Water
 Most
abundant
 71% of surface
 Hydrologic cycle
Phase changes
-transfers of latent energyGas (water vapor)
Solid (ice)
Liquid
Phase changes
-transfers of latent energyGas (water vapor)
Solid (ice)
Liquid
Phase changes
-transfers of latent energyGas (water vapor)
LATENT HEAT
OF
VAPORIZATION
+ 597 cal/gm
Solid (ice)
Liquid
Phase changes
-transfers of latent energyGas (water vapor)
LATENT HEAT
OF
VAPORIZATION
+ 597 cal/gm
Solid (ice)
Liquid
melting
Phase changes
-transfers of latent energyGas (water vapor)
LATENT HEAT
OF
VAPORIZATION
+ 597 cal/gm
Solid (ice)
Liquid
melting
+ 80 cal/gm
LATENT HEAT OF FUSION
Phase changes
-transfers of latent heat energyGas (water vapor)
LATENT HEAT
OF
VAPORIZATION
+ 597 cal/gm
Solid (ice)
Liquid
melting
+ 80 cal/gm
LATENT HEAT OF FUSION
Phase changes
-transfers of latent heat energyGas (water vapor)
597
80
100
+777 cal/gm
Solid (ice)
LATENT HEAT
OF
VAPORIZATION
+ 597 cal/gm
Liquid
melting
+ 80 cal/gm
LATENT HEAT OF FUSION
Phase changes
-transfers of latent heat energyGas (water vapor)
+777 cal/gm
+ 597 cal/gm
freezing (-80)
Solid (ice)
Liquid
melting
+ 80 cal/gm
Expressions of atmospheric
humidity:
1. vapor pressure
2. relative humidity
3. dew point
4. mixing ratio
Vapor Pressure
 Water
vapor is one of the gases in the
atmosphere that contributes to air
pressure.
 The
total pressure of a mixture of gases
equals the sum of the pressures exerted
by the constituent gases.

Dalton’s Law
 Vapor
pressure is the portion of the air
pressure due to water vapor.
Saturation concept
Constant two-way interchange at surface;
vapor
liquid
EVAPORATION: more molecules enter gas phase.
CONDENSATION: more enter liquid phase.
EQUILIBRIUM: equal amount become liquid
and gas.
At equilibrium, vapor pressure is SATURATION
VAPOR PRESSURE
Saturation vapor pressure
At higher temperatures, the amount
of water vapor in the atmosphere
(and therefore the saturation vapor
pressure) is higher.
Raising temperature increases
energy of molecules and more
readily escape surface as gas.
Initially evaporation prevails, but
eventually a new equilibrium is established.
Higher temp, higher water vapor conc,
higher equilibrium/saturation V.P.
Saturation vapor pressure
At higher temperatures, the amount
of water vapor in the atmosphere
(and therefore the saturation vapor
pressure) is higher.
Notice the difference between vapor
pressure and saturation vapor pressure
What if….
temperature is 30ºC and vapor
pressure is 20 mb?
Net evaporation or
net condensation?
Saturation vapor pressure
At higher temperatures, the amount
of water vapor in the atmosphere
(and therefore the saturation
vapor pressure) is higher.
if temperature drops to 20º ?
Saturation vapor pressure
At higher temperatures, the amount
of water vapor in the atmosphere
(and therefore the vapor pressure) is
higher.
There is less energy available for
evaporation ; condensation will occur,
then a new equilibrium is reached.
“Warm air can hold more
water vapor than cold air”
“When temperature is
higher, there’s more
thermal energy for
evaporation , so there is
more water vapor in the air”
“Warm air expands and can “There’s always plenty of
hold more water vapor”
room for water vapor
molecules”. (Average
intermolecular distance is 145 cm.)
Now look at inset graph:
Now look at inset graph:
Below freezing, sat. vapor pressure
over ice is different than saturation
vapor pressure over (supercooled)
water.
Now look at inset graph:
Below freezing, sat. vapor pressure
over ice is different than saturation
vapor pressure over (supercooled)
water.
Which is greater?
Does it take more water molecules
to saturate air over water or over ice?
Over water
Relative humidity
 Amount
of moisture in air relative to
saturation amount, expressed as %.
actual amount water vapor X
saturation amount
100
If we have a parcel of air
at 10ºC and the vapor
pressure is 6 mb,
what is the relative
humidity?
6 X 100 = 50%
12
 If
temperature increases (and actual
amount of water vapor stays same),
what happens to relative humidity?
Dew Point Temperature
 Temperature
at
which cooling air
becomes
saturated.
 Dew
 Clouds
 Frost
a closer look at dew point
temperature :
Even “clear” air contains microscopic water
droplets,
BUT evaporation > condensation so drops
don’t survive long enough to clump and
grow into cloud droplets.
Why do droplets
have to grow to
become cloud droplets?
As temp drops, evaporation rate decreases
REMEMBER! Evaporation requires energy!
There comes a temperature at which
net condensation > net evaporation :
“dew point temperature”
…then a droplet can grow into a cloud
particle.
 Air
is saturated when dew point and air
temperature are same.
 Once
air is saturated, dew point and air
temp stay same
 Dew
point changes as absolute amount of
water vapor changes.

NOT temperature-dependent
 Therefore
dew point is better measure
than R.H. of how much moisture is in air.
Vapor Pressure= 10 mb
Morning:
Temperature
Relative
humidity
Dew point
10°C
?
?
Vapor Pressure= 10 mb
Morning:
Temperature
Relative
humidity
Dew point
10°C
83%
?
Vapor Pressure= 10 mb
Morning:
Temperature
Relative
humidity
Dew point
10°C
83%
5ºC
Vapor Pressure= 10 mb
Morning:
Temperature
10ºC
Relative
humidity
83%
Afternoon:
30ºC
?
Dew
point
5ºC
?
Vapor Pressure= 10 mb
Morning:
Temperature
10ºC
Relative
humidity
83%
Afternoon:
30ºC
24%
Dew
point
5ºC
?
Vapor Pressure= 10 mb
Morning:
Temperature
10ºC
Relative
humidity
83%
Afternoon:
30ºC
24%
Dew
point
5ºC
5ºC
Vapor Pressure= 10 mb
Morning:
Temperature
10ºC
Relative
humidity
83%
Afternoon:
30ºC
24%
Dew
point
5ºC
5ºC
Has the amount of water vapor changed?
Dew points in US rarely exceed 80°F (27°C).
Highest dew point ever recorded:
Eritrea : 94°F (34°C)
(temp. was 115°F (46°C))
 Why
do we never hear of relative
humidities = 100% ?
(even if it is raining)

Because RH is measured at surface; not at
level of clouds.
• For same reason there can be clouds in sky and a
reported RH < 100%
Mixing ratio
Mass of water vapor per mass of dry air
(g/kg)
Saturation mixing ratio: mixing ratio of
saturated air.
Skew T chart
Using these lines on
chart:
At given temp, can
find sat. ratio.
At given dew point,
can find actual ratio.
Can compute
relative humidity.
Shows dew pt.
changes in rising air.
Sling psychrometer
measures relative humidity
Psychrometric tables
 Wet
 If
bulb depression = dry minus wet
you sling the psychrometer and the air
temperature (dry bulb) is 40° and the wet
bulb temperature is 36°, what is the
relative humidity?
 Relative humidity table
 What is the dew point?
 Dew point table