Section H: Atmospheric water and precipitation

hydrosphere

global water system

what is the key role of the hydrosphere and how is it powered

redistributing energy flows via atmospheric/oceanic circulation. powered by radiant energy from the sun

what is the percentage of saline and freshwater on earth

97.2% saline, 2.8% freshwater

hydrologic cycle

movement of water between various storage location (reservoirs). amount of water is finite. total amount evaporated equals the total precipitated globally

hydrogen bonding

bond between H2O molecules due to slightly -ve O charge and slightly +ve H charge

cohesion

H2O molecules attracted to each other (water sticking to other water)

adhesion

H2O is attracted to another substances

as water cools what happens

• contracts when cooling to 4°c

• expands by around 9% between 4-0°C

  • ice floats on water- less dense compared to liquid water

  • expansion in weathering of rock

specific heat

amount of energy required to raise the temperature of 1g of a substance by 1°C

what is the universal solvent

water

latent heat

energy that is absorbed (stored) or released when a substance changes state (without a change in temp)

• forms or breaks molecular bonds

• cannot be felt or measured

condensation

water vapour to liquid water - energy released

evaporation

transformation of liquid water into water vapour

transpiration

loss of water vapour directly from leaf pores (stomata) in plants into the atmosphere

evapotranspiration

evaporation + transpiration

evapotranspiration rates depends on

• net radiation which increases heating

• air temperature which influences maximum humidity

• relative humidity and moisture capacity of air

• wind speed

humidity

amount of water vapour in the air

• varies spatially and temporally (supply and demand)

measures of humidity

• vapour pressure

• specific humidity

• relative humidity

• dew point temperature

precipitable water

measurement of how much moisture could theoretically precipitate given the right conditions

where is high and low vapour pressure found

high vapour pressure is found close to a water surface and air at higher altitudes has lower vapour pressure

how does water vapour move

moves along a vapour pressure gradient from high vapour pressure to low vapour pressure

how does wind move moist air

it moves it away from the source

warm air can hold ___ H2O than cold air

much more

cold dry air can have close to ____% H2O vapour

0%

warm tropical air may have ____% H2O vapour

4-5%

explain water density at the latitudes

high latitudes have low temps thus low water density

low latitudes have high temps thus high water density

specific humidity

(g/kg) actual amount of water vapour in a given amount of air. direct measure.

• ex: 10 grams of H2O vapour in a kg of air

maximum specific humidity

(g/kg) maximum amount of H2O vapour that a body of air can hold at its current temperature. (increase temp = increase MSH)

saturation curve

describes the relationship between maximum humidity and temperature

relative humidity

percentage of actual H2O vapour in the air compared to the maximum amount the air could hold at that temperature

• saturation depends on temp

• describes how close the air is to saturation at its current temp

• expressed as %

relative humidity can change 2 ways

1. temperature changes - changes the maximum specific humidity

2. water vapour changes (gains or losses) - changes the specific humidity

equation of relative humidity

specific humidity/maximum specific humidity x 100

at 100% relative humidity it means that

the air is saturated (it’s holding all the water vapour it could hold at that temperature)

temperature and (relative) humidity have an _____ relationship

inverse

heating

temperature increases, relative humidity decreases

cooling

temperature decreases, relative humidity increases

explain humidity at lower and higher lats

• lower lats: specific humidity is high (hot air holds more w.v.)/relative humidity is low (higher amount of w.v. so could hold way more air)

• higher lats: specific humidity is low/relative humidity is high (close to saturation cuz holds less)

the stability of the atmosphere depends on

1. relative temperature

2. density

clouds

visible masses of tiny suspended water droplets or ice crystals

two necessary conditions for cloud formation

1. air must be saturated (100% RH)

   • either by cooling below the dew point or by adding w.v. to the air

2. there must be a substantial quantity of hygroscopic condensation nuclei (small aerosols) for water vapour to collect (ex. dust, sea salts)

cirro form cloud

thin, wispy (made of ice crystals)

cumulo form cloud

taller, puffy cloud, round (like drawing clouds)

strato form cloud

layers, blanket, sheet, covers whole sky

nimbo form cloud

precipitation

height range of clouds

1. low - stratus

2. middle (alto)

3. high (cirrus)

4. vertically developed

how are sun dogs created

caused by the refraction of sunlight by the hexagonal ice crystals in cirrus and cirrostratus clouds

typical when sun is close to horizon

rainbows caused by sunlight striking liquid drops rather than ice crystals

where does precipitation form

within clouds when cloud droplets or ice crystals grow large enough to overcome updrafts and fall to earth

rain

liquid H2O droplets

snow

ice crystals

sleet

rain freezes before hitting the ground

freezing rain

rain freezes on impact with the ground

hail

ice crystals melt and refreeze before falling

virga

rain that evaporates before reaching the ground

how many cloud droplets are needed to form a rain drop heavy enough to fall to earth

1 million

raindrop size is determined by

drop size and air resistance

where does precipitation originate from

parcels of moist air that have been adiabatically cooled

• condensation and cloud formation start at the lifting condensation level (LCL)

adiabatic cooling occurs through the lifting of air from the surface to higher levels in the atmosphere by

1. convective uplift

2. orographic uplift

3. convergent uplift

4. frontal (cyclonic) uplift

convective uplift

• spontaneous

• unequal heating of surface causes air parcel to rise and expand

• pressure of unstable air decreases as it rises (air cools adiabatically)

• condensation occurs at LCL

• hot lower density air

orographic uplift

air forced to rise by a mountain

• windward side - air cools at dry adiabatic rate to dewpoint, clouds forms (LCL) and air continues to cool at SAR

• leeward side - air descends - warming DAR, creates rain shadow (dry conditions)

chinook wind

occurs when a steep pressure gradient develops in mountainous regions (canadian rocky mountains)

• high pressure on windward side

• low pressure on leeward side

• air is warmed adiabatically as it moves over the mountain and down leeward slope

convergent uplift

collision of similar air masses moving horizontally

• low pressure systems at ITCZ

• warm moist tropical air rises in hadley cells

• NE/SE trade winds converge towards ITCZ

• uplift is initiated, energy released which augments process

• produces cloud cover and rain

frontal uplift

different air masses collide forming a front and uplift of air (some is forced up and other part stays down)

precipitation variability

the expected departure from an area’s average annual precipitation in any given year

acid rain

aka acid precipitation or acid deposition. the deposition of either wet or dry acidic materials from the atmosphere on earth’s surface.

sources of acid precipitation

sulfuric and nitric acid. sulfur dioxide (human induced) emissions from smokestacks, nitrogen oxides (NOx) from motor vehicle exhaust. these may drift hundreds of thousands of kilometers away by wind.

how is acidity measured

pH scale, based on the relative concentration of hydrogen ions (H+). lower end = acidic, higher end = basic

how does acid precipitation damage stuff

damages aquatic ecosystems, forest diebacks, buildings and monuments destroyed

how to reduce acid rain

reduce so2 and nox from coal burning electricity generating plants, reducing volatile organic compounds, reducing particulate matter