Download saturation vapor pressure

Water cycle in the environment
Role of water in the
atmosphere
• Clouds and precipitation
(weather)
• Impact on radiative balance
• Impact on thermodynamic
processes and vertical stability
of the atmosphere through
evaporation and
condensation.
• Cleaning of the atmosphere
• Participation in chemical
processes.
Water cycle in the environment
type
volume (km3)
surface water
230 580
underground
water
8 406 720
ice
water vapor in
the atmosphere
29 190 000
12 900
share (%)
0.0171
0.625
2.15
0.001
oceans
1 321 890 000
97.2
sum
1 359 700 000
100.0
Obieg wody w przyrodzie
Moisture Variables
• There are numerous ways to quantify the
amount of water vapor in the air.
• 1. Vapor pressure e - partial pressure of
water vapor. Also a saturation vapor
pressure. This is the fundamental way to
measure the amount of vapor.
• 2. Vapor density ρv - defined by equation
of state for vapor. Also called absolute
humidity
The absolute humidity changes as air rises and descends
with the same amount of water vapor in the
parcel of air, an increase in volume
decreases the absolute humidity, while an
decrease in volume increases the absolute
humidity
parcel size
H2O vapor
content
absolute
humidity
2 m3
10 g
5 g/m3
1 m3
10 g
10 g/m3
• 3. mixing ratio r - mass of water vapor per unit
of dry air. Usually expressed in g/kg, but most
correct when unitless (i.e. kg/kg). Typical range
for globe = 0-25 g/kg.
• 4. specific humidity q – mass of water vapor per
unit mass of moist air,
mv Rd
e
e
r


 0.622 
md Rv p  e
pe
mv
Rd
e
e
q


 0.622 
md  mv Rv p  (1  Rd Rv ) e
p
where: Rd=287.05 J·kg-1·K-1 – gas constant for dry air
Rv=461.51 J·kg-1·K-1 – gas constant for water vapor
The absolute humidity and mixing ration do not
change as air rises and descends
H2O vapor
parcel weight
weight
specific
humidity
1 kg
1g
1 g/kg
1 kg
1g
1 g/kg
• The amount of water vapor the atmosphere can
‘hold’ is limited. This limit is given by the
saturation vapor pressure, which is an
exponential function of temperature only.
• When SVP is equaled or exceeded, vapor moves to
the liquid phase and latent heat is released. The
possibility of supersaturation exists, but is only
observed in atmosphere to a small degree
Reaching Saturation
• There are several different
processes that a parcel of air may
undergo in order to reach
saturation
• These processes define
temperatures which can be used to
indicate the amount of moisture in
the air
What happens at saturation?
• Though not entirely obvious, water begins to
condense.
• As vapor condenses, latent heat is released
• Lapse rate of parcel changes dramatically.
phase changes of water
•
•
•
evaporation / condensation (Lp=2462·103 J ·kg-1)
icing / melting
(Lt = 334·103 J ·kg-1)
sublimation / resublimation (Ls=2834·103 J ·kg-1)
specific heat of water: 4·103 J ·kg-1
Water vapor pressure
The total pressure inside the parcel of air is equal to
the sum of pressures of the individual gases. The
partial pressure of water vapor is called the actual
vapor pressure e.
Actual vapor pressure is measured in hPa, kPa or mb.
The greatest values of actual vapor pressure are
observed near equator, where they can reach 40 hPa.
On average at 2 m 20 hPa.
Low values of actual vapor pressure are observed
are observed near the poles in winter. Usually it
doeas not exceed 2 hPa. Extremely low values were
noted on Syberia in Vierchojansk, where during
winter the actual vapor pressure can drop even to
0.03 hPa
Relative Humidity
• 5 Relative Humidity RH --Ratio of mixing ratio
to its saturation value. Expressed in percent
e
f  100%
E
• This is what many instruments measure
Relative Humidity
• most common
measure, but not as
useful as many others
• Function of two
variables –
temperature and the
amount of water
vapor
Virtual Temperature
• 6 Virtual Temperature TV – temperature of dry
air having the same density as that of a sample
of moist air at the same pressure
Tv = (1 + 0.61q) · T
Dewpoint
• 7 Dewpoint Td - temperature to which moist air
must be cooled, holding p and w constant, in
order to reach saturation wrt water.
B
Td  Td w, p  
ln  A / wp 
• ws at dewpoint = w of moist air.
Water vapor in the atmosphere
Earth surface is a main source of water
vapor in the atmosphere. That is why the
greatest amount of water vapor is observed
in the lower troposphere.
Water vapor content near the surface
amounts on average 0.2 % near the poles
and 2.5% near the equator.
In some cases in can reach even 4%
Changes of water vapor content
with the height
Above boundary layer the water vapor content decreases
with the height ezponentialy (faster than other
components of air).
At altitude 13-20 km in temperate region the content od
water vapor is about 10-6 (mass of water vapor per unit of
mass of dry air).
Above 20 km water vapor content increases slightly with
the heightnand at 23-50 km the pearl clouds are observed
build from ice crystals.
Above 70 km water molecules break up because of solar
radiation at wavelenghts 0.1657 m and water vapor
content decreases to 10-8 ..
Trace amount of water vapor are observed up to 90 km.
Saturation water vapor pressure
For each temperature there is an amount of water vapor
saturated the air – E.
If water vapor pressure becomes greater than
saturation water vapor pressure then the condensation
occurs. The condensation nucleus are necessary. So
sometimes it can happen that e > E.
Saturation water vapor pressure is:
- greater for concave surfaces than for the flat ones
- lower for roztworów soli than for pure water
Saturation water vapor pressure
Clausius-Clapeyron equation describes the impact of
temperature on saturation water vapor pressure
L
E  E0  exp 
 Rv
 1 1 
    
 T0 T  
where E0=6.11 hPa, T0=273 oK.
Ratio L/Rv is different for
supercooled water (5423oK)
and ice (6139oK) (because of
differences in heat of evaporation
and heat of sublimation).
that is why saturation water
vapor presssure is greater for
supercooled water than for ice.
Mixing ratio and specific humidity
at saturation
r
q
1 r
and
q
r
1 q
Because usually r,q < 0.04 than r  q
Absolute humidity of saturated air
Other characteristic of humidity
Saturation deficit, d, it is a difference between the
saturation vapor pressure maksymalnym at given
temperature E and actual pressure e:
d=E–e
Dew point temperature,  (or Td), temperature to which
it is necessary to cool the air to saturate it relative to flat
surface.
Dew point deficit, , the difference between given
temperature T and the dew point temperature Td :
 = T – Td
Precipitable water
Precipitable water is the total content of water in the
atmosphere in the of the air. It is equal to the layer of
water if all the water from the atmosphere condense near
the surface. It is measured in kg·m-2 or w mm.
Annual course of precipitable water over Central Poland
Precipitable water
styczeń
Mean values of
precipitable
water in the
period 19582003 in
[kg/m2] ([mm])
lipiec
Daily humidity course
Daily course of relative
humidity is oposite to the
course of temperature
with one maximum and
one minimum.
In daily course of vapor
pressure and specific and
absolute humidity two
maximas and two
minimas can be
distinguished.
• morning minimum is
caused by temperature
• afternoon minimum is
caused by convection.
This minimum is not
observed at the seaside
and in the mountains.
Dobowy przebieg różnych charakterystyk
wilgotności na stacji Łódź-Lublinek – wartości
uśrednione z 74 letnich dni z pogodą radiacyjną
f 
e
E
Annual course of humidity
Annual course of water vapor pressure (as well as
absolute humidity, specific humidity and mixing
ratio) is parallel to the annual course of
temperature; in summer the content of water vapor
is the greatest and in winter the smallest. Is is
caused by the relation of E (saturation vapor
pressure) on temperature.
Annual course of the relative humidity is oposite to
the course of temperature. But in monsun regions
the relative humidity is much greater during
summer than in winter. It is related to different
features of air mases approaching these regions in
winter and summer.
Evaporation
Evaporation takes place were the body change phase
from liquid to gasous. It happens in each temperature.
The evaporation from plants is called transpiration.
Potential evaporation (or evaporative capacity) is the
maximum possible evaporation at given temperature, not
restricted by the amount of water.
Actual evaporation is an amount of water which really
evaporate.
The rate of evaporation Fw is measured in kg·m-2·s-1 or
mm·day-1 and is:
- proportional to saturation deficit (E-e),
- opposite proportional air pressure p,
- relate to shape of surface of evaporating body (coefficient
A),
- related to wind speed (function f(v))
E e
Fw  A 
 f (v )
p
CLOUDS
A cloud is a visible aggregate of tiny water droplets or
ice crystals suspended in the air.
criterions of classification
 composition
 way of developing
 appearance
Composition:
Liquid clouds are built from water droplets only. They
develop in above 0°C temperatures as well as at
temperatures slightly lower than 0°C .
Ice clouds are composed from ice crystals only. They
exist in temperatures below -40°C .
Mixed clouds are composed from both water droplets
and ice crystals. They exist in temperatures below -0°C
but above -40°C .
Appearance:
According to International Cloud Classification ten
principal cloud froms are divided into four groups. Each
group is identified by the height of the cloud's base above
the surface: high clouds, middle clouds and low clouds .
The fourth group contains clouds showing more vertical
than horizontal development.
This system was introduced by Abercromby and
Hildebrandsson who expanded the original Howard
classification.
Cloud types
Cirrus (Ci)
Cirrostratus (Cs)
high clouds
Cirrocumulus (Cu)
Altostratus (As)
Altocumulus (Ac)
middle clouds
Nimbostratus (Ns)
Stratocumulus (Sc)
Stratus (St)
Cumulus (Cu)
Cumulonimbus (Cb)
low clouds
clouds with vertical
development
Approximate height of cloud base
polar
temperate
tropical
high clouds
3-8
5 - 13
6 - 18
middle clouds
2-4
2-7
2-8
low clouds
<2
< 2
<2
Cloud types
CIRRUS
thin wispy cloud blown by high winds
into long streamers called mares'
tailes. They are build from ice crystals
only. They do not give any
precipitation.
CIRROCUMULUS
Appear as small, founded, white puffs that may occur individually or in long rows.
When in rows they have a rippling appearance that distinguishes them from the
silky look of cirrus and sheetlike look of cirrostratus.
CIRROSTRATUS
The thin, sheetlike high clouds that often cover the entire sky. They are so
thin that the moon or sum can be seen through them. They are composed
from ice crystals, so refract the light passing through them producing a
halo.
ALTOSTRATUS
Middle level cloud. It is
a grey or blue-grey
(never white) cloud that
covers oten the entire
sky. In thinner parts the
sun (or moon) can be
dimly visible
(translucidus). If the
cloud is thick (opacus)
than the sun light could
not be seen through the
cloud.
ALTOCUMULUS
Middle clouds that appear as gray,
puffy masses sometimes rolled
out in parallel waves or bands.
The sky can be seen
between individual particles
of the cloud.
NIMBOSTRATUS
Dark gray "wet " – looking cloud layer associated with more or less continuous
precipitation. The intensity of precipitation is usually low to moderate. The base
of the cloud is usually difficult to define
STRATUS
A uniform grayish cloud that often covers the entire sky. Its base is very low
over the groud sometimes resembling the fog. It gives no precipitation or drizzle
nebulosus (St neb – foggy,
murky) – foggy, uniform
curtain, without any details;
fractus (St fra – frayed)–
cloud with unregular shape
and frayed appearance
STRATOCUMULUS
A low lumpy cloud layer. It appears in
rows , in patches or as rounded
masses with blue sky visible betwen
individual cloud elements. The color of
stratocumulus ranges from light to
dark gray.
the processes of
saturation:
• evaporation,
• cooling
• mixing
Clouds appear when air becames supersaturated
Usually it happens when air mass ascends and cool.s
Ascending air mass expands and its temperature drops.
When temperature is decreasing, its relative humidity is increasing
up to the moment the air becames saturated (relative humidity
=100%)
When rising of air parcel continues part of water vapor condences
or resublimates.
Water vapor condences on small particles called aerosol. If the
aerosol is hydroscopic than condensation or resublimation c an
happen at saturation lower than 100%
Convective clouds
Orographic clouds
Orographic clouds
moist
air
condensation level 3
dry air
moist
air
condensation level 2
dry air
condensation level
Frontal clouds
Ci
Cs
As
Ns
500-1000 km
Convective clouds made by convergence
Fogs
radiative
advective
orographic
from evaporation
Many Deserts Are Located Where a High Percentage of
the Precipitation Moisture Originates Over Land – These
Areas are Not Supplied With Much Moisture
Climate feedbacks
• Water vapor feedback
• Ice/snow albedo feedback
• Cloud feedback
Water vapor feedback
Surface
temperature
Atmospheric
H2O
(+)
Greenhouse
effect
 Positive feedback loop
Snow/ice albedo feedback
Surface
temperature
Snow and ice
cover
(+)
Planetary
albedo
 Another positive feedback loop
What about clouds?
Some reflection
10 km
Cirrus clouds
(Thin)
More reflection
Altitude
Cumulus/stratus clouds
(Thicker)
What about clouds?
 Tc 4
10 km
Cirrus clouds
High and cold
 Tc 4
Altitude
Cumulus/stratus clouds
Tw4
Low and warm
Tw4
Tc
Temperature
Tw
Ts4
Ts
What about clouds?
•
Cumulus and stratus clouds
–
–
–

•
Low and warm
Small greenhouse effect
Big effect on albedo
These clouds cool the climate
Cirrus clouds




High and cold
Large greenhouse effect
Smaller effect on albedo
These clouds warm the climate
Cloud feedback
• Most models predict that cloudiness should
increase as the climate warms
– If low clouds increase the most, then the feedback will
be negative
– If high clouds increase the most, then the feedback
will be positive
• The balance of evidence suggests that cloud
feedback is negative. However, this is highly
uncertain, as clouds are sub-grid-scale in size
and are therefore difficult to model.
: orbit-net.nesdis.noaa.gov/arad/ gpcp/maps/frontmap.gif