Transcript Weather Dynamics

Weather Dynamics
Ch. 13, 14, 15
Basics of Heat Transfer
• Conduction – The mechanism of heat
transfer in which highly energetic atoms or
molecules collide with less energetic atoms
or molecules, giving them some energy.
• Convection – The mechanism of heat
transfer in which highly energetic molecules
move from one place to another.
• Radiation – The mechanism of heat transfer
in which atoms or molecules emit
electromagnetic waves.
Solar Energy
• Space contains very little matter, so the
energy that arrives at Earth’s outer
atmosphere is about the same as when
it left the sun.
• The amount of energy that reaches us
is called the solar constant, 1367 J/m2s.
• The solar constant is the amount of
radiant energy that hits 1 m2 of Earth’s
outer atmosphere per second.
• This constant applies only to rays that
hit perpendicular to the surface. Some
of this energy is reflected back into
space or is absorbed.
Refer to
diagram on
Page 424.
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Albedo
Part of the solar energy that comes to
Earth is reflected back out to space in
the same, short wavelengths in which it
came to Earth.
The fraction of solar energy that is
reflected back to space is called the
albedo.
Albedo is a measure of the reflectivity
of a surface.
30% of incoming radiation is reflected
back to space
Questions ????
• So how does the Earth maintain a
relatively constant temperature since
some energy is absorbed, some is
reflected?
• Why does the Earth not lose all its heat
to the cold depths of space?
• Why do the oceans not boil due to the
constant absorption of energy?
• The answer lies in our atmosphere that
creates a “Greenhouse Effect”
Estimating the Magnitude
of the Natural Greenhouse Effect
• If you consider the energy reaching
the earth from the sun it only provides
enough energy to heat the earth to a
temperature of -19C.
However, average global surface
temperature is + 14C
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Natural greenhouse effect warms the
surface by 33C
The Greenhouse Effect
• As solar radiation reaches the Earth it
is absorbed and heats up the surface.
This heat is then radiated into the
atmosphere as infrared radiation.
• There are gases in our atmosphere
such as water vapour, CO2, methane
and others that are very good at
absorbing infrared radiation (heat).
• These gases then reradiate that heat
back towards the surface thus further
heating up the Earth.
Global Warming
• Global warming is the increase in the average
measured temperature of the Earth's nearsurface air and oceans since the mid-20th
century, and its projected continuation.
What evidence do we have that climate change
occurs naturally?
• The Ice Ages (Last one was 15 000 years ago)
• El Nino
• Volcanic Eruptions
• Climate change occurs naturally but the concern
now is that humans are accelerating the rate of
global warming.
Global Warming
• Most experts agree that human activities
are enhancing the natural greenhouse
effect of the atmosphere and accelerating
global warming.
• Climate change is more than a warming
trend.
• Increasing temperatures will lead to
changes in many aspects of weather, such
as wind patterns, the amount and type of
precipitation, and the types and frequency
of severe weather events.
How will climate change affect us?
• Not all regions of the world will be affected
equally by climate change.
• Low-lying and coastal areas face the risks
associated with rising sea levels.
• Scientists have also determined that
warming will be greater in polar regions
than nearer to the equator, and that
continental interiors will experience greater
warming than coastal areas.
How will climate change affect us?
• Climate change is a global problem, affecting all
countries. While greenhouse gases (GHGs) form
naturally, many human activities add additional
GHGs to the atmosphere.
• Heating and cooling buildings, using energy at
home and work, driving vehicles to move people
and goods, powering industrial processes – most
things we do that consume energy contribute to
the problem.
• In Canada, climate change will affect fishing,
farming, forestry, lakes, rivers, coastal
communities and the North
Projected Global Surface Temperature Change
Evidence that climate is changing…….
Observations over recent decades also show…
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Evaporation & rainfall are increasing;
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More of the rainfall is occurring in downpours;
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Permafrost is melting;
Corals are bleaching;
Glaciers are retreating;
Sea ice is shrinking; 2007 & 2008 have been the
years with the most sea ice melt in history.
Sea level is rising;
Wildfires are increasing;
Storm & flood damages are soaring.
Assignment
• Read pg. 422- 426
• Do Questions 1-4 pg. 426.
Energy and Water
• Only about 30% of the Earth’s surface is land
and most of the land is covered by cloud a
large portion of the time.
• This means that most of the solar energy that
reaches the Earth strikes water. As a result
the interactions between solar energy and
water have major influences on the Earth’s
weather.
• Water absorbs 93% of the energy that
reaches it, and yet the average temperature
of most bodies of water on our planet does
not vary greatly.
Energy and Water
• The temperatures of oceans and large lake
remain relatively constant for many
reasons.
• Some of the reasons relate to water’s
unique properties such as it large specific
heat capacity.
• This is defined as the amount of heat that
is required to raise one gram of a
substance 1oC.
Specific Heat Capacity
• Chemists have measured the specific heat
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capacity (c) of many substances.
Using these measured values you can calculate
the amount of heat (Q) required to raise the
temperature of an amount of a substance (m) by
a given temperature (∆T).
Q = mc ∆T
Q = heat (J); m = mass in grams; c = specific
heat capacity (J/ g oC); ∆T = change in
temperature (oC)
Specific heat capacities of many substances are
in Table 13.1 on pg. 427.
Example #1
1. Calculate the amount of heat needed to
increase the temperature of 250g of
water from 20oC to 56oC.
Q = mc ∆T
Q=?
m = 250g
c = 4.18 J/g oC (from table 13.1)
∆T = 56 oC - 20oC = 36oC
Q = 250 g x 4.18 J/g oC x 36oC
Q = 37 620 J = 38 kJ
Example #2
2. Calculate the specific heat capacity of copper
given that 204.75 J of energy raises the
temperature of 15g of copper from 25oC to 60oC.
Q = mc ∆T
c=?
Q = 204.75 J
m = 15g
∆T = 60oC - 25oC = 35 oC
c = Q/m ∆T
c = 204.75 J/15g x 35 oC
c = 204.75 J/ 525 g oC
c = 0.39 J/ g oC
• The higher the specific heat capacity of a
substance, the more heat it can absorb
and the more time it will take for it to
release that heat.
Heat of Vaporization
• The second unique property of water that
helps maintain constant temperatures is its
heat of vaporization.
• This is the amount of energy required to
convert 1.0 grams of a substance from a
liquid state to a gaseous state.
Heat of Vaporization
• If the substance returns to a liquid state
from the gaseous state, the same amount
of energy that was used to convert it
originally will be released.
• Scientists call this the latent heat of
vaporization because the thermal energy
that is used to evaporate the liquid does
not become thermal energy again until the
gas condenses back into a liquid.
Heat of Vaporization
• Not all of the energy that strikes water is used to
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heat it up. Some of it is used to vaporize the
water.
You can calculate the amount of heat that is
required to evaporate a given mass (m) of liquid
by using the values in Table 13.2 (pg. 428) and
the following formula.
Q = m ∆Hovap
Q = heat
m = mass in grams
∆Hovap = heat of vaporization of 1g of a
substance. Table 13.2.
Example
Sam has a large pothole in his driveway
where rainwater collects. If the hole
contains 2.7 kg of water, how much energy
is required to evaporate all the water?
Q = ? m = 2.7 kg
∆Hovap = 2260 J/g
Q = m ∆Hovap
Q = 2700 g x 2260 J/g
Q = 6 102 000 J or 6.1 x 106 J
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Heat of Fusion
A third unique property of water is its large
heat of fusion.
This is the amount of heat that is required
to melt 1g of a solid into a liquid.
In reverse, it is also the amount of energy
that will be released when a liquid freezes.
The formula is the same as for the heat of
vaporization except that heat of fusion is
used instead of heat of vaporization.
Q = m ∆Hofus
Example
• A huge ice sculpture is delivered to a
spring carnival. The sun delivered 3.0 x
108 J of energy before it completely melted
away. How much did the ice sculpture
weigh when it was delivered?
Q = m ∆Hofus
m=?
Q = 3.0 x 108 J
∆Hofus = 333 J/g
m = Q/ ∆Hofus
m = 3.0 x 108 J / 333 J/g
m = 900 000 g or 900 kg
Summary
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Remember these points when deciding
which of the 3 formulas to use:
1. If there is a change in temperature of a
substance use Q = mc ∆T.
2. If a substance is changing state from a
liquid to a gas or from a gas to a liquid
use Q = m ∆Hovap
3. If a substance is changing state from a
liquid to a solid or from a solid to a liquid
use Q = m ∆Hofus
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Water in the Air
Why doesn’t all of the water in the world
evaporate into vapor?
There is a finite amount of water that the
air can hold.
Air at different temperatures can hold
different amounts of water. Warm air can
hold more vapor than cold air.
When there is as much water vapor in the
air as possible at a given temperature, we
say the air is saturated.
Water in the Air
• Humidity is a measure of the amount of
water vapour in the air.
• Absolute humidity is the actual amount of
water vapour in the air, expressed in units
such as grams of water vapour per
kilogram of air.
• Relative humidity is the percentage of
water vapour in the air compared to the
amount of water vapour that air would
contain if it were saturated.
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An example of Relative Humidity
Why is the air so dry in homes in the winter?
The temperature outside is say -12oC and
the relative humidity is 75%. That air is
pulled into your house and heated to 20oC.
What is the RH now?? (using table 13.4)
Remember that cold air can hold less
moisture that warm air.
So at 75% RH at -12oC the air would
contain 0.75 x 1.53g water/kg air = 1.15 g.
Air at 20oC can hold 15.0g water/kg air. So
the RH would now be 1.15 g/15.0 g = 7.7 %
Water in the Air
• Dew Point: The dew point is the temperature to
which a given parcel of air must be cooled for
water vapor to condense into water. The
condensed water is called dew. The dew point is
a saturation point.
• It is associated with relative humidity. A high
relative humidity indicates that the dew point is
closer to the current air temperature. Relative
humidity of 100% indicates that the dew point is
equal to the current temperature (and the air is
maximally saturated with water). When the dew
point stays constant and temperature increases,
relative humidity will decrease.
(Pg. 433)
Assignment
• Read pg. 427- 433
• Do Questions 1-8 pg. 434.
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Atmospheric Pressure
Every layer of air exerts pressure on the
air below because every molecule in the
air is pulled toward the Earth by gravity.
Consequently, the lower layers are
compressed by all the layers above.
This is called atmospheric pressure.
As you increase in altitude, the pressure
decreases due to less atmosphere above
you.
Interactions of Solar Energy with Land and Air
• As we learned earlier, different substances
have different heat capacities.
• This determines how much heat a substance
can hold, and how fast it will dissipate that
heat. For example water has a higher heat
capacity than asphalt. Thus water will take
longer to warm up, but it will also cool off
slower than asphalt.
• Convection produces many effects on air.
• As air heats up due to conduction and
radiation from the surface, parcels of air
react in a predictable way.
As air heats up at the surface it does several
things:
1. As air heats up at the surface it becomes less
dense than the air around it, thus it will rise to
air that is the same density.
2. As the air parcel rises it begins to expand.
This is due to the lower atmospheric pressure
on the parcel as it gets higher in the
atmosphere.
3. This expansion also causes the air to cool at
a rate of 10oC/1000m.
These processes will be important when we
discuss clouds.
The Atmosphere
The atmosphere is divided into 5 layers
1. Troposphere: This is the layer from
the surface up to 10 km above. This
is the layer in which all weather
occurs. Temperature drops as you
increase in altitude in this layer.
2. Stratosphere: This layer
contains most of the ozone
which filters out much of the
harmful ultraviolet (UV)
radiation from the sun.
Temperature increases as you
rise in altitude.
3. Mesosphere: Temperature again drops as you
increase in altitude in this layer. This is the
layer that meteors burn up in.
4. Thermosphere: This layer begins 100 km
above the surface. It is extremely hot in this
layer due to oxygen absorbing high energy UV
light.
5. Ionosphere: A layer of charged particles within
the thermosphere and mesosphere. This layer
is a collection of electrons that have been
ejected from high energy molecules. This
allows radio waves to be reflected off this layer
and received large distances form their source.
Assignment
• Read pg. 435- 446
• Do Questions 1-6 pg. 448.