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Agricultural Science
Climatology
Semester 2, 2007
Richard Thompson
http://www.physics.usyd.edu.au/Ag/agschome.htm
Course Coordinator: Mike Wheatland
Chapter 4
Water in our Atmosphere
Water in the atmosphere (1)
• Water regulates temperature of plants & animals
because of the large latent heat of vaporisation.
• How water vapour (in air) moves depends on its
density, which is related to its temperature.
• A mass of air that is less dense than its
surroundings will rise in the atmosphere.
• A mass that is more dense will sink.
• This is the effect known as buoyancy.
Water in the atmosphere (2)
• Archimedes Principle – object feels a buoyancy
force equal to the weight of the fluid (liquid or
gas) it displaces.
• A body floats (on water) if its weight equals the
weight of the water displaced. This is how a
duck floats.
This stone can ‘t displace enough
water to equal its weight so it sinks if the
string breaks.
What role does pressure play?
• Pressure (p) at a given level in a liquid or gas is
the same, but it decreases as you rise in the
atmosphere (less weight of air above you).
• Result is a net force on a body because of the
different pressure. This is the buoyancy force.
• To calculate changes in pressure:
p = po - ρgh (po is at sea level; h is altitude)
This assumes gas is not compressible! Only
good for small heights, small changes
The Barometric Formula (1)
P = P0 exp(-h/H) where H = kT/(mg)
If we take: k=1.38x10-23 kg m2 s-2 K-1 ;
g=9.8 m s-2 ; m=29x1.67x10-27 kg;
T=300K; then
H = 87,000 (8.7 km)
This is ‘scale height’ at which pressure falls to
1/e (37%) of its surface value
Compare: Mt Everest is 8.85km high
This method comes from the Ideal Gal Law and assumes constant
Temperature – also an imperfect model!
The Barometric Formula (2)
Pressure vs Height
1200
Pressure (mb)
1000
800
600
400
200
0
0
2
4
6
Height (km)
8
10
12
Atmospheric Pressure at Sea Level
• Standard atmospheric pressure is defined as
101,325 Pascals (1 atm) which is actually a
typical sea level pressure for mid latitude
regions on Earth
• Pressure is force per unit area. A Pascal is a
Force of 1 newton/square metre. A mass of 1 kg
has a weight of 9.8 newtons
• Synoptic (weather) charts show pressure in units
of hectopascals (hPa). Another unit is millibar
and 1 hPa = 1 millibar
• 1 atm = 1013.25 hPa
Weather Map for July 13, 2006
Some thermodynamics
• Ideal gas equation generally holds:
PV = nRT
• If 2 masses of gas have same temperature,
volume and pressure, they must contain the
same number of molecules – water molecules
weigh less than oxygen or nitrogen, so moist air
is less dense than dry air.
• Remember: it is also true that warm air is less
dense than cold air.
PV diagrams show how the different
processes change the properties of gas
ADIABATIC – no heat transfer
ISOTHERMAL – constant temperature
ISOBARIC – constant pressure
ISOCHORIC – constant volume
Work = PΔV
Dynamics in the atmosphere (1)
• Warm air is less dense – it rises
• Cool air is more dense – it falls
• Most changes occur adiabatically – no heat is
transferred during process
• If air rises, it will then be in a region of lower
pressure - the gas expands, cools down
• If air sinks, it will then be in a region of higher
pressure - gas is compressed, heats up
Dynamics in the atmosphere (2)
• Moist air is less dense than dry air
• Both kinds cool as they rise, but moist air
cools more slowly (some condensation
occurs which releases heat)
• Rate of change in temperature with height
is called adiabatic lapse rate: dry (DALR)
is -10 °C/km and moist (MALR) is -6 °C/km
Adiabatic Lapse Rate (1)
• This is the rate of change of temperature of
atmosphere as a function of height assuming no
heat exchange with surrounds (definition of
adiabatic)
• Determines stability of atmosphere (e.g.
thermals) and formation of clouds
• Meteorologists measure lapse rate using
radisondes and compare with predicted rates
• It is a negative value – air cools as it goes higher
Adiabatic Lapse Rate (2)
• Dry Adiabatic Lapse Rate (DALR) is rate of
temperature change with height for a rising
parcel of dry air. For atmosphere, this is -9.8
degrees C/km.
• Moist Adiabatic Lapse Rate (MALR) is similar
but for moist air (saturated) and is -4.9 degrees
C/km – the difference due to the latent heat
released when water condenses
• Environment Lapse Rate (ELR) is the actual rate
for a stationary atmosphere. Typically -6.5
degrees C/km but this varies from day-to-day
and location.
Adiabatic Lapse Rate (3)
• Look at absolute values of rates
• If ELR is larger than DALR and MALR then air is
always unstable - increasing formation of
cumulus clouds and thunderstorms. Common in
afternoon over land when air is heated.
• If ELR is less than DALR and MALR then air is
always stable and clouds formation is unlikely.
Common in morning when air has cooled
overnight.
• If ELR is between the two values for DALR &
MALR, then dry air stable, moist air unstable.
Example of intermediate situation
U nstable air – parcel rises
2000 m
12 °C
14 °C
E nergy release during
C ond ensation (latent h eat)
M oist air lapse rate
= - 6 °C /km
10 °C
Stable air – parcel falls
D ry air lapse rate
= - 10 °C /km
1000 m
20 °C
E nvironm ental lapse rate
= - 8 °C /km
Less dense air rises, m ore dense air sinks
Examples of different Lapse Rates:
cooling in °C/km of increasing elevation
ELR
(actual)
-4
MALR
(moist air)
-6
DALR
(dry air)
-10
Result
-8
-6
-10
Dry air stable
Moist air unstable
-12
-6
-10
Always unstable –
Rising air doesn’t
cool fast enough
Always stable –
no clouds form
Pressure systems and rain
• Falling air – compression, temperature
rises, clouds less likely, no rain. High
pressure system (deserts).
• Rising air – expansion, temperature falls,
condensation and clouds more likely, more
rain. Low pressure system.
Why do clouds form?
• In atmosphere - continual evaporation and
condensation between gas (water vapour)
& liquid (water drops)
• Evaporation is easier from small droplets
• Condensation needs small particles in air
to begin process
• Dew point – air is saturated; evaporation &
condensation rates are equal
• Clouds: water drops condensed on grains
Cumulus Clouds
Thunderclouds
Precipitation and Clouds
• Adiabatic cooling occurs when moist air rises
• Air becomes saturated, allowing condensation and
precipitation as rain, hail or snow
• Different types of cloud associated with rain, some
contain ice
• High clouds (> 5 km) – e.g. cirrus, ice crystals
• Middle clouds (2-7 km) – e.g. altostratus, mostly water
• Low clouds (< 2 km) – e.g. stratus (rain), nimbostratus
(snow)
• Vertically developed clouds (via convection) – e.g.
cumulus, cumulonimbus (thunderstorms)
What other options besides rain?
• Snow – lower temperatures produce ice crystals
in upper cloud layers
• Hail – ice crystals rise and fall in cloud several
times resulting in layered growth
Golf-ball hail in the USA
Humidity (1)
• Absolute Humidity is the ratio of the mass of water
vapour to the volume of the air (i.e. kg/m3)
• Specific Humidity is mass of water vapour to the mass
of dry air
• Relative Humidity is the ratio of the vapour pressure to
that of saturated air.
• For a given amount of water vapour in the atmosphere,
the saturated air pressure is less at lower temperatures
so the Relative Humidity is higher at dawn!
• Relative humidity is often mentioned in weather reports
as an indicator of rain
• Relative Humidity is a measure of comfort – as it is
harder to lose heat by perspiration when there is high
humidity
Humidity (2)
Fogs - dew – frosts (1)
• Condensation occurs when temperature falls
below dew point
• If air does not become saturated until
temperature below freezing – frosts occur
• If air dry & cold – plant cells rupture when
temperature gets to dew point; called a killing
frost
• Avoid killing frosts by spraying water – adds
moisture – ice forms on plants and insulates
internal cell fluids against freezing
Fogs - dew – frosts (2)
• Water vapour in the air is invisible
• Fog & mist are very small water droplets
• Water gets into atmosphere by
evaporation (oceans, lakes, rivers) and
transpiration (animals, plants)
• Water goes back to Earth by precipitation
(rain, hail, snow)