Transcript EG1104: Earth Systems

EG4508: Issues in environmental science
Meteorology and Climate
Dr Mark Cresswell
Introduction, Astronomy, Energy,
Radiation & Temperature
Suggested References #1
Text Books:
• Ahrens, C. Donald. (2000) Meteorology today : an introduction to
weather, climate, and the environment.
• Harvey, Danny. (2000) Climate and global environmental change
• Burroughs, William James. (2001) Climate change : a multidisciplinary
approach.
• Climate change 2001 : The scientific basis / edited by J.T. Houghton
• McGuffie K and Henderson-Sellers A. (1997). A climate modelling
primer. Published by John Wiley, England.
Suggested References #2
Scientific Journals:
• Quarterly Journal of the Royal
Meteorological Society
• Monthly Weather Review
• Meteorological Applications
• Journal of Climatology
SEE UKSCIENCE METEOROLOGY PAGES FOR MORE INFO
Suggested References #3
Internet:
• KNMI climate explorer:
–
http://climexp.knmi.nl
• Royal Meteorological Society:
–
http://www.royal-met-soc.org.uk/
• The Met. Office:
–
http://www.meto.gov.uk/
• NOAA-ENSO:
–
http://nsipp.gsfc.nasa.gov/enso/
WWW.UKSCIENCE.ORG  EGS UNITS  EG4508 LINK
General Points
• The atmosphere behaves like a fluid
• The atmosphere is a mixture of
different gases, aerosols and particles
• The atmosphere remains around the
earth as an envelope because of gravity
• Much of the observed motion in the
atmosphere results from solar radiation
Basic Astronomy
• For most of the Earth, energy varies on
daily (diurnal) and seasonal (annual)
time-scales.
• Changes from daytime to night and
progression through the four seasons
depends on the configuration of the
Earth-Sun orbit
Quantity of solar radiation may vary as a result of solar activity
Solar wind increases in magnitude at times of high sunspot activity
Basic Astronomy
• The Earth completes a single rotation about its axis in approx 24
hours (23.9345 hours!) - this period is known as a day
Typical Diurnal Air Temperature
25
Temperature (Celcius)
20
15
10
5
0
00:00
06:00
12:00
18:00
00:00
06:00
12:00
Synoptic Hour
18:00
00:00
06:00
12:00
18:00
00:00
Basic Astronomy
• The Earth completes a single revolution around the Sun in
approx 365 days (365.256 days) - period is known as a year
Average Daily Maximum Temperature (London)
mean daily maximum temperature (celsius)
25
20
15
10
5
0
January
February
March
April
May
June
July
Month
August
September
October
November December
Basic Astronomy
• Energy received at different points on the
earth’s surface is not constant
• As we move from the equator to the poles
the quantity of energy decreases
• This is due to Earth curvature
• The same amount of energy is spread over a
greater area and has to pass through a
thicker layer of the atmosphere
Basic Astronomy
• Axis about which the earth rotates tilts
Spring
Summer
Winter
Autumn
Basic Astronomy
SUMMER (N. Hemisphere)
WINTER (N. Hemisphere)
Basic Astronomy
• The Earth does not spin perfectly about its axis - but tilts to
trace a cone in space - caused by the combined Sun and Moon’s
gravitational pull and is called precession
• This tilt angle varies - between about 22º and 25º - it is
currently 23.5º
Basic Astronomy
• In addition to tilt - the elliptical orbit the Earth takes
around the Sun varies also
• Mean distance between the Earth and the Sun is 1AU
(1.496 x 108 km). Minimum distance is 0.983AU and
the maximum distance is 1.017AU
Basic Astronomy
• In addition, the Earth’s path along its ellipse will vary slightly
due to differential gravitation pull. This is known as the
eccentricity of the orbit.
Basic Astronomy
• The combination of factors such as orbital
eccentricity, precession, tilt angle, distance from the
Sun etc greatly affect our climate by varying the
quantity of solar energy received
• Collective term is Orbital Forcing
• May have influenced the magnitude and period of
past ice ages
Basic Astronomy
• The gravitational pull of the
moon affects our tides and
also moderates energy levels
in the oceans
• Ocean dynamics greatly
influences our Earth’s climate
system
Composition of the atmosphere
GAS
PERCENT*
Nitrogen
78.08
Oxygen
20.95
Argon
0.93
Neon
0.0018
Helium
0.0005
Hydrogen
0.00006
Xenon
0.000009
* = Percent by volume dry air
** = Percent by volume
GAS & PARTICLES PERCENT**
ppm
Water vapour
0 to 4
Carbon dioxide
0.036
365
Methane
0.00017
1.7
Nitrous oxide
0.00003
0.3
Ozone
0.000004
0.04
Particles
0.000001 0.01-0.15
Chloroflourocarbons 0.00000002
0.0002
From Ahrens C. D, 2000
Vertical structure of the atmosphere
• Weight is the mass of an object
multiplied by the acceleration of gravity
Weight = mass x gravity
• An object’s mass is the quantity of
matter in the object
Vertical structure of the atmosphere
• The density of air is determined by the
mass of molecules and the amount of
space between them
Density = mass/volume
• Density tells us how much matter is in a
given space (or volume)
Vertical structure of the atmosphere
• Each time an air molecule bounces
against an object it gives a tiny push
• This small pushing force divided by the
area on which it pushes is called
pressure
Pressure = force/area
Vertical structure of the atmosphere
• In meteorology we discuss air pressure
in units of hectopascals (hPa)
(previously called millibars mb)
• The average atmospheric pressure at
the Earth surface is 1013.25 hPa
• We can sense sudden changes in
pressure when our ears ‘pop’ such as
that experienced in old aircraft
Relationship between pressure and height
• As we climb in elevation (up a mountain
or in a hot air balloon) fewer air
molecules are above us:
atmospheric pressure always decreases
with increasing height
Relationship between temperature and height
Introduction to the Oceans
• The oceans occupy 71% of the earth’s
surface
• Over 60% of global ocean surface is in
the southern hemisphere
• Three quarters of the ocean area is
between 3,000 and 6,000 metres deep
Structure of the Oceans
• The thermocline is a layer characterised
by decreasing temperature and
increasing density with depth
• The thermocline is stratified and inhibits
vertical mixing
Structure of the Oceans
• Below the thermocline layer is the deep
layer of cold, dense water
• Deep layer motion is mostly driven by
density variations due to salinity change
Average Ocean Currents for Atlantic and Pacific
Energy: basic laws and theory
• Energy is the ability or capacity to do
work on some form of matter
• Energy is transformed when it interacts
with matter - e.g. potential energy is
transformed into kinetic energy when a
brick falls to the ground
• Matter can neither be created nor
destroyed - only change form
Energy: basic laws and theory
• The energy stored in an object determines
how much work it can do (e.g. water in a
dam). This is potential energy
PE = mgh
PE = potential energy
m = mass of the object
g = acceleration of gravity
h = object’s height above the ground
Energy: basic laws and theory
• A volume of air aloft has more potential
energy than the same volume of air above
the surface
• The air aloft has the potential to sink and
warm through a greater depth of the
atmosphere
• Any moving object possesses energy of
motion or kinetic energy
Energy: basic laws and theory
• The kinetic energy of an object is equal to
half its mass multiplied by its velocity
squared:
KE = ½ mv2
• The faster something moves, the greater its
kinetic energy. A strong wind has more kinetic
energy than a light breeze
Energy: basic laws and theory
• Temperature is a measure of the average
speed of the atoms and molecules, where
higher temperatures correspond to faster
average speeds
• If a volume of air within a balloon were
heated the molecules would move faster and
slightly further apart - making the air less
dense
• Cooling air slows molecules down and so they
crowd together becoming more dense
Energy: basic laws and theory
• Heat is energy in the process of being
transferred from one object to another
because of the temperature difference
between them
Temperature scales
• Hypothetically, the lowest temperature
attainable is absolute zero
• Absolute zero is -273.15 ºC
• Absolute zero has a value of 0 on a
temperature scale called the Kelvin
scale - after Lord Kelvin (1824-1907)
• The Kelvin scale has no negative
numbers
Temperature scales
• Although Kelvin is the preferred scale
for scientists and physicists there are
two more commonly used scales
• Fahrenheit was developed in the
1700s by Daniel Fahrenheit who
assigned the value 32 to the
temperature at which water freezes and
212 to the temperature at which water
boils
Temperature scales
• The zero point of the Fahrenheit scale
is the lowest temperature possible
when mixing ice, salt and water. The
180 equal divisions between the
freezing and boiling points are known
as degrees
• A thermometer calibrated with this
scale measures temperature in degrees
Fahrenheit (ºF)
Temperature scales
• The Celsius scale was introduced in
the 18th century. The value of 0 is
assigned to the freezing point of water
and the value 100 when water boils at
sea-level
• An increasing temperature of 1 ºC
equals an increase of 1.8 ºF
Specific heat and latent heat
• Liquids such as water require a
relatively large amount of heat energy
to bring about just a small temperature
change
• The heat capacity of a substance is
the ratio of the amount of heat energy
absorbed by that substance to its
corresponding temperature rise
Specific heat and latent heat
• The heat capacity of a substance per
unit mass is called specific heat
• Specific heat is the amount of heat
needed to raise the temperature of one
gram (g) of a substance by one degree
Celsius
• 1g of liquid water on a stove would
need 1 calorie (cal) to raise its
temperature by 1 ºC
Specific heat and latent heat
• When water changes its state (solid to
liquid, liquid to gas etc) heat energy will
be exchanged
• The heat energy required to change a
substance from one state to another is
called latent heat
• Evaporation is a cooling process
• Condensation is a warming process
Energy transfer in the atmosphere
• Conduction: transfer of heat from
molecule to molecule (hot spoon)
• Convection: transfer of heat by the
mass movement of a fluid (such as
water and air)
• Radiation: Movement of energy as
waves - the electromagnetic spectrum
Electromagnetic
spectrum with
enhanced detail for
visible region of the
spectrum
Note the large range of
wavelengths
encompassed in the
spectrum - it is over
twenty orders of
magnitude!
EMR and the Sun-atmosphere system
• About 50% of incoming solar radiation
is lost by the atmosphere: scattered
(30%) and absorbed (20%)
• Scattering involves the absorption and
re-emission of energy by particles
• Absorption (unlike scattering) involves
energy exchange
EMR and the Sun-atmosphere system
• Wavelengths less than and greater than
0.8µm (800nm) are often referred to as
shortwave and longwave radiation
respectively
• The shortwave solar radiation consists
of ultraviolet and visible
• The terrestrial longwave component is
known as infrared
EMR and the Sun-atmosphere system
• Just under 50% of the radiation
reaching the Earth’s surface is in the
visible range
• Components of visible light are referred
to as colours
• Each colour behaves differently and
white light can be separated out by use
of a prism
EMR and the Sun-atmosphere system
• The human eye cannot see infrared
radiation
• Infrared radiation is absorbed by water
vapour and carbon dioxide in the
troposphere
• The atmosphere’s relative transparency
to incoming solar (SW) radiation, and
ability to absorb/re-emit outgoing
infrared (LW) radiation is the natural
greenhouse effect
The Earth’s energy balance
• Incoming solar (shortwave) energy
should be balanced by outgoing
terrestrial (longwave) energy
• Without a balance the Earth would heat
up or cool down uncontrollably
• Energy may take a tortuous path from
Sun to ground and back to space.
Greenhouse effect
• The natural greenhouse effect
maintains a stable climate for life on
earth
• Outgoing radiation (longwave) is
absorbed by molecules such as water
vapour and carbon dioxide
• Energy is then re-emitted in all
directions - forming a blanket
Greenhouse
Effect