Transcript energy and the water cycle

Science 10 Daily Notes
Section 13.2 pgs 427-434
Energy and Water
Most of sun’s energy hits the oceans
Amount of reflected solar radiation
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Snow: 90%
Ice: 50%
Liquid water (lakes, rivers, oceans): 7%
Why do bodies of liquid water not change
temperature, even though they absorb
93% solar radiation?
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Due to unique properties of water
Water has many unique characteristics.
One of these properties is water’s unusual ability to absorb large
quantities of heat without much change in temperature.
This characteristic is the
specific heat capacity.
Unique Properties of Water
1. Specific heat capacity – next slide
2. Heat of vaporization – amount of energy
required to convert water from a liquid to
a gas (evaporation)
3. Heat of fusion – amount of energy
required to convert a solid to a liquid
(melting)
Specific Heat Capacity
Amount of heat required to raise the temp of 1g
of a substance by 1 degree C
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Formula: Q = mc∆t
Q = amt of heat
M = mass of substance
C = specific heat capacity
∆t = temp change
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If a substance has a high specific heat capacity, then
the substance will need a lot of energy to change its
temperature
Water has a high specific heat capacity
What does it mean to have a high
specific heat capacity?
It means water has the capacity to hold a
lot of heat without changing temperature
The temperature of water increases much
less than the temperature of most similar
substances after gaining the same amount
of energy
Large specific = small temperature
heat capacity
increase
When water absorbs 4.184 Joules of heat, the
temperature of one gram of water will increase
by 1 C°.
Relatively speaking, this is an enormous amount
of heat energy.
Coastal areas like Nova Scotia, which are
surrounded by water, maintain relatively stable
climates with moderate temperatures.
This is because water can absorb or release a
great deal of heat without experiencing huge
fluctuations in temperature.
By comparison, look at the heat capacity of copper.
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1 gram of copper will rise in temperature by 1 C° when just 0.385 Joules of
heat is absorbed.
This low specific heat capacity indicates that copper is a good
conductor of heat.
You might predict that applying a small amount of heat will make the
temperature of a gram of copper skyrocket while the same amount of
heat hardly makes the temperature of one gram of water rise at all.
Q = mcΔT
Use this to find out the mass, energy, or
temperature change of a specific
substance as it gains or loses energy.
Chemistry: Specific Heat Capacity
Q = mΔH0vap
Use this to find the mass or energy of a
specific substance as it changes state
from liquid to gas (vaporization)
When vaporization occurs, the
temperature will remain the same.
A negative energy would occur for
condensation (gas to liquid)
Q = mΔH0fus
Use this to determine the mass or energy
when a substance changes from solid to
liquid (fusion = melting)
There is no temperature change while the
substance is melting
A negative Q (energy) would be recorded
if the substance solidified (liquid to solid)
Water in the Air
Saturated = full to capacity
Unsaturated = not full to capacity
When there is as much water vapour in the
air as possible at a given temperature, we
say that the air is saturated.
*Warm air can hold more water vapour than
cool air
Formation of Haze, Fog, and Clouds:
Condensation Nuclei
The process of condensation of
vapor -> water to form a cloud
drop is not that simple in the
atmosphere
NEED Condensation Nuclei to
form cloud drops
Condensation Nuclei:
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small particles in air created
from/by:
dust
volcanoes
factory smoke
forest fires
ocean salt
sulfate particles from
phytoplankton in ocean
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They are most abundant in lower
troposphere over urban areas
They are quite small relative to a
rain drop or cloud drop
QUESTION FOR THOUGHT:
Why does the relative humidity seldom
reach 100% in polluted air?
Formation of Haze
Two types:
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dry haze - large/giant particles in the air
(smoke, smog, dust)
wet haze - H2O condenses onto hydroscopic
CCN - can occur at RH's as low as 75%
wet haze has a dull gray, white color
Formation of Fog, Introduction
Forms as the RH increases to 100% haze particles grow into fog (cloud)
particles near the ground
is really a cloud near the ground
International definition: Visibility is less
than 1 km
Fog in polluted areas can be a health
problem since it becomes acidic
Formation of Radiation Fog Radiational Cooling
Need:
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shallow, moist air near surface
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clear/calm nights
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although light winds will bring more air in contact with ground
Radiational cooling allows the temperature to drop to the dew
point
Once the temperature reaches the dew point, radiation
fog begins to develop
Common in the fall - especially when our weather is
dominated by high pressure
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often forms in valleys first since this is where the coldest air is called valley fog
Humidity
ABSOLUTE HUMIDITY
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A type of humidity that considers the mass of water vapor present per unit
volume of space. Also considered as the density of the water vapor. It is
usually expressed in grams per cubic meter.
RELATIVE HUMIDITY
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Describes the amount of water vapor that exists in a gaseous mixture of air
and water expressed as a percentage of the maximum amount of water vapor
that could be present if the vapor were at its saturation conditions.
Relative Humidity
The most common way that the amount of water in the
air is reported is relative humidity.
RH is a comparison between the amount of water in the
air with the amount of water that could be in the air when
evaporation and condensation are equal. Mathematically
RH is a percentage of "the maximum amount of water
possible at that temp.
For example: 75% r.h. means if air in a room could have
20 liters of water in it, it really only has ________ liters.
If temp increases without adding any water, rh
decreases. If temp decreases, rh increases. Why? The
amount of water that COULD be in the air changes, so
with the same amount of water, percent changes.
Dewpoint and Relative Humidity
Calculating the RH requires the correct equation(s).
The RH is the amount of moisture in the air (via
moisture mass or vapor pressure) divided by the
maximum amount of moisture that could exist in the
air at a specific temperature (via max moisture mass
or saturation vapor pressure).
RH is expressed as a percentage and has no units
since the units in both the numerator and
denominator are the same. The percentage is found
by multiplying the ratio by 100%. The RH is NOT the
dewpoint divided by the temperature.
For example, if the temperature was 60 F and the
dewpoint was 30 F, you would not simply take
(30/60)*100% = 50% RH.
Dewpoint is the temperature at which
condensation occurs. The more water
vapor that is in in air, the higher this
dewpoint temperature will be. In other
words, high humidity will result in a higher
dewpoint temperature. The dewpoint
temperature of air is important in weather
forecasting because it indicates how high
air must rise in order for clouds to begin
condensing from that air.
Use your water vapor (dewpoints) graph:
1. How much water (at most) could be in the air at 20
degrees: _________ . If it really only has 7 g/kg, then the
rh is: __________ .
2. This air has a dewpoint of ________ .
3. If the air in the previous example rose into the
atmosphere and cooled at the rate of 7 degrees Celcius
per kilometer of altitude, at what altitude would
condensation and cloud formation occur?
Dewpoint Graph
http://www.omega.com/temperature/Z/pdf/
z102.pdf
What is “Dew Point”?
The warmer air is, the more water vapor it
can "hold."
Dew point is a measure of how much
water vapor is actually in the air.
Relative humidity is a measure of the
amount of water in the air compared with
the amount of water the air can hold at the
temperature it happens to be when you
measure it.
Now, let's see how dew point and relative humidity work.
Imagine, that at 3 p.m. you measure the air's
temperature at 30 degrees and you measure its humidity
at 9 grams per cubic meter of air.
What would happen if this air cooled to 10 degrees with
no water vapor being added or taken away?
As it cools to 10 degrees the air becomes saturated; that
is, it can't hold any more water vapor than 9 grams per
cubic meter.
Cool the air even a tiny bit more and its water vapor will
begin condensing to form clouds, fog or dew - depending
on whether the air is high above the ground, just above
the ground, or right at the ground.
Back at 3 p.m., when we made the measurements, we
could say that the air's dew point is 10 degrees C. That
is, if this particular air were cooled to 10 degrees at
ground level, its humidity would begin condensing to
form dew.
How about relative humidity?
At 3 p.m. the air has 9 grams of water vapor per
cubic meter of air. We divide 9 by 30 and
multiply by 100 to get a relative humidity of 30%
In other words, the air actually has 30% of the
water vapor it could hold at its current
temperature.
Cool the air to 20 degrees. Now we divide 9, the
vapor actually in the air, by 17, the vapor it could
hold at its new temperature, and multiply by 100
to get a relative humidity of 53% (rounded off).
Finally, when the air cools to 10 degrees, we
divide 9 by 9 and multiply by 100 to get a relative
humidity of 100% - the air now has all the vapor
it can hold at its new temperature
Water Cycle
Evaporation and condensation are
continually taking place
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Source of evaporation (water vapor in air):
Oceans, rivers, lakes, moist ground, living plants
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Source of condensation:
Air moves to cooler regions or the air temp lowers
Water condenses to form clouds or ice crystals
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Condensed water eventually falls down to
earth as precipitation and returns the water to
the lakes, oceans, etc to be evaporated again
Answers to homework #1-4 pg 434
1. How specific heat capacity influences the
temp of a large lake.
A: It has a high specific heat capacity which
means that it takes more solar energy to
change the temperature than air or land.
Also, water mixes and flows freely which
helps maintain a constant temp.
Homework…
2. Heat of vaporization: amount of energy
required to covert 1g of a substance from
a liquid to a gas
3. Converting from a liquid to a solid (water
freezing) is called heat of fusion.
4. When saturated air cools water will
condense out of the air and form clouds.