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Atmospheric Water. and Weather. 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

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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
Water source

Percent oftotal water



Ice caps, glaciers


Ground water


Fresh-water lakes


Inland seas


Soil moisture






Total water volume


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
  • 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

An iceberg is 91%

below water surface

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
  • 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. temperature changes

2. evaporation

3. condensation

4. 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
  • 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:
  • Hair hygrometer
  • 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

  • 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


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


Rising parcel of air remains warmer and less dense than surrounding atmosphere

Stable conditions


Rising parcel of air becomes cooler and denser than surrounding air, eliminating the upward movement

Conditionally unstable conditions


Lifted parcel

is theoretically

cooler than

air after lifting

ELR = 

DALR = 


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

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