1. 1. Background for Cloud Physics
1.1 Overview of atmospheric thermodynamics
1.2 Overview of cloud observations

2. 1.1 Atmospheric Thermodynamics
* Measures of vapor content
Vapor pressure: The partial pressure of water vapor in the atmosphere
Vapor density: Mass of water vapor per volume of air
Mixing ratio: Mass of water vapor per mass of dry air
Specific humidity: Mass of water vapor per mass of air
Relative humidity: The ratio of the mixing ratio to its saturation value
at the same temperature and pressure
Virtual temperature: Accounts for both the effects of temperature and
humidity on pressure and density of the air

3. * Clausius-Clapyron equation
a. Saturation vapor pressure of water exceeds that of ice for all temperature
below 273K;
b. Any atmosphere saturated with respect to water is supersaturated relative
to ice;
c. The degree of supersaturation increases with the supercooling.
Table (1): Saturation Vapor Pressure Over Water and Ice, and Latent Heats of
Condensation and Saturation
T (oC)
es (Pa)
ei (Pa)
L (J/g)
Ls (J/g)
-40
19.05
12.85
2603
2839
-35
31.54
22.36
-30
51.06
38.02
2575
-25
80.90
63.30
-20
125.63
103.28
2549
2838
-15
191.44
165.32
-10
286.57
259.92
2525
2837 -5
421.84
401.78
611.21
611.15
2501
2834 5
872.47
2489 10
2477 15
2466 20
2453 25
2442 30
2430 35
2418 40
2406

4. * Ways of reaching saturation
Dewpoint temperature: temperature at which saturation is reached
if cooled isobarically
Wet-bulb temperature: temperature at which saturation is reached
if cooled by evaporating water into it at constant pressure
Equivalent temperature: temperature which moist air would attain
if all the moisture were condensed out at constant pressure
Isentropic condensation temperature: temperature at which saturation
is reached when moist air is cooled adiabatically with mixing ratio held
constant

5. * Mechanisms for Cooling the Air
(1) Lifting – most clouds form when air is lifted.
a) Convergence – low press center – stratus
b) Mountains – lifting by terrain
c) Fronts – lifting over denser air.
d) Warm air relative to surroundings
i) Fires, volcanoes – cumulus
ii) Latent heat
(2) Mixing – seeing your breath on cold day
(3) Contact – with cold surface – advection fog
(4) Radiation – ground fog

7. 1.2 Clouds, Dew, Fog, and Precipitation
As rising air cools, the temperature will approach the dew point, which is
when condensation will occur.
Clouds: Clouds are condensed water droplets, either in liquid or ice form.
Dew: When high relative humidity air comes into contact with a cold
surface, the closest air cools to the dew point, depositing the water
as droplets onto the surface.
Frost: If the temperature of the surface is below the freezing point (0° C),
then water vapor may condense directly into ice crystals, by-passing
the liquid phase altogether. Frozen dew is slightly different, in that
the water first condenses into a liquid, and then further cooling
freezes the liquid water into ice.
Fog: A fog is nothing more than a low/surface layer cloud. The ground level
humidity has reached saturation and water vapour condenses into water
droplets, forming clouds.

8. Types of Precipitation
Drizzle: Small (< 0.5 mm) raindrops falling from low lying stratus clouds.
Rain: Larger drops (> 0.5 mm), usually falling from nimbus, or nimbostratus
clouds. Such clouds will have more vigorous up-drafts and “convective
cells”.
Freezing Rain: Rain that falls onto surfaces with sub-zero temperature, so the
rain freezes on contact. Results in layers of ice forming on all
surfaces.
Snow: Falling ice crystals. Larger snowflakes will form when cloud temperatures
are greater than -10 °C and supercooled water aids in the growth of the ice
crystal.
Ice Pellets / Sleet: Forms when rain falls through cold air, freezing the droplets.
Hail: Forms in clouds with vigorous convective cells, which allow repeated layers
of water to freeze onto ice balls. The more vigorous the up-drafts (cells), the
larger the hail stones.
Virga: precipitation that evaporates before it hits the ground.

9. Dew
Frost
Sleet
Hail