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Lecture 8Daily temperatureVapor pressure
Saturation vapor pressureDew point temperature
Relative Humidity
2
Daily temperature
Daily Temperature VariationsQ: When is the minimum
temperature during a day?
Q: When is the maximum temperature during a day?
E=sT4
because Stefan-Boltzmann law
Outgoing LW radiation emitted by the earth should be similar to the daily temperature, why?
Net radiation
Whylag?
Q: Why there is a lag between maximum incoming SR and
temperature? Q: What determines
temperature variations?
Net radiation Net R
Net R=Incoming SW- Outgoing LWif Net R >0,
surface is warmingif Net R <0,
surface is cooling
+Net R
-Net R
-Net R
Similar reason for why the warmest period of a year is in July/August and not on June 21 (summer solstice)
Pressure = Force per unit area
the weight of the column of air will be about 14.7 lbs.
Sea-level pressure is also given in other units:14.7 lbs/inch2 =1013.25 millibars (mb)
Hence, the pressure at sea level is = force/area = 14.7 lbs/inch2
Lecture 1 - 7
Air pressure = total air weight per unit area of the atmospheric column above z
High pressure
Low
pre
ssur
e
Dalton's law
Total pressure exerted by a gaseous mixture is equal to the sum of the partial pressures of each individual component in a gas mixture
2 2 2...H O CO Np p p p
2
2
2
2
2
2
total pressure of the gas mixture
partial pressure of water vapor (H O)
partial pressure of carbon dioxide (CO )
partial pressure of nitrogen (N )
H O
CO
N
p
p
p
p
P = P1 + P2 +P3
For example
Gas mixture
P
=
Per unit area
Gas 1P1
Gas 3P3
Gas 2P2
Per unit area Per unit areaPer unit area
Example Given a parcel of air comprised of only nitrogen, oxygen, and water vapor
N2 78%O2 21%H2O1%
PTotal = PN2+ PO2 + PH2O1000 mb = 780 mb + 210 mb + 10 mb
Fig. 11-9, p. 306
No water condensationNo weather
Under what conditions does condensation occur?
Temperature = Dew point temperature
Relative humidity = 100%
Relative Humidity
NYC daily Weather
Temperaturedew point temperature
Relative humidity
Wind
Rain
Temperature = 43oF Dew point = 31oF
Relative humidity
How much water vapor can be held in
the air?
What determines the capacity?
0 5 10 15 20 25 30 35 40 45 500
10
20
30
40
50
60
70
80
90
100
110
120
Temperature (oC)
Vap
or p
ress
ure
PH
2O
(m
b)
Vapor Pressure 2H Op
Water
Dry Air
T=30°C
Total weight of water vapor per unit area
20H OP
Saturation Vapor Pressure P*H2O
0 5 10 15 20 25 30 35 40 45 500
10
20
30
40
50
60
70
80
90
100
110
120
Temperature (oC)
Vap
or p
ress
ure
(m
b)
Vapor Pressure 2H Op
20H OP Water
Moist Air
H2Ovapor
T=30°C
evaporation
Total weight of water vapor per unit area
Saturation Vapor Pressure P*H2O
0 5 10 15 20 25 30 35 40 45 500
10
20
30
40
50
60
70
80
90
100
110
120
Temperature (oC)
Vap
or p
ress
ure
(m
b)
2
*H OP
2
*H OP
Saturation Vapor Pressure 2
*H oP
Total weight of water vapor per unit area
T=30 oCSaturation: balance between the number of water molecules entering and leaving the water surface.
=
Air is “saturated”
2H OP
2
17.27* 273.3( ) 0.6108
T
TH OP T e
Q: what does this red curve tell us?
0 5 10 15 20 25 30 35 40 45 500102030405060708090100110120
Temperature (oC)
Satu
rati
on
vap
or
pre
ssu
re (
mb
)
T=30 oC
T=44 oC
T=10 oC
This curve indicates how much water vapor can be held in the air at a given temperature
2
*H OP
Temperature determines the capacity of holding water vapor in the air
2
17.27* 273.3( ) 0.6108
T
TH OP T e
20
Why does Hurricane always generate in
these areas?
Hurricane = Typhoon = Tropical CycloneAtlantic and eastern Pacific: hurricaneWestern Pacific: typhoonIndian Ocean & Australia: tropical cyclone
0 5 10 15 20 25 30 35 40 45 500
10
20
30
40
50
60
70
80
90
100
110
120
Temperature (oC)
Satu
rati
on
vap
or
pre
ssu
re (
mb
)
2
*H OP
Relative Humidity
Total weight of water vapor per unit area
T=30 oCSaturation: balance between the number of water molecules entering and leaving the water surface.
=
Air is “saturated”2
*H OP
2H OP
2
2
*100%H O
H O
PRH
P
0 5 10 15 20 25 30 35 40 45 500
10
20
30
40
50
60
70
80
90
100
110
120
Temperature (oC)
Satu
rati
on
vap
or
pre
ssu
re (
mb
)
2
*H OP
Saturation Vapor Pressure 2
*H oP
Total weight of water vapor per unit area
T=30 oCSaturation: balance between the number of water molecules entering and leaving the water surface.
2
2
*100%H O
H O
PRH
P =
Air is cooled toT=20 oC2H OP
Dew point
2
*H OP
Dew point temperature
Temperature to which a given parcel of air must be cooled for water vapor to condense into water.
0 10 20 30 40 500
10
20
30
40
50
60
70
80
90
100
110
120
Satu
rati
on v
apo
r pr
essu
re (
mb
)2
2
*100%H O
H O
PRH
P
2
1H OP
2
*H OP
Tdew Dew point
o10 C
2
2H OP
o26 C o34 C
2
3H OP
NYC daily Weather
Temperature indicates the capacity of holding water vapor in the air
Dew point temperature indicates how much water vapor is in the airRelative humidity
0 5 10 15 20 25 30 35 40 45 500102030405060708090100110120
Temperature (oC)
Satu
ratio
n va
por p
ress
ure
(mb)
This curve indicates how much water vapor can be held in the air at a given temperature
( )se T17.625
( ) 6.1exp243.04s
Te T
T
T=10 oCT=30 oC
T=44 oC
Hot weatherHot weather The atmosphere can hold more water vapor.
The atmosphere can hold more water vapor.
The water-holding capacity of the atmosphereThe water-holding capacity of the atmosphere
Increased flooding
Increased flooding
Global warmingGlobal warming
It takes longer to recharge
It takes longer to recharge
It releases more latent heat
It releases more latent heat
It produces heavier rainsIt produces
heavier rains
More violentMore
violentIncreased
droughtIncreased
drought
Extreme weatherExtreme weather
increased
The water-holding capacity of the
atmosphere
The water-holding capacity of the
atmosphereincreased
The carbon-storage capacity of the
terrestrial biosphere
The carbon-storage capacity of the
terrestrial biosphere
Global warming
Global warming
Extreme weather
Extreme weather
increased
altering
Feedback by changing atmospheric CO2
How ?
