A Brief History of Water
Outgassing
•Water on Earth formed within the planet
•Massive quantities outgassed into early atmosphere
•Torrential rains created lakes and oceans
•Flows of water over land carried dissolved and
undissolved elements to oceans
•Present volume of water 1.36 billion km3 reached
about 2 billion years ago
•Volume of water is quite stable (loss to
space/compounds equalled by supply from below)
Some Simple Facts about Water
71 % of Earth's surface is water (by area)
The weight of water is 1kg/L
Sea Levels
Eustatic sea level change is controlled by:
water temperature and ice sheet/glacier volume
Mean sea level is currently rising (interglacial)
Sea level was 100m lower 18,000 BP
Distribution of Water on Earth
•97.2% of all surface water is oceanic
•2.8% is non-oceanic
•Most of Earth's freshwater is frozen in ice
sheets/glaciers
•Rest is in lakes, rivers, groundwater or soil
moisture
Oceans
Ice caps, glaciers
Percent of
total water
97.24%
2.14%
Ground water
Fresh-water lakes
Inland seas
0.61%
0.009%
0.008%
Water source
Soil moisture
Atmosphere
Rivers
Total water volume
0.005%
<0.001%
<0.0001%
100%
Source: U.S. Geological Survey
The Unique Properties of H2O
1.
A Solvent
•Water molecules attracted to one another
-
 side (2H) attracted to side (O) of another molecule
H-bonds form between molecules - cause of surface tension and
capillarity
2.
Heat Properties
Three phases - solid, liquid, vapour
Phase changes
Melting: Solid  Liquid
Freezing: Liquid  Solid
Evaporation/Vaporization: LiquidVapour
Condensation: Vapour Liquid
Sublimation: SolidVapour
Deposition: VapourSolid
Frozen H2O
•Ice takes up as much as 9% more space than the same
number of liquid H20 molecules
An iceberg is 91%
below water surface
•Ice floats because it weighs only 91% as much as water
•To melt, heat energy must increase molecular motion until
H-bonds break
•Latent heat of fusion is large compared to heat necessary
to heat ice or water without a phase change
Liquid H2O
•Pure water is most dense at 4C
Water expands above or below that temperature
•Fills its container, but non-compressible
H20 Vapour
•Water that evaporates must absorb energy –
latent heat of evaporation
•The dominant cooling process in the Earth's energy budget
•Water vapour that condenses liberates energy
latent heat of condensation
Humidity
•Water vapour content of air is its humidity
•Warm air holds more water as vapour than cold air
•Relative humidity: A ratio that compares the amount of water
vapour in the air to the maximum water vapour capacity at that
temperature
•The relative humidity of saturated air is 100%
RH = [H20 vapour content/H20 capacity] x 100
What affects relative humidity?
1.
2.
3.
4.
temperature changes
evaporation
condensation
advection
At saturation, any decrease in temperature or addition of water
vapour results in condensation
Dew point temperature:
the temperature at which air becomes saturated
When RH = 100%, the air temperature and the dew point
temperature are the same
RH is highest at dawn and lowest in the afternoon (warmer).
How to Express Humidity
1. Vapour pressure:
the portion of atmospheric pressure that is made up of
water vapour molecules (mb or kPa)
•
water evaporates from a moist surface until the increasing
vapour pressure in air causes some molecules to return to
the surface
•
maximum capacity of air to hold moisture referred to as
saturation vapour pressure, the maximum pressure that
water molecules can exert
•
Saturation vapour pressure changes with temperature
(almost doubles with each 10C rise)
Specific humidity:
the mass of water vapour (g) per mass of air (kg)
Maximum specific humidity is the maximum mass
of water vapour that can be held by 1kg of air at a
given temperature
Humidity Measurements:
1. Hair hygrometer
2. Sling psychrometer
Sling psychrometer
Wet-bulb/Dry bulb thermometers
•wet bulb thermometer has its bulb moistened with a wick and
air is passed over it
•the temperature depression is determined by dryness
•temperatures the same when relative humidity = 100%
•wet bulb measures a much lower temperature if the air is
dry (due to evaporation)
•Psychrometric chart is required
Atmospheric Stability
A 'parcel of air' is a body of air that has particular temperature
and humidity characteristics.
Warm air has a lower density
Cold air has a higher density
A parcel of lower density air will rise and expand as external
pressure decreases
A parcel of higher density air will descend and be compressed
by higher external pressure
Stability
The tendency of a parcel to remain in place or change vertical
position by ascending or descending
To measure stability we need to understand the temperature
distribution at a range of heights
Measured with an instrument package called a radiosonde
Normal lapse rate: 6.4C/km
Environmental lapse rate: ?.? C/km
In the absence of external heating and cooling…
•Ascending air cools with expansion
•Descending air heats due to compression
“adiabatic”
Dry adiabatic lapse rate:
The rate at which dry air cools by expansion or warms by
compression with a change in height.
DALR = 10C/1000m
Moist adiabatic lapse rate:
The rate at which moist ascending air cools by expansion
MALR typically about 6C/1000m
Varies:
4C/1000m in warm air
near 10C/1000m in cold air
Latent heat of condensation liberated as parcel rises
Unstable conditions
ELR > DALR
Rising parcel of air remains warmer and less dense than
surrounding atmosphere
Stable conditions
ELR < MALR
Rising parcel of air becomes cooler and denser than
surrounding air, eliminating the upward movement
Conditionally unstable conditions
DALR>ELR>MALR
Lifted parcel
is theoretically
cooler than
air after lifting
ELR = 
DALR = 
Source: http://www.atmos.ucla.edu
Lifted parcel
is theoretically
warmer than
air after lifting
ELR = 
DALR = 
Lifted parcel
is the same
temperature as
air after lifting
Note: Conditionally-unstable conditions
occur for m <  < d
Cloud Formation
•Air rises to altitude where RH=100%
•H2Ovap  H2Oliq on condensation nuclei
Cloud Types
•Stratiform - layered
•Cumuliform - globular or puffy
•Cirroform – wispy, always composed of ice
Rain clouds: nimbostatus (light), cumulonimbus (heavy)
Mid-level clouds: altostratus, altocumulus
High-level clouds: cirrus, cirrostratus, cirrocumulus
Fog
Ground-level cloud
Visibility less than one kilometre
Advection fog
1. Warm, moist air passes over cooler surface
2. Cold air flows over warm body of water
(evaporation or steam fog)
3. Upslope fog (hills force moist air upward)
4. Valley fog (cool air settles into low-lying areas)
Radiation fog
Radiational cooling on clear nights brings air
temperature to dew point near the ground
Air Masses
Continental Polar – cP
Maritime Polar – mP
Continental Tropical – cT
Maritime Tropical – mT
Atmospheric Lifting Mechanisms
Convectional lifting
Convergence lifting
Orographic lifting
Review: Cold fronts, warm fronts and mid-latitude
cyclones