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GEO 344 Weather and Climate
Prof. Stuart Evans
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Lecture 5
Energy Transfer
Announcements
• Homework #1 due 11:59pm Sunday (syllabus will be updated shortly)
• Reading Quiz #4 is available, due next Tuesday before class
• It’s a little harder than usual because Chapter 5 is really important and I want to
give you some practice on something that only counts a little bit
• I will be posting the activity pages in Course Documents as a study resource
Homework #1
Save your answers as you go!
You can submit multiple attempts, but we will only grade the last one.
You must write your own answers to short-answer questions! Submitting an answer
that is the same as someone else’s or from a webpage is plagiarism.
You’re better off trying to figure out the answers based on class materials (lectures,
activities, the book, etc) than trying to Google the answers. I find a lot of wrong
answers if I Google the questions.
“Anomaly” = difference from the average
From the National Weather Service yesterday:
What happened Normal Anomaly
What happened – Normal = Anomaly
The Ideal Gas Law
For gases (like the atmosphere), temperature, pressure, and volume are all relat
ed
pV= ??∗?
pressure volume number of
molecules
temperatureconstant
If you change one of the variables, at least one of the other
ones will have to change to compensate
The Ideal Gas Law
pV= ??∗?
If you change one of the variables, at least one of the other
ones will have to change to compensate
Understanding the relationships: pretend everything = 1
pV= ??∗?
1*1 = 1*1*1
Now change something: T = 2
1*1 = 1*1*2
✓
✗
How could you balance this? Three ways:
1*1 = 0.5*1*21*2 = 1*1*22*1 = 1*1*2
p=2 V=2 n=0.5
✓ ✓ ✓
Energy!
• There are numbers and equations here!
• You don’t have to memorize them.
• I won’t make you do anything that requires a calculator on midterms.
• You will have to use them on homework.
• You will have to understand how to use them, what they tell you,
what their inputs and outputs are.
• My hotplate outputs 900 W –
almost as much as a 1 kW
microwave
• We will run it on low. I guess that’s
about 200 W coming out of the hot
plate. The rock will get hot.
• I calculate 200 W into a 0.2 kg rock
for 40 minutes = 3,000°C
• Will it get to 3,000°C?
Rock on a hot plate
What is energy?
Energy is that which allows work to be done
Make something move
Heat something up
Make an object
change shape
Make a chemical
reaction happen
Make something
melt or evaporate
Energy has to be added to make any of the happen
What is energy?
Energy comes in lots of forms!
Electricity
Gravity
Motion
Heat Light
Chemical
We’re going to
focus on heat and
especially light
Check on the rock!
What is energy?
Energy comes in units of joules.
B
ut we actually don’t care about joules!
In this class, we will study the rate at which energy is used or
received.
The rate of energy use or flow is called power, and we
measure it in watts (abbreviated W).
James Joule
James Watt
Common language: power = big vague concept
Scientific language: power = energy per second
What is power?
The rate of energy use or flow is called power, and we
measure it in watts (abbreviated W).
1 watt = 1 joule per second
Energy
(Joules)
Po
w
er
(W
at
ts
)In this picture →
• if you think of the water as energy,
• then the rate of the pour is the power.
• The rate of water transfer to the glass is
like power, the rate of energy transfer
from object to object.
Check on the rock!
How much is a watt?
1 watt each
60 watts
(incandescent)
80 watts
(resting
metabolism)
1000 watts
= 1kilowatt
250 horsepower
= 109,000 watts
= 109 kW
2,500,000 watts
= 2.5 megawatts
18,000,000,000,000 watts
= 18 terawatts
(global energy use)
400,000,000,000,000,000,000,000,000
watts
4,900,000,000 watts
= 4.9 gigawatts
Ways to transfer energy
Conduction Convection Radiation
They all happen in your kitchen!
Why the pan handle gets hot Why water boils all at once How a broiler works
Check on the rock!
Conduction – transferring heat energy by contact
Only the bottom of the pan
is exposed to the flames, but
all of the pan gets hot
Somehow the energy
gets from here…
… to here
Conduction is heat transferred by molecules in contact
with each other. Basically the hot molecules bump
against the cold ones and heat them up.
The longer things are in contact,
the more heat is transferred
Conduction – transferring heat energy by contact
Some materials are good at conduction, like metal
You only have to touch the hot
metal for a moment for enough
heat to be conducted to burn you
Some materials are bad at conduction, like air
You can reach into a 500° oven
and the air doesn’t burn you
Check on the rock!
Conduction – transferring heat energy by contact
Warm ground heats the atmosphere from below (like the pan on the stove heats food)
Convection– transferring heat energy by circulation
Heating from below
Circulation is created
A boiling pot is the classic example
Check on the rock!
Conduction + Convection = heated atmosphere
Warm ground heats the atmosphere from below (like the pot on the stove), creating a circulation that
moves heat upward (like the water in the pot)
Convection is important to rainfall. We’ll talk more about it in the coming weeks.
Light energy
Everything with a temperature gives off light.
à everything has a temperature
à everything gives off light
We call this blackbody radiation or thermal radiation
The hotter something is, the more light it gives off
hot hotter hottestnot hot
Check on the rock!
Light energy
Question: If everything gives off light, how come everything doesn’t
glow in the dark?
Answer: Unless things are really hot (1000 0F or more) they give off
light at wavelengths our eyes can’t detect.
there’s light we can’t see?
The electromagnetic spectrum
Light comes in a huge range of wavelengths
Our eyes can detect this part
Short wavelength = high energy
à ultraviolet (UV) gives us
sunburns
à X-rays damage our cells
Long wavelength = low energy
à Radio waves don’t cook us
Check on the rock!
Color Bluer Redder
Wavelength Short wavelengths Long wavelengths
Energy High energy Low energy
Names
Ultraviolet, X-Ray,
Gamma
Infrared,
Microwave, Radio
The electromagnetic spectrum
How I remember that there’s more energy in blue light:
“ultraviolet” sounds like “super purple”, so it must be beyond the blue end of the spectrum, and
“ultra” sounds like it has lots of energy. I also know that UV has so much energy it burns me.
A brief aside
Light is also called electromagnetic radiation.
Scientists refer to sunlight as “solar radiation” or “insolation”.
Radiation is not radioactivity!
≠
Check on the rock!
m
or
e
lig
ht
e
m
itt
ed
0 0.5 1.0 1.5
wavelength (microns)
Objects giving off light
So what wavelengths does an object emit light at?
It depends on the temperature of the object
Hotter things à more total power
peaks at shorter (bluer) wavelengths
Notice the temps are in Kelvin
What’s Kelvin???
visible range
Each curve represents an object at a different temperature
Temperature scales
Julien Emile-Geay USC, 2013
The three temperature scales
Kelvin
Absolute, logical
Celsius
Relative, logical
Fahrenheit
Relative, illogical
William Thomson, 1st Baron Kelvin
(1824 – 1907)
There’s a third temperature system!
K stands for Kelvin.
A degree of Kelvin is the same as a degree of Celsius.
0 K is absolute zero. Nothing can ever be colder than this.
Kelvin = Celsius + 273.15
William Thomson
(Lord Kelvin) derived
absolute zero in 1848.
Check on the rock!
m
or
e
lig
ht
e
m
itt
ed
0 0.5 1.0 1.5
wavelength (microns)
Wien’s Law
Hotter things à more total power
peaks at shorter (bluer) wavelengths
visible range
Wilhelm Wien
(derived law in 1893)
Question: which part of the lava is hottest?
A.
B.
C.
Check on the rock!
Question: which star is hotter?
A
B
There’s an equation for this (for interest only)
wavelengthmax = 2898 / T
Surface temperature of the sun: ~5800 K
2898 / 5800 = 0.5 microns
Wien’s Law
Hotter things à more total power
peaks at shorter (bluer) wavelengths
Green!
Check on the rock!
m
or
e
lig
ht
e
m
itt
ed
0 0.5 1.0 1.5
wavelength (microns)
Stefan-Boltzmann Law
Hotter things à more total power
peaks at shorter (bluer) wavelengths
visible range
Ludwig Boltzmann and
Josef Stefan, derived law
in 1884 and 1879
E = power (W/m2)
σ = 5.67 x 10-8
T = temperature (°K)
Total energy emitted per second
per square meter
E = σ T4
An infrared animal for everyone
More energy (in this case infrared) is being emitted by the hot parts of the animal
Dog noses are cold.
Check on the rock!
Stefan-Boltzman Law
Hotter things à more total power
peaks at shorter (bluer) wavelengths
Power emitted goes up with the
4th power of temperature.
If you double the temperature,
the power emitted becomes 16
times larger!
Practice with Google
Let’s practice with the rock!
http://google.com/
Calculate how much energy the rock was giving off !
(Final temp = 84.9 °C)
Power
into
rock
from
hot
plate
Power out of rock
through radiation
Hot plate
1 micron0.1 microns 10 microns 100 microns
GEO 344 Weather and Climate
Prof. Stuart Evans
Lecture 6
Humidity and Condensation
Updates
• Homeworks will get graded as quickly as possible
• Next week’s reading quiz will be posted tonight
Phases of water
Gas Liquid Solid
You can’t actually see water vapor
– it’s invisible
Clouds are made of tiny water drops We’re pretty familiar with this
Water: if you can see it, it’s not a gas
Phases of water
What is inside the bubbles? What phase do you see here?
Measures of Water Vapor
We have three ways to define the amount of water vapor in the air:
• vapor pressure, e
• relative humidity, RH
• dew point, TD
Not actually a measure of water vapor in the air:
• saturation vapor pressure
Vapor pressure
• Pressure review:
• Pressure = Force / area
• Pressure is additive (Dalton’s Law)
• Total pressure = pressure from dry air
+
pressure from water vapor
•
What if we JUST considered the water vapor?
• This is what we call water vapor pressure, e
• e tells you the actual amount of water in the air
Column of air
(both dry air and water vapor)
nitrogen, oxygen, etc.
Vapor pressure
• Pressure review:
• Pressure = Force / area
• Pressure is additive (Dalton’s Law)
• Total pressure = pressure from dry air
+
pressure from water vapor
• What if we JUST considered the water vapor?
• This is what we call water vapor pressure, e
• e tells you the actual amount of water in the air
Column of air
(both dry air and water vapor)
nitrogen, oxygen, etc.
Vapor pressure
• Total pressure = pressure from dry air
+
pressure from water vapor
What if we JUST considered the water vapor?
• This is what we call water vapor pressure, e
• Above example: e = 10 hPa
• What if there were twice as many vapor molecules? e = 20 hPa
1000 hPa of pressure from dry air
+ 10 hPa of pressure from water vapor
= 1010 hPa total pressure
Vapor pressure
Vapor Pressure: pressure due to the water vapor in the air, e
Saturation vapor pressure: pressure due to water vapor in the air,
in the hypothetical scenario that the air is saturated, es
What is saturation?
Saturation
• Saturation is an equilibrium situation where:
evaporation = condensation
• Consider dry air over a body of water…
Dry, no water vapor Water vapor increases Water vapor stays same
evaporation condensation
water
SATURATION!
Saturation
Water evaporating out of glass into the air
(and some being condensed into it)
All the water evaporated
means there was no equilibrium
means air was not saturated
Later
What does saturation look like?
Saturation means liquid water must exist
Vapor pressure
Vapor Pressure: pressure due to
only water vapor, e
Saturation vapor pressure:
pressure due to only water vapor,
if the air were saturated, es
es depends only on temperature!
Vapor pressure
Saturation vapor pressure: pressure
due to only water vapor, if the air
were saturated, es
es is how much water vapor
there could be
It’s a property of air: for a particular
temperature, there’s a maximum
amount of water vapor possible.
How much water
there could be
Relative humidity
Relative humidity tells you how
close the vapor pressure, e, is to
saturation
RH = 100 * e / es
RH tells you how close to the red
line you are.
How much water
there could be
Relative humidity
Examples from the reading quiz
es for air that is 30 °C = 42 hPa
e for air that is 30 °C and 50% RH = 42*50% = 21 hPa
es for air that is 10 °C = 12 hPa
e for air that is 10 °C and 100% RH = 12*100% = 12 hPa
Warm air can have more water in it even when not close to
saturated than totally saturated cold air
How much water
there could be
Relative humidity
You get more snow when it’s only a little
below freezing
How much water
there could be
Where is there water vapor?
Satellite image of upper atmosphere
(about 300 hPa) water vapor.
Bright shades are more water vapor,
dark is less.
The world, animated
https://a.atmos.washington.edu/~ovens/wxloop.cgi%3Fwv_moll+/14d/
Dew Point
Dew point: the temperature at
which RH = 100% would occur
Dew point tells us how much water
is in the air (another way of
getting e).
T – TD is called the dew point
depression, and tells you how
much you would need to cool to
create condensation
When the temperature equals the dew point
If the dew point is below 0 °C you get frostCondensation occurs on surfaces at TD
When the temperature equals the dew point
Morning fog over a field
• By what process did the ground cool at night?
• By what process did the air cool?
• What is the dew point depression right now?
• Why does this only happen first thing in the morning?
Steam fog in Chicago
Lake water – right about
freezing. Air A result of
“warm” water exposed to
cold air.
Dew point changes with weather
Warm air mass
High dew point
Cool air mass
Low dew point
Dew point changes with height
Which air mass is more humid?
At what pressure levels would
you be most likely to find
clouds?
When the temperature equals the dew point
At what altitude
does T = TD?
As air cools
cooling air
Why doesn’t this change?
Why is it dropping now?
es only depends on
temperature. No formula for
it, just something you need to
look up in a table like this.
As air cools
cooling air
Why doesn’t this change?
Why is it dropping now?
A sample box of air
Box of air starts here…
then starts cooling
Temperature
Dew Point
Humidity
Pressure
Cloud in a bottle!
GEO 344 Weather and Climate
Prof. Stuart Evans
Lecture 7
Latent Heat Hurricane Florence (2018), powered by latent heat
Announcements
• Reading Quiz for Tuesday (6.1 – 6.5)
• This will be the end of material for the first midterm
• Midterm coming up (we’ll do some review on Tuesday)
• About 30 multiple choice, about 3 short answer
• Nothing that requires a calculator
• More details Tuesday
Announcements
In-class Activities are available as a study tool
Announcements
Answers to reading quizzes are available via the Gradebook
This is a link to the
questions/answers
Relative humidity
Relative humidity tells you how
close the vapor pressure, e, is to
saturation
RH = 100 * e / es
RH tells you how close to the red
line you are.
How much water
there could be
Relative humidity
Relative humidity tells you how
close the vapor pressure, e, is to
saturation
RH = 100 * e / es
How much water is in the air How much water there could be
(hypothetical, depends on temperature)
You can change RH by:
• Changing e
o more water → higher e → higher RH
o less water → lower e → lower RH
• Changing es
o Higher temperature → higher es → lower RH
o Lower temperature → lower es → higher RH
Why is the air dry in an airplane cabin?
• Air brought in from outside the
plane.
• Outside air is ~ -50 °C
• Air is warmed up to ~ 20 °C
• es for -50 °C is 0.06 hPa
• es for 20 °C is 2.4 hPa
• es increases by factor of 40!
• If outside is 100% RH, inside
would be 2.5%.
• Airplanes have to humidify the
air they bring in
Dew Point
Dew point: the temperature at which
RH = 100% would occur
Dew point tells us how much water is
in the air (you can get it from e).
T – TD is called the dew point
depression, and tells you how
much you would need to cool to
create condensation
Why do the inside of your car windows fog when it’s cold?
The air in the car is humid
Imagine 16 °C and 67% RH.
16 °C means es = 18 hPa (Table 5.1)
18 hPa * 67% = 12 hPa = e
How cold does it need to get to reach saturation?
For what temperature would 12 hPa be saturated?
12 hPa is the es for air that is 10 °C
→ 10 °C is the dew point
If the windows are cold enough to cool the cabin
air to 10 °C you’ll get condensation.
Vapor pressure
• Total pressure = pressure from dry air
+
pressure from water vapor
What if we JUST considered the water vapor?
• This is what we call water vapor pressure, e
• Above example: e = 10 hPa
• What if there were twice as many vapor molecules? e = 20 hPa
1000 hPa of pressure from dry air
+ 10 hPa of pressure from water vapor
= 1010 hPa total pressure
Vapor pressure
• Total pressure = pressure from dry air
+
pressure from water vapor
What if we JUST considered the water vapor?
• This is what we call water vapor pressure, e
• Above example: e = 10 hPa
• What if there were twice as many vapor molecules? e = 20 hPa
• The more water vapor in the air, the more pressure it exerts
• It can be odd to think of an amount of water as a pressure – but we can visualize the pressure
from water vapor if we make it really big.
Visualizing vapor pressure
A really big can: https://www.youtube.com/watch?v=JsoE4F2Pb20
Total pressure = pressure from dry air
+
pressure from water vapor
Total
pressure
outside
Total
pressure
outside
Total
pressure
inside
https://www.youtube.com/watch%3Fv=JsoE4F2Pb20
Phases of water
Gas Liquid Solid
You can’t actually see water vapor
– it’s invisible
Clouds are made of tiny water drops We’re pretty familiar with this
Water: if you can see it, it’s not a gas
Phase changes
These get most
of our attention
What is energy?
Energy is that which allows work to be done
Make something move
Heat something up
Make an object
change shape
Make a chemical
reaction happen
Make something
melt or evaporate
Energy has to be added to make any of the happen
• The molecules of a liquid are more energetic than those of a solid
→ melting takes energy to make it happen
• The molecules of a gas are more energetic than those of a liquid
→ evaporating takes energy to make it happen
Why does it take energy?
Make something
melt or evaporate
• If you go the other way (condensation or freezing)
you get the energy back (energy is released)
The energy you have to put in or get back is called
latent heat
Latent heat
Energy is required to evaporate water
Using energy for evaporation takes that
energy away from the environment,
cooling it.
Latent heat is why dogs pant and people sweat
Water evaporates off the dog’s tongue.
Energy was needed to make the evaporation happen
That energy came from the dog’s body heat
à The dog’s tongue is cooled down by the evaporation
Evaporating sweat off our skin takes energy
out of our skin, cooling it.
If you were coated in 0.1 mm water and
evaporated it all in 5 minutes, that would
require ~1000 W!
As plants photosynthesize,
they evaporate water out of
pores in their leaves. You can see the water
being released if you trap
it inside a bag
Latent heat is why trees and plants keep a place cool
• Evaporation uses up energy, cooling the leaves (and thus the air around them)
• Evaporation of water in the soil cools the ground
As plants photosynthesize,
they evaporate water out of
pores in their leaves. You can see the vapor
being released if you trap
it inside a bag
Latent heat is why trees and plants keep a place cool
• Evaporation uses up energy, cooling the leaves (and thus the air around them)
• Evaporation of water in the soil cools the ground
New York City temperature map
à Central Park is several degrees cooler
In hot dry places, evaporative cooling is very effective
Swamp coolers are popular in the Southwest DIY “air conditioning” is popular?
Air is cooled by evaporating water in the pad Air is cooled by evaporating condensation on the bottles
Latent heat
Condensing water releases energy
You get the energy back when you
condense water vapor to liquid water.
Latent heat release warms up your drink
• Air next to the cold can cools by conduction
• Colder air → lower saturation vapor pressure → higher relative humidity
• Eventually saturation is reached,
which causes condensation
• A 0.1 mm thick layer of condensation
warms a 12 oz. drink by 4.9 °C
Latent heat release is why steam burns you, even though it’s only 212 °F
You could put your hand in a 500 °F oven and not get burnt Your hand over a boiling pot or kettle gets burnt quickly
The water vapor condenses on your skin and releases energy.
Latent heat release helps power thunderstorms (more later in the course)
Phase changes
Use energy, cool the environment
Release energy, heat the environment
A weather example
Sometimes rain falls through a dry layer and evaporates before it hits the ground. This is called virga.
A weather example
What is happening to the air temperature where the evaporation happens?
A weather example
What is happening to the air temperature where the evaporation happens?
Evaporation requires energy, which it takes from the air, so:
COOLINGCOOLING
A weather example
Cold air sinks because it is dense…
COOLINGCOOLING
A weather example
Cold air sinks because it is dense and then spreads out…
COOLINGCOOLING
We call this a gust front, and is the cold wind you feel before an intense rain happens
The gust front picks up
dirt/debris which shows
up on radar
