The Water Cycle
Ecclesiastes 1:7
All the rivers run into the sea; yet the sea is not full; unto the place from
whence the rivers come, thither they return again
Fig. 4-1, p.78
Structure of Water
Figure 5.1
Figure 5.2
Water's unique molecular structure and hydrogen bonds enable
all 3 phases to exist in earth's atmosphere.
Sublimation & deposition describe the non-incremental changes
between solid and vapor phases.
Evaporating Water into Air
Evaporation:departures of
water molecules from its
surface
Condensation : arrivals of
molecules from adjacent
vapor
When air is saturated,
evaporation and
condensation are in
equilibrium.
Figure 5.3A
NRG is taken from the
environment during
evaporation and NRG is
released to the environment
by condensation
‘Latent Heat’ is the term that describes the NRG transferred/stored
by water changing phase
Water in the atmosphere…
Absolute & Specific Humidity
Absolute Humidity: mass of water vapor/volume of air
For a given mass of water vapor
in an air parcel, the absolute
humidity changes as the parcel
volume changes (e.g., lifts or
descends).
Specific Humidity: mass of water vapor/total mass of air
Specific humidity is concerned
with the mass of vapor to mass of
air, and is not affected by changes
in parcel volume.
Mixing ratio:
 Mass of water vapor/mass of dry air
 g/kg
 Mixing ratio and specific humidity do not
change as long as water vapor is not added
or subtracted from the parcel of air.
Specific Humidity vs. Saturation
Warm air can absorb more
vapor than cold air.
For a given parcel of air,
specific humidity declines
from its highest in the
tropics to its lowest in the
colder poles.
Figure 5.9
Determining Vapor Pressure
Average atmospheric pressure
of 1013 mb is comprised in part
by the weight of vapor
molecules.
Warmer air can absorb more
vapor than cooler air before it
saturates.
Figure 5.10
Relative Humidity
 Water vapor/capacity to hold water vapor
 This is a measure of how much water vapor the air
is holding divided by the amount of water vapor
the a given parcel of air CAN hold.
 Expressed as a percentage
 Changes in RH occur by changing the water vapor
content or changing the temperature of the air
parcel
Relative Humidity Trends
Figure 5.11
Relative humidity (RH) indicates air parcel proximity to saturation.
Saturation can be achieved, or RH increased, by adding more water
or dropping the air temperature.
Dew point is the temperature at which saturation occurs.
Seasonal Dew Point Maps
January, July, dew point
Dew Point vs. Relative Humidity
Dew point is the temperature for
saturation, and used with a vapor
pressure curve reveals the mass of
vapor in the air.
While relative humidity may be
higher in polar air, more water is
actually absorbed in desert air
(warm air holds more water).
Relative and Specific Humidity
Figure 5.14
Relative humidity (RH) as an
indicator of saturation reveals
that desert air is far from
saturated, and that cold polar
air nears saturation.
Graphs of RH contrast with
specific humidity in the deserts
and poles.
Figure 5.9
Sources of Moisture
Figure 5.15
Patterns of US humidity are strongly governed by wind direction
and ocean temperatures.
Cooler Pacific waters create lower humidities in the west, while
warmer Gulf waters generate high humidity along the southeast and
east coast.
Relative Humidity and Comfort
Unsaturated air may absorb more water from the
evaporation of human sweat.
The departure of fast moving, and by definition higher
temperature, water molecules into the vapor phase cools
the human skin.
In winter, this process can make a dry house extra
chilly.
Heat Index & Safety
Human perception of
temperature is distinct
from measured air
temperature, and is
particularly different
at higher humidities
when the human body
is less efficient at
sweating and selfcooling.
Figure 5.16
On hot days, fans that
move saturated air
away from the skin
help humans avoid
unwanted heat
syndromes.
Measuring Relative Humidity
 The simplest tool is the sling psycrometer
 This tool uses two thermometers
 A standard scale is provided in your text in
AppendixD
One is dry the other has a small ‘sock’ a cloth cover
that is soaked with water
When the instrument is spun around, liquid water
evaporates from the ‘sock’ lowering the
temperature… the amount of water that
evaporates indicates the amount of water that
CAN evaporate… thus indicating the relative
humidity.
Sling Psychrometer
Figure 5.17
Wet bulb temperature indicates how cool a surface will become by
evaporating water into the air, and when compared with the dry
bulb, or regular, air temperature it indicates relative humidity.
These two temperatures are measured by this instrument.
Hair & Other Hygrometers
Figure 5.18
Human and horse hair becomes roughly 2.5% shorter as relative
humidity drops from 100% to 0%, which is the principle operating
the hair hygrometer.
Other hygrometers are based on electrical resistance, infrared
absorption, and dew point condensation.
Formation of Dew & Frost
Figure 6.1
Figure 6.2
As air cools to its saturation, or dew point, vapor molecules slow
down and can adhere as dew on the ground surface or as frost
when air temperature drops below freezing.
Daily temperature lows often occur by radiational cooling,
forming dew at night or early morning.
Haze & Water Seeking Nuclei
Figure 6.3
Above the ground surface, cooling and slowing water vapor instead
condenses upon condensation nuclei.
Hygroscopic particles such as salt or dust seek condensing vapor,
and can form a wet white haze when relative humidity is above
75%, or a dry blue haze when drier.
Fog & Human Safety
Foggy days in the US
have a predictable
distribution due to
ocean and mountain
influences.
Fog can help crop
growth in California,
but can also cause
severe automobile,
airplane, and boating
accidents.
Figure 6.8
As a result, there are
several fog dispersal
experiments.
Condensation & Moisture Sources
Figure 6.6
FOG DRIP
Fog that is
filtered by tree
branches, or
condensation of
vapor directly
onto vegetation
or the ground
surface,
provides an
important water
source for
ecological
processes.
Ground Based Radiation Fog
Figure 6.4
Fog is condensed vapor droplets at a density that severely restricts
visibility.
It may form by radiation fog, which occurs at the ground when dew
point temperature is reached by radiational cooling.
Acid fog threatens humans health because the droplets combine
with gaseous pollutants.
Advection Fog
Figure 6.5
Warm moist air that moves, or advects, above a cold surface may
become cooled to its dew point temperature, creating an advection
fog.
This fog often forms above the ocean due to mixing currents, or
when warm ocean air rolls into the cooler waters at the Pacific
Coastline.
Evaporation or Mixing Fog
Fog can form by
mixing warm
unsaturated air
with cool
unsaturated air,
which can occur
during evaporation.
Steam fog is on
example of this
mixing process and
occurs when warm
pools of water are a
source for vapor
that condense into
the cooler air above.
Figure 6.7
Cloud Groups & Types
Clouds are water droplets suspended in the
atmosphere.
Clouds are grouped by their elevation as high, middle,
low, and those that vertically stretch across many
altitudes.
There are several cloud types in these 4 groups.
Condensation onto Nuclei
Condensation of
water vapor into
liquid water is more
likely to occur when
the vapor cools and
slows, and attaches to
nuclei.
Heated vapor moves
so fast that it bounces
away from
condensation nuclei
Common condensation
nuclei in the
atmosphere: dust, salt,
smoke (acid rain)
Figure 5.4A
Cooling the Air….
 Cool air can carry less moisture
 When air is cooled, water vapor condenses
 Condensation forms clouds and is the source of
precipitation.
 When air is lifted, it cools.
 Sources of lifting:
–
–
–
–
Orographic lift
Frontal lifting
Convective lifting
Convergent lifting
Orographic Lift
Jet Contrails
Jet engine
exhaust provides
vapor and nuclei
for condensation
trails (contrails),
which evaporate
quickly in dry
air, but linger
with higher
relative
humidities.
Figure 6.25
P-51 Mustangs of the 325th Fighter Group Fly top cover for B-24s en
route to targets in German occupied Europe (photo by John Gaston)
The much faster fighters flew long curving flight paths in order to stay back with the
bombers they were tasked to escort.
Summary of Cloud Types
Figure 6.20
Stratocumulus Clouds
Figure 6.15
Low clouds with rounded patches that range in color from light to
dark gray.
With your hand extended overhead, they are about the size of your
palm.
Cirrus Clouds
High clouds (above 6000 m in middle
latitudes) that are thin and wispy
and comprised mostly of ice crystals.
Figure 6.9
Cirrocumulus Clouds
High clouds
that are
rounded
puffs,
possibly in
rows, are
less common
than cirrus.
Figure 6.10
Cirrostratus Clouds
High clouds
that thinly
cover the
entire sky
with ice
crystals.
Light
passing
through
these
crystals may
form a halo.
Figure 6.11
Altocumulus Clouds
Middle clouds
(between 2000
and 7000m in
middle latitudes)
that are puffy
masses of white
with gray edges.
Figure 6.12
With your hand
overhead, they
are about the
size of your
fingernail.
Altostratus Clouds
Middle
clouds that
cover the
entire sky
and may
create a
dimly visible
or watery
sun and
diminish
formation of
shadows.
Figure 6.13
Nimbostratus Cloud
Low clouds
(below 2000m)
with
precipitation
that reaches the
ground.
Figure 6.14
Shredded parts
of these clouds
are called
stratus fractus
or scud.
Stratus Clouds
Figure 6.16
Low clouds that resembles a fog, but does not reach the ground, and
can generate a light mist or drizzle.
Cumulus Humilis Clouds
Figure 6.17
Clouds with vertical development that take a variety of shapes,
separated by sinking air and blue sky.
Shredded sections are called cumulus fractus.
Cumulus Congestus Clouds
Figure 6.18
Clouds with vertical development that become larger in height, with
tops taking a ragged shape similar to cauliflower.
Pileus Cloud
An unusual
cloud that
forms above
a building
cumulus by
deflected
moist winds.
Figure 6.23
Mammatus Clouds
Figure 6.24
An unusual cloud that hang like sacks, formed by sinking air with a
high water content.
Nacreous Clouds
Figure 6.26
An unusual cloud best viewed at winter in the poles and forms in the
stratosphere.
Noctilucent Clouds
Figure 6.27
An unusual wavy cloud that is best viewed at the poles and forms in
the upper mesosphere.
Ground based Viewing of Clouds
Figure 6.28
Clouds directly overhead will appear to be less densely packed than
clouds at the horizon due to viewing angles.
Lenticular Clouds
An unusual
cloud that
has a lens
shape and
forms in the
crest of a
wave.
Figure 6.21
Figure 6.22
A lenticular cloud that forms downwind of a mountain peak and is
regularly replenished by condensing water vapor.
Photo by Chris Terry