see next Slide). No stream can cross it.
http://www.geographyalltheway.com/ib_geography/ib_drainage_basins/the_drainage_basin.htm
Havasu Falls, Grand Canyon
Near the source of rivers (which may be
a spring that comes out of the ground)
there are waterfalls and rapids and
sediment is eroded. Further downstream
the river broadens out onto a floodplain,
ow slows and the river begins to
meander and ultimately split or braid in a
complex of channels, where it deposits
the sediments. This often happens when
the river nears the sea, where it spreads
out into a delta as it empties into the sea.
As sediments are deposited in the
floodplains or deltas the main river often
changes course.
The next slides show the act and impact
of erosion, deposition, and ground water
Alexandre Hogue. Mother Earth Laid Bare, 1936
Dual impact of poor farming practices and a severe Drought - the “Dust Bowl”
(ca. 1930-36) on the Great Plains
Erosion during downbursts creates gullies in exposed soft sediments of
semiarid regions Cappadocia, Turquía
Braiding on sediment choked stretch of Alaska’s Matanuska
River. Sediments are weathered and eroded by glaciers and
running water. The ridge on the right is a moraine or mound
of rubble plowed and pushed by an ancient glacier.
Braided River,
Alaska
Groundwater and the Water Table
The enormous volume of water stored underground is not stored in vast caverns but in
tiny pores between grains of soil or cracks in rock. In sand the fraction of the volume of
the pores or Porosity is about 36% of the total volume. Ground water, which we are now
depleting, provides the water for hundreds of millions of wells around the world.
Mamshit Israel, the Nabatean City of Memphis. The sediment behind the dam has large
porosity so it holds much water and also keeps it from evaporating. In that way the
Nabateans thrived in the deserts of Israel and Jordan. The Nabateans also built PETRA.
Ground Water
Nabesna Glacier, Wrangell Mountains, Alaska
Note Medial and Lateral Moraines
Much of the world’s fresh water is
locked up in ice, not all of which is clean
Glaciers are like rivers of ice. They form when snow accumulates in the high mountains
and gets packed down. The ice then begins to flow slowly downhill, grinding out the
valleys and dragging the sediments as lateral and medial moraines. The sediments also
pile up into a terminal moraine at the lower end of the glacier, where the ice melts in the
so-called zone of ablation. When climate warms the glacier retreats because the melting
occurs faster than the downstream flow. Melted glaciers leave behind classical Ushaped valleys (see next slide of Yosemite Valley).
Where the ice is brittle it cracks to form large crevasses. This is much like earthquakes.
Nearer the bottom the ice is ductile – it flows like plastic. Material buried at the head of
the glacier can appear at the foot many years later. This was the case with a plane that
was lost over the Andes in 1948 or so. It crashed and has recently reappeared.
As ice crystals fall, they pile up on the ground and turn our eyes from the sky
above to the ground below, where more beauty and wonder reside.
The Antarctic Ice Sheet shows what Yosemite Valley looked like 20000 years ago.
Terminal Moraines of Long Island
At the height of the last Ice Age about 21000 years ago the ice sheet covering Canada
and the northern USA reached Long Island (which was then not an island) and left
behind a long rubble hill or moraine, now up to 400 feet above sea level. Long Island, as
a result is a long ridge of loose dirt and rocks.
http://people.hofstra.edu/J_B_Bennington/research/long_island/li.html
Block Island Lighthouse – Cliff is the Remnants of the Terminal Moraine
Water vapor is the most variable gas in the atmosphere because it readily
evaporates from water or sublimes from ice, and then condenses to water or
deposits to ice. And since water is the staff of life, vapor is often treated
differently from all other gases in the atmosphere even though it is just a gas.
The way to condense vapor is to cool the air. This happens every night near
the ground, on the grass (where dew is formed). Beautiful strings of dew drops
that condense at
night often appear on
spider webs. (see
next slide.)
One way to remove
almost all vapor from
the air is to chill it by
pumping air through a
pipe immersed in dry
ice or liquid nitrogen
(see 2nd slide). The
vapor deposits or
crystallizes directly to
ice inside the pipe.
Droplets condensing on a spider web overnight
Measures of Vapor and Definitions of Humidity
Temperature (T)is a measure of the kinetic energy of the molecules. Two objects have the same
temperature when no heat is transferred between them when they are brought into contact.
Mixing Ratio (w) is the mass fraction of vapor in the air. It is often expressed as the number of
grams of water vapor per kilogram of dried air, or parts per thousand (‰).
Vapor Pressure (e) is the part of air pressure due to the pressure of the water vapor molecules.
When divided by air pressure, it gives the fraction of vapor molecules in the air.
Relative Humidity (RH) is the vapor content of the air divided by the vapor capacity expressed
as a percent (%).
Saturation occurs when the vapor content of the air is at its capacity. Any more vapor quickly
condenses into water or deposit to ice. When air is saturated with water vapor RH = 100%
Dew Point Temperature (Td)is the temperature at which the air becomes saturated after cooling
without adding water vapor. Further cooling causes cloud droplets or dew (condensation).
Wet Bulb Temperature (Tw) is the temperature at which the air becomes saturated after cooling
by evaporating water. (When RH < 100%, Td < Tw < T).
Relative Humidity
Relative humidity (RH) increases when vapor content increases or vapor
capacity decreases. Since vapor capacity increases with T, falling T mean
falling vapor capacity and rising RH.
Ah! Now I finally understand why RH is lowest in mid afternoon and
highest around dawn, when fog and dew are most likely.
High T
Large Capacity
Low T
Small Capacity
Vapor Content Constant
Morning
Low T, High RH
Afternoon
High T, Low RH
Condensing Vapor – Producing Precipitation
Vapor
Capacity
All clouds and rain are formed by the process of cooling air.
As air cools its water vapor capacity decreases. When
vapor capacity falls below the original vapor content the
excess vapor condenses to form liquid water or ice.
20
12 13
12
9
9
7
7
6
6
Vapor
Content
HOT
6
3
5
Cooling….Produces Condensed Water
COLD
Vapor Pressure and Condensation
2500.0
Vapor Pressure (Pa)
Vapor capacity roughly doubles for
every T increase of 10ºC.
2000.0
es(T)
1500.0
Excess Vapor
Condensed
Original
Td
1000.0
Original
Vapor Content
500.0
When air is cooled below the saturation point,
(i. e, dew point, Td) the excess vapor condenses
0.0
273
278
283
288
293
Temperature (K)
The next Slide shows how to calculate the Amount of Water Vapor that
condenses when air is cooled.
Vapor Fraction (w), Relative Humidity
(RH), and Condensation of Vapor.
The table shows the saturated (capacity) mass percent of water
vapor in air at p = 105 Pa ( sea level pressure). Thus,, 100 g of
air at 20C can hold 1.49% × 100 g = 1.49 g of vapor. [300 g of
air at 20C can hold 1.49% × 300 g = 4.47 g of vapor]. RH equals
actual vapor content (wact) divided by vapor capacity (wsat).
Example 1: Find RH of 100 grams of air at T = 20C that actually
contains wact = 0.5 grams of water vapor.
Solution:
RH 
w
content
0.5
 act 
 33.6%
capacity wsat 1.49
Meteorologists must predict how much rain or snow will fall. To
do this they must calculate how much vapor will condense
when air is cooled. If final vapor capacity, wsat(final) is less than
initial vapor content, wact(initial) the amount that condenses is the
difference between the two times the mass of air.
Example 2: If the air in Example 1 air is chilled to T = -10C
calculate how much vapor will condense.
Solution:
w
actinitial

 wsatfinal  mair  0.5%  0.179% 100 g  0.321 g
T(o C) % wsatl(liq) *% wsatl(ice)
40
4.957
0
35
3.708
0
30
2.756
0
25
2.034
0
20
1.488
0
15
1.078
0
10
0.772
0.00
5
0.546
0.00
0
0.382
0.382
-5
0.263
0.251
-10
0.179
0.162
-15
0.119
0.103
-20
0.078
0.064
-25
0.050
0.040
-30
0.032
0.024
-35
0.020
0.014
-40
0.012
0.008
-45
0.007
0.005
-50
0.004
0.002
* At T < 0 C there are two values for vapor capacity – a lower value (wsat(ice)) if ice is present
and a higher value (wsat(liq)) for supercooled liquid water. When both supercooled droplets
and crystals are present , crystals will steal vapor from droplets and grow at their expense!
Supercooled Water, Ice, and Precipitation
The charts and tables on the previous slides show that when T < 0C, vapor capacity at the
surface of ice is less than at the surface of liquid water because fewer vapor molecules have the
additional energy they need to escape from ice. This difference implies that when supercooled
water (water that remains liquid below 0C) and ice are both present in a cloud, ice crystals will
grow at the expense of the supercooled droplets (which will shrink and evaporate). The ice
crystals then grow large enough to fall, as we see in cirrus clouds or in the altocumulus clouds on
the next slide. This mechanism, called the Wegener-Bergeron process is responsible for most rain
except in air over the tropical oceans. Alfred Wegener (of Continental Drift fame) was the first to
point out that ice reduced vapor capacity and Tor Bergeron realized it accounted for most rain
produced outside the tropics
In the case of the altocumulus clouds shown in the next slide, the supercooled water droplets
freeze when an airplane flies through the cloud and chills the air around the wingtips.
Ice crystals grow at the expense of
supercooled droplets and fall out of the cloud.