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The role of water in the
atmosphere
Terms and definitions for
atmospheric moisture
The impact of moisture
on comfort
Formation of dew, frost,haze, fog, and clouds
Classification of fog and
cloud types
Humidity, Condensation, and CloudsHumidity, Condensation, and Clouds
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Distribution of Water on EarthDistribution of Water on Earth
1.3109 km32.8107 km3
8.1106 km3
0.2106 km3
1.3104 km3
0.6103 km3
Total hydrosphere 1.4109 km3
OceansGlaciers
Ground water
Lakes/rivers
Atmosphere
biosphere
(=97.3%)
(=2.7%)(=0.001%)
The atmosphere contains only ~ 1 week supply of precipitation!
Oceans
Glaciers
Groundwater
Lakes/rivers
atmosphere
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Most abundant trace gas; Large variability:
0 4%
Large amounts are found close the surface, decreasing quickly aloft.
Properties of WaterProperties of Water
Winter Summer
Altitude [hPa]Altitude [hPa]
5 km
1.5 km
Global distribution of H2Oin the northern hemisphere
[g H2O/ kg air]
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Most abundant trace gas; Large variability: 0 4%
Large amounts are found close the surface, decreasing quickly aloft.
Source: evaporation from the ocean and land. Sink: precipitation.
Properties of WaterProperties of Water
Hydrologic Cycle
Evaporation & transpiration by plants liquid water into atmospheric vapor.
Condensation converts water vapor back to a liquid droplet, which maythen fall as precipitation to ground or surface water supplies.
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Most abundant trace gas; Large variability: 0 4%
Large amounts are found close the surface, decreasing quickly aloft.
Source: evaporation from the ocean and land. Sink: precipitation.
Surface Tension: highest of all common liquids
Properties of WaterProperties of Water
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Most abundant trace gas; Large variability: 0 4%
Large amounts are found close the surface, decreasing quickly aloft.
Source: evaporation from the ocean and land. Sink: precipitation.
Surface Tension: highest of all common liquids Heat Capacity: highest of all common solids and liquids (1 cal g-1 C-1)
Properties of WaterProperties of Water
c
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Most abundant trace gas; Large variability: 0 4%
Large amounts are found close the surface, decreasing quickly aloft.
Source: evaporation from the ocean and land. Sink: precipitation.
Surface Tension: highest of all common liquids Heat Capacity: highest of all common solids and liquids (1 cal g-1 C-1)
Most important greenhouse gas
Radiative Properties:
- transparent to visible wavelengths
- virtually opaque to many infrared wavelengths
- large range of albedo possible
Properties of WaterProperties of Water
water 10 % (daily average)
Ice 30 to 40%
Snow 20 to 95%
Cloud 30 to 90%
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Structure of Water
Water's unique molecular structure and hydrogen bonds enable
all 3 phases to exist in earth's atmosphere.
Properties of WaterProperties of Water
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Structure of Water
Also: hydration possible
Hydration
H2O permanent dipol: H2O taken
up by ions
particle
growth
increasingwater content
of particles
Properties of WaterProperties of Water
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Properties of WaterProperties of Water
Water on earth exists in all 3 phases
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Phase changes of water:
Properties of WaterProperties of Water
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Describing atmospheric moisture
Atmospheric water vapor can
be defined in different ways,
including
absolute humidity v,
specific humidity q,
mixing ratio r,
vapor pressure e,
dew point temperature Td, relative humidity RH
Atmospheric MoistureAtmospheric Moisture
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Absolute Humidity v, Specific Humidity q, Mixing ratio r
Specific humidity =
mass of water vapor/total mass of air
Specific humidity q is not affected
by changes in parcel volume.
Vmvv =
a
v
m
mq=
Mixing ratio = mass of watervapor/mass of dry air
usually, q r
d
v
m
mr =
Absolute humidity =
mass of water vapor/volume of air
For a given mass of water vapor
in an air parcel, the absolute
humidity v
changes as the
parcel volume changes
(e.g., lifts or descends).
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Vapor pressure e:
Air molecules all contribute to air pressure p
Each subset of molecules (e.g., N2, O2, H2O) exerts a partial pressure
The vapor pressure, e, is the pressure exerted by H2O vapor molecules in air
the larger the vapor pressure is, the more H2O vapor molecules in air
2-30 mb common at surface
Atmospheric MoistureAtmospheric Moisture
= q qa pp
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In general: water molecules move between the liquid and gas phases
Atmospheric MoistureAtmospheric Moisture
Unsaturated air
Saturation vapor pressure es:
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In general: water molecules move between the liquid and gas phases
Saturated air (e = es):
for every water molecule evaporated into the air, a vapor molecule wouldcondense to liquid water (equilibrium)
in this case: water vapor pressure (e) = water saturation vapor pressure (es)
The air can not hold more water vapor
Atmospheric MoistureAtmospheric Moisture
Unsaturated air Saturated air
Saturation vapor pressure es:
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Condensation, Evaporation, Saturation Vapor Pressure (SVP)
Number of molecules escaping particle = number of molecules entering particle
in this case: partial gas pressure = saturation vapor pressure (SVP)
partial gas pressure > SVP SVP > partial gas pressure
Atmospheric MoistureAtmospheric Moisture
EQULIBRIUM
What happens, if there is no equilibrium?
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Note: Water saturation vaporpressure decreases with altitude
Atmospheric MoistureAtmospheric Moisture
Saturation vapor pressure es:
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Relationship between temperature and saturation vapor pressure:
Clausius-Clapeyron equation
Saturation vapor pressure es increases exponentially with temperature
At higher T, faster water molecules in the liquid escape more frequently
causing saturation water vapor amount to rise
We sometimes say: Warmer air can hold more water vapor
es [kPa] = saturation vapor pressure
L [J Kg-1] = latent heat of vaporization or deposition
e0 [kPa] = 0.611 kPa at T0=273.15 K
Rv
[J K-1 kg-1] = gas constant for water vapor
T [K] = temperature
= )
11(exp
0
0TTR
Lee
V
s
Atmospheric MoistureAtmospheric Moisture
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Relationship between temperature and saturation vapor pressure:
Clausius-Clapeyron equation
Atmospheric MoistureAtmospheric Moisture
Note:
Es(water) > Es(ice) at all times!
this difference is reason for rain-
drop growth in cumulus clouds
Evaporation
condensation
partial gas pressure > SVP
SVP > partial gas pressure
partial gas pressure > SVP SVP > partial gas pressure
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Atmospheric MoistureAtmospheric Moisture
Dew-Point Temperature Td:
Temperature to which air must becooled to become saturated
(at constant pressure and water vapor content).
Always: Td T
Difference between relative humidity (RH) and dew-point temperature Td:
RH: measure of how close the air is to saturation,
Td : measure of the airs actual moisture content.
The higher Td
, the more water vapor in the air.
Dew point depression: T- Td
The largerthe dew point depression is, the drierthe air is, or the air is
farther away from saturation
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Atmospheric MoistureAtmospheric Moisture
Dew-Point Temperature Td:
Average surface dew-point temperature (F)
January July
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Dew-Point Temperature Td:
Td [K] = dew-point temperature
L [J Kg-1
] = latent heat of vaporization or depositione0 [kPa] = 0.611
Rv [J K-1 kg-1] = gas constant for water vapor
T0 [K] = 273.15
1
00 )ln(
1
=
e
e
L
R
TT
sV
d
Atmospheric MoistureAtmospheric Moisture
= )
11
(exp 00
dVs TTR
L
ee
Dew-point temperature Td as function of saturation vapor pressure esThe higher Td, the more water vapor in the air.
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Relative humidity (RH):
The ratio of the actual amount of water vapor in the air compared to the
maximum amount of water vapor the air can hold.
RH [%] = relative humidity
es, qs, s, rs = pressure, specific humidity, absolute humidity,
mixing ratio of water vapor at saturation vapor
pressure
100100100100 ===ssss r
r
q
q
e
eRH
Atmospheric MoistureAtmospheric Moisture
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Relative Humidity Trends
RH indicates air parcel proximity to saturation.
Higher RH does not necessarily mean more water vapor in the air
RH increases by adding more wateror dropping T.
Dew point Td is the temperature at which saturation occurs (i.e. RH=100%).
Atmospheric MoistureAtmospheric Moisture
Td ?
RH=100% ?
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A common misconception:
Air with high RH must have a greater water vaporcontent than air with lower RH.
Which place has more water vapor in the air?
International Fall, Minnesota:
T= - 10oC and RH = 100%
or
Phoenix, Arizona:T= 20oC and RH = 30%
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14.0020
10.0015
7.0010
5.005
3.500
2.00-10
0.75-20
Saturation mixing ratio rs(g/kg)
Temperature
(C)
International Fall:
T = -10 oC; rs = 2 g/kg,
RH = 100% = r / rs
Actual mixing ratio r:
r = RH x rs = 1 x 2 g/kg
= 2 g/kg
Phoenix:
T = 20 oC; rs = 14 g/kg,
RH = 30% = 0.3 = r / rs
Actual mixing ratio r:
r = RH x rs= 0.3 x 14 g/kg
= 4.6 g/kg
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Specific Humidity vs. Saturation
Atmospheric MoistureAtmospheric Moisture
Which environment has
higher water vapor inthe air:
Desert air or polar air?
While RH may be higher
in polar air (or duringwinter-time), more wateris actually absorbed in
desert air (or during
summer).
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Specific Humidity vs. Saturation
Warm air can absorb more vapor than cold
air, so for a given parcel of air, specific
humidity declines from its highest in the
tropics to its lowest in the colder poles.
Desert air is far from saturated, cold polar
air nears saturation.
Atmospheric MoistureAtmospheric Moisture
Specific Humidity(latitudinal distribution)
Relative Humidity(latitudinal distribution)
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Specific Humidity vs. Saturation
Relationship between
specific humidity and relative
humidity and its dependanceon air temperature
Atmospheric MoistureAtmospheric Moisture
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Exercise:
a chilly morning in Houston:
Temperature: 10oC; RH = 100%.
In the afternoon, the air warms to 30oC
Assumption:
Same air mass has been in the area, i.e. themoisture content in the air has not changed
much from the morning to the afternoon.
Would RH increase or decrease in the afternoonfrom the morning?
What would be the RH in the afternoon?
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emorning = RHmorning x es = 100% x es
What is the vapor pressure, e,
in the morning?
at 10C, es = 12 mb
100=
se
eRHHouston morning:
T=10C; RH=100%
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emorning = RHmorning x es = 100% x es
emorning = RHmorning x es = 12 mb
What is the vapor pressure, e,
in the morning?
at 10C, es = 12 mb
100=
se
eRHHouston morning:
T=10C; RH=100%
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at 30C, es = 42 mb
emorning = RHmorning x es = 100% x es
emorning = RHmorning x es = 12 mb
What is the vapor pressure, e,
in the morning?
at 10C, es = 12 mb
RHafternoon = (emorning/es afternoon) x 100
= (12 mb / 42 mb) x 100
= 28%
100=
se
eRH
In the afternoon:
Houston morning:
T=10C; RH=100%
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Sources of Moisture
Atmospheric MoistureAtmospheric Moisture
Why is the southwest coast of the US hot and dry,while the Gulf coast is hot and moist?
Both are adjacent to large bodies of water
Both experience onshore wind flow on a regular basis
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Sources of Moisture
Atmospheric MoistureAtmospheric Moisture
CoolerPacific waters create
lowerhumidities along the W coast
warmerGulf waters generate high
humidity along the SE and E coast.
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Relative Humidity and Comfort
Unsaturated air (low RH) 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 (effect increases with
decreasing RH).
Air close to saturation (high RH) lowers cooling of human skin due to less
evaporation.
Atmospheric MoistureAtmospheric Moisture
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Heat Index & Safety
Atmospheric MoistureAtmospheric Moisture
Human perception of
temperature is distinct
from measured air
temperature, and is
particularly different at
higher RH when the human
body is less efficient at
sweating and self-cooling.
On hot days, fans that
move saturated air away
from the skin help humans
avoid unwanted heat
syndromes.
Apparent temperature
[Heat Index] takes into
account ambient air
temperature with RH.
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Heat Index & Safety
Atmospheric MoistureAtmospheric Moisture
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Sling Psychrometer (also aspirated Psychrometer available)
Wet bulb temperature indicates how cool a surface will become by
evaporating water into the air. When compared with the dry bulb, or regular air temperature (T), it
indicates relative humidity (RH) and dew point (Td).
Measurement of Atmospheric MoistureMeasurement of Atmospheric Moisture
Why necessary to whirl or aspirate a psychrometer?
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Hair & Other Hygrometers
Human and horse hair becomes roughly 2.5% shorter as RH drops from
100% to 0% (used as the principle of the hair hygrometer). Other hygrometers are based on electrical resistance (humidity changes
resistance), infrared absorption (by water vapor), and dew point
condensation (cooling surface of a mirror until condensation forms).
Measurement of Atmospheric MoistureMeasurement of Atmospheric Moisture
http://catalogue.museogalileo.it/multimedia/Hygrometer.html
S f i t i bl
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Summary of moisture variables
Vapor pressure the pressure exerted by the water vapor moleculesin a given volume of air
Specific humidity the ratio of mass of water vapor in a given
air parcel to the total mass of air in the parcel
Mixing ratio - the ratio of mass of water vapor in a given air
parcel to the total mass of dry air in the parcel
Relative humidity The ratio of the amount of water vapor in
the air compared to the amount of required for saturation
Dew point temperature - Temperature to which air must becooled (at constant pressure and constant water vapor content)
to become saturated
Wet bulb temperature - The lowest temperature that can be
obtained by evaporating water into the air.
Absolute humidity the density of water vapor which is the ratio
of mass of water vapor and the volume of air.
SUMMARYSUMMARY
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SUMMARY
1. Saturation exists when the number of water molecules evaporating from a
liquid equals the number condensing.
2. In our atmosphere, condensation occurs primarily when the air is cooled.
3. Absolute humidity describes the mass of water vapor in a fixed volume of air,
or the water vapor density.
4. The airs actual (water) vapor pressure is an indication of the airs water vapor
content.
5. Relative humidity, expressed as a percent, does not tell us how much water
vapor is actually in the air, rather it tells us how close the air is to being
saturated.
6. Without changing the airs water vapor content, as air cools the relativehumidity increases, and as air warms the relative humidity decreases.
7. The dew-point temperature is a good indicator of the airs water vapor content.
High dew points indicate high water vapor content and vice versa.
8. When the air temperature and dew point are close together, the relative
humidity is high; when they are far apart, the relative humidity is low.
9. Summertime is generally more humid in the eastern half of the United States
because of the air flow off the warm Gulf of Mexico.
10. High relative humidity in hot weather can make us feel it is hotter than it
actually is by retarding the evaporation of perspiration