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Lecture 1
Topic 1
The atmosphere: radiation
and moisture
Earth Systems 2: The Hydrosphere
ENS00203
Dr Kathryn Taffs
School of Environment, Science and Engineering
Southern Cross University
Lecture outline
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•
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•
•
•
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•
Some basic definitions
The atmosphere
Solar radiation
Electromagnetic spectrum
Atmospheric influences
atmospheric interaction
reflection
absorption
scattering
net radiation balance
Atmospheric moisture
•
•
•
Temperature
• Temperature is:
• the amount of energy (or heat)
held by any substance
• It is measured in:
• Celius, Farenheit or Kelvin
Air pressure
• The pressure exerted by the air above it.
• Force exerted against a surface by the
collision of gas molecules.
Measuring air pressure
Mercury barometer
Aneroid barometer
Change of pressure with
elevation
Higher ……………………. Lower
The Atmosphere
• Most weather
occurs in the
tropopause
~10,000 metres
above the Earth’s
surface.
Energy drives our
world
•
•
•
The energy for life on Earth, and to
drive the atmospheric circulation,
comes from the Sun.
Hence we need to understand the
structure and composition of the
atmosphere to understand how
solar energy effects life on the
surface of Earth.
Solar radiation
• Average temperature of the Sun’s surface is 6000 K
• The Sun emits energy known as electromagnetic
radiation.
Electromagnetic radiation
• Electromagnetic
radiation travels in
a wave pattern
• Each wave has a
different length
between the
troughs and crests
respectively
Electromagnetic spectrum
• The electromagnetic spectrum can be
broken into segments according to the
wavelength
Atmospheric influences
• There are a number of factors that affect
the solar constant (how much energy
reaches the top of the atmosphere)
• solar output
• solar distance
• angle of incidence
• Length of day
Solar output
• Energy from the sun varies over
time
• this is related to sun spot activity
• Sun spots are dark areas of the sun
Sun spot activity on 20th
August (from http://
www.lmsal.com/YPOP/
ProjectionRoom/
latest_SXT_full.html)
associated with increased energy
output, particularly in the ultraviolet wavelengths
• this is a minor influence on the
amount of radiation received by the
atmosphere
Solar distance
• Amount of radiation varies according to
distance between the Sun and the Earth
which varies seasonally as the Earth’s orbit
is elliptical
Angle of incidence
tilt of the Earth has a
• The
more dramatic effect on the
amount of radiation received
at the top of the atmosphere
higher the latitude the
• The
less radiation received
because of the greater
surface area over which it is
spread
the thicker the
• (and
atmosphere it must travel
through)
• The greater the angle of insolation, the
greater the surface area over which it is
spread
• This significantly affects the amount of
radiation received
Length of day
• The tilt of the
Earth’s axis
also affects the
length of day.
• This obviously
affects the
duration that
solar radiation
is received
Atmospheric interaction
with radiation
• Once radiation reaches
the top of the
atmosphere it may be:
• scattered
• reflected, or
• absorbed
From: www.teoptics.kulgun.net/Blue-Sky/xt
Scattering
• Air molecules and fine dust in
the path of insolation deflects
it.
• When the particles are < the
wavelength it is known as
Rayleigh scattering.
• When the particles are > the
wavelength it is known as Mie
scattering.
Absorption
small part of insolation is
• Aabsorbed
by atmospheric
•
gases
amount absorbed depends
of the wavelength and the
gas type
• For example:
and ozone absorb
• oxygen
small wavelengths
vapour and carbon
• water
dioxide absorb longer
wavelenths
Reflection
insolation interacts with the surface
• Finally,
of the Earth
reaching the surface is either
• Radiation
absorbed or reflected.
• The proportion reflected is the albedo
varies according to the nature of
• Albedo
the material, its colour, its roughness,
whether it is wet or dry and the angle of
incidence of the insolation
Global energy budget
• Scattering, reflection and absorbance all
occur to differing degrees
Global energy budget
• and the long wave radiation ...
Spatial variations of
insolation
• The amount of insolation received varies
spatially across the globe. It is primarily
determined by:
• latitude, and
• cloud cover
Effect of latitude
• January
Effect of latitude
• July
Cloud Cover
• Clouds have a high albedo reflecting about
half of radiation received
• Clouds are predominantly concentrated in
the equatorial belt.
• Hence maximum insolation received at the
earth’s surface actually occurs in the subtropics at 20°N and 20°S
Global distribution of insolation
Australian solar
radiation
From: http://www.bom.gov.au/climate/averages/climatology/solar_radiation/IDCJCM0019_solar_exposure.shtml
Balancing global heat
•
There is a surplus of heat in the equatorial belt (more
radiation received than lost by cooling)
•
At the poles more radiation is lost by cooling than that
received
•
To balance the global heat balance 2 processes occur:
•
•
latitudinal heat transfer
vertical heat transfer
•
These two processes and the basic cause of all of our
weather
•
leading onto topic 2 on global winds
Atmospheric moisture
• Convection is the
vertical movement of air.
• It transfers heat from
the warm ground
surface upwards.
• Clouds form where the
air temperature equals
the dew point
temperature
Properties of atmospheric
moisture: definitions
GAS
Motion + collisions => partial pressure
partial pressure
the pressure applied by the collision of gas particles in
the atmosphere
vapour pressure
the pressure applied by the collision of water vapour
molecules in the atmosphere
•
•
Properties of atmospheric
moisture: definitions
saturated
•
mass of air holding the maximum
amount of water vapour at a given
temperature
unsaturated
•
when a mass of air is containing
less than the saturated amount.
Properties of atmospheric
moisture: definitions
•
•
•
•
absolute humidity
mass of water vapour in a unit
volume of air.
relative humidity
percentage ratio between the
actual vapour pressure and the
saturation vapour pressure
mixing ratio
mass of water vapour per unit mass of dry air
dew point temperature
temperature at which air must be cooled for saturation to occur
Measuring humidity
• Humidity is measured using wet
and dry bulb thermometers.
• The difference in temperature
between the 2 values enables us
to calculate relative humidity.
• In the prac we used Tables, there
are also conversion programs
available, eg. www.bom.gov.au/lam/
humiditycalc.shtml
Daily Relative Humidity
The amount of
water vapour
the air can hold
increases with
temperature.
Condensation
For condensation to occur:
•
air must be saturated (cooled
to the dew point)
•
there must be a surface to
condense on (condensation
nuclei)
•
a trigger for the process such
as a front or orographic uplift.
Lapse rates •
•
•
The rate at which air temperature
changes with height is known as the
environmental lapse rate
(ELR).
•
0.65 °C per 100 m
When that air is unsaturated this is
known as the dry adiabatic
lapse rate (DALR)
•
1 °C per 100m
When that air is saturated it is
known as the wet adiabatic
lapse rate (WALR)
•
0.64 °C per 100m
Aerological
diagrams
Wind
speed
Height above
and
ground surface in
HPa, and ft and m direction
Temperature
Temperature
Dew point temperature
Previous balloon release date and time
Balloon release date and time
Orographic uplift
• As air parcel
rises, it cools.
• When dew point
is reached,
condensation
occurs
• results in cloud
development
Calculation of cloud level
2500m-1600m=900m, 9x0.64ºC=5.24ºC, thus temperature at C=15ºC-­‐5.24ºC=9.6ºC
600 m
25ºC-15ºC=10ºC, @1ºC/100m = 1000m, hence clouds form at 1000+600m=1600m
We can calculate the cloud base level and air temperature
of a moving air mass using the adiabatic lapse rates
Inversions
• Occasionally, at some
altitudes the
temperature abruptly
begins to increase with
height.
• This occurs if a warm
layer of air overlies a
colder layer
• After a short vertical
distance the temperature
in the warm layer will
begin to cool again
Condensation: 1. Clouds
Latin derivations:
•
cumulus = “heap” to
describe a puffy cloud
•
cirrus = “curl of hair” to
describe a wispy cloud
•
stratus = “layer” to
describe a sheet like
cloud
•
nimbus = “violent rain”
to describe a rain cloud
Cloud formation
• There are 4 main ways that moist air can
be lifted to form clouds
Orographic uplifting
Convective lifting
Interaction of air masses
Mechanical turbulance
Cloud categories
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•
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There are 10 principal cloud types:
High clouds; cirrus, cirrostratus, cirrocumulus
Middle clouds; altostratus, altocumulus
Low clouds; nimbostratus, stratocumulus, stratus
Clouds with vertical development; cumulus,
cumulonimbus
High level clouds
Cirrus
Cirrocumulus
Cirrostratus
No precipitation from high level clouds
Mid level clouds
Altocumulus
May produce light showers and if
appears on a summer morning may
indicate a thunderstorm in the evening
Altostratus
Often results in precipitation and
sometimes snow
Low level clouds
Cumulus
Showers of rain or snow
Stratocumulus
Often results in drizzle
Stratus
Often results in drizzle
Multi-layer clouds
Nimbostratus
Heavy rain or snow
Cumulonimbus
Thunderstorms, lightning,
squalls, heavy rain
Condensation 2. Fog
Fog
Cloud formation at sea level
Smog
Mixture of fog and industrial pollution
Dew
Condensation on cold surfaces such as glass blades
Frost
Formation of ice crystals instead of dew
Fog
There are 2 main types of
fog:
Advection fog: air flows from
warmer to colder area and
reaches dew point
Radiation fog: Moist air in
contact with cooling land
surface to reach dew point
Practical 1
• Water vapour in the atmosphere
• relative humidity, dew point
• environmental lapse rates
• wet and dry adiabatic rates
• Cloud classification
• Climate Assignment
Climate Assignment
• Bring lap top or mobile device
• MyGrades for BoM station
• Instructions in practical book pracs 1-4
• Podcasts and help files in MySCU,
Assignment Resources
• Reading 1.1
Readings
•Lutgens, F.K. and Tarbuck, E.J. 2007. Heating
Earth’s Surface and Atmosphere. In, The Atmosphere.
Pearson Prentice Hall, New Jersey. Pp. 32-63.
•Reading 1.2
Tarbuck, E.J and Lutgens, F.K. 2009. Moisture,
•Clouds
and Precipitation. In, Earth Science. 12th
Ed. Pearson, Prentice Hall, New Jersey. Pp.
477-511.
•Quiz 1
Topic 1 Quiz available through Assessment
•Resources