Announcements

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Announcements
Lab kits will be distributed today at the end of
the lecture (little after 930). See homework
assignments page. Lecture notes are now also
posted in PowerPoint format
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When is the Earth closest to the sun?

1. Northern hemisphere winter
2. Northern hemisphere spring
3. Northern hemisphere summer
4. Northern hemisphere fall
5. The Earth is the same distance from the sun year
   round.

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Summary of Lecture 7
The three orbital parameters of the Earth are the
eccentricity, precession, and obliquity. Most
relevant to the discussion of the seasons is the
obliquity, or tilt of the Earth with respect to
its orbital plane (at 23.5). The intensity of
solar energy depends on the zenith angle. If the
sun is directly overhead the zenith angle is
equal to zero and the solar energy is most
intense. Solar energy is further attenuated at
high zenith angles due to the fact that the solar
bean has more atmosphere to pass
through. Earths obliquity causes variation in
solar radiation by changes in the zenith angle
and length of day through the yearand thus is
the cause of the seasons. Special latitudes are
associated with the solstices and equinoxes.
Know what these special latitudes are and what
they physically mean. Know dates when the
solstices and equinoxes occur.
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NATS 101 Section 4 Lecture 8

• Temperature Variations

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Today well see how and why temperature varies
through the day and across the globe

• Many of the concepts we looked
• at in previous lectures, such as modes of heat
  transfer and specific heat capacity, will prove
  helpful in the discussion

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Lets start with how daytime and nighttime
temperatures are affected by the three modes of
heat transfer
For simplicity, well a consider a clear and calm
dayand there are lots of those in Arizona since
we live in desert!
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Concept of a surface energy budget
ENERGY IN
ENERGY OUT
GROUND
ENERGY IN ENERGY OUT ? Temperature
constant ENERGY IN gt ENERGY OUT ? Temperature
increases ENERGY IN lt ENERGY OUT ? Temperature
decreases
ENERGY TRANSPORT CONVECTION, CONDUCTION,
RADIATION
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Daytime Energy Budget Morning
Tground increases Tair increases
Incoming solar radiation
Outgoing terrestrial radiation
Conduction (from ground to air)
GROUND
Incoming solar radiation exceeds outgoing
terrestrial radiation. The temperature of the
ground warms. Conduction heats the air, but
because air is a poor conductor, this only
happens in the few centimeters above the ground.
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Daytime Energy Budget Midday and Afternoon
Tground increases Tair increases
Incoming solar Radiation (maximum at noon)
Outgoing terrestrial radiation
Convection
Conduction (from ground to air)
GROUND
Convection begins near the surface, in the form
of thermals, and helps to redistribute the heat
upward. As long as the incoming solar
radiation exceeds the sum of the outgoing
terrestrial radiation, conduction, and
convection, the air temperature continues to
increase.
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Typical Temperature Profile on a Calm, Clear
Summer Day in Arizona
Why is the thermometer located within a shelter
that is elevated, painted white, and have vents
on the side?
COTTON REGION SHELTER
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Convective Thermals
On really warm days, the convective thermals can
cause dust devils. This is a common occurrence
in Arizona during the summer. Well talk about
how these occur later in the semester.
Arizona Dept. of Transportation photo
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Time of Maximum Temperature
The maximum temperature occurs when the incoming
solar energy is equal to the outgoing earth
energy Outgoing earth energy is the sum of the
outgoing terrestrial radiation, conduction, and
convection. Maximum temperature typically occurs
in mid-afternoon, several hours after the maximum
in incoming solar energy.
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So how does the surface energy budget and
temperature change at night?
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Nighttime Energy Budget
Tground decreases Tair decreases
WARM
Outgoing terrestrial radiation
Conduction (from air to ground)
Solar radiation shut off
COLD
GROUND
Solar radiation is shut off and the air and
ground begins to cool by radiating terrestrial
radiation. This process is called radiational
cooling. Because the ground radiates more
effectively then air, it cools faster. The air
just above the ground transfers additional energy
to ground by conduction.
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Typical Nighttime Temperature Profile
Radiational cooling of the surface causes the air
near the ground to be colder than the air
above. When temperature increases with height,
this is called an inversion. Well see next
time that a certain type of fog may occur in the
inversion.
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Time of Minimum Temperature
The minimum temperature occurs right around
sunrise, after the Earths surface has
radiationally cooled during the entire night.
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Tucson Morning Sounding for 1-15-07
-10C isotherm
TEMPERATURE
0C isotherm
LITTLE OR NO WIND NEAR SURFACE
INVERSION (surface to 850 mb)
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So far weve considered the diurnal evolution of
temperature for the idealized case of clear and
calm conditions. What are some factors that
would make conditions less than ideal?
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Wind Mixes the air by forced convection
Wind provides a mechanical mixing mechanism to
transfer heat away from the surface during the
day and to the surface at night. Temperature
variation with height is more uniform.
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Clouds Affect solar and terrestrial radiation
ABSORPTION, SCATTERING AND REFLECTION OF SOLAR
RADIATION
VERY EFFECTIVE ABSORBERS AND EMITTERS OF
TERRESTRIAL RADIATION
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Daytime Energy Budget With Clouds
Solar radiation reflected, scattered, absorbed
Terrestrial radiation to ground
Decreased solar radiation to ground
Convection
Outgoing terrestrial radiation
Conduction (from ground to air)
GROUND

The effect of reflection of solar radiation is
most dominant, so the presence of clouds
typically results in cooler surface temperatures
during the day.
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Nighttime Energy Budget With Clouds
Terrestrial radiation to ground
Solar radiation shut off
Conduction (from air to ground)
Outgoing terrestrial radiation
GROUND
Because of their emission of terrestrial
radiation, the presence of clouds results in
warmer surface temperatures at night.
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Effect of Surface Moisture Latent Heating
Incoming solar radiation
Evaporation and transpiration (latent heat)
Outgoing terrestrial radiation
Convection
Conduction (from ground to air)
GROUND
The presence of water, causes some of the surface
energy to be partitioned to evaporation and
transpiration of moistureor latent heating.
This effect decreases the surface temperature
during the day.
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What controls temperature variations from place
to place?
Latitude Land and water distribution Ocean
currents Elevation
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Global Surface Temperature Variability Latitude
Effect of latitude Temperatures decrease with
increasing latitude. Effect is more pronounced in
wintertime. Largest annual changes occur at
higher latitudes. Temperature does not change
much in the low-latitude tropics Hottest places
are in the sub-tropics, where most deserts are
January
July
Maximum Temperature
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Global Surface Temperature Variability Land and
water distribution
Effect of oceans and continents Greatest
temperature swings are in the interior of
continents. Siberia 60 F summer -50 F
winter Reason Water has a higher specific heat
capacity than land, so it heats and cools slowly.

SIBERIA
January
SIBERIA
July
Maximum Temperature
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FlashbackHigh Heat Capacity of Water
Heat capacity of water is 4 to 5 times greater
than rock or soil!
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Continental vs. Maritime Climate
Two things to notice here 1. For both places,
the hottest and coldest month DO NOT occur at the
time of the winter and summer solstices. 2.
The place located on the ocean has less of an
annual temperature range and its hottest month
occurs about a month or two after the more
continental location.
Continental
Maritime
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Global Surface Temperature Variability Ocean
currents

SIBERIA
Some areas at high latitudes are quite warm
relative to other places at the same latitude,
especially in winter. Most of northern Europe,
including England, Germany, France, and
Scandanavia is at the same latitude as Siberia or
Canada!
CANADA
EUROPE
January
July
Maximum Temperature
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European Climate and the Influence of the Gulf
Stream
The Gulf Stream current in the North Atlantic
transports warm water from the tropics toward
Europe. As a result, Europe is much warmer
than it otherwise would be. Well discuss this
topic in more detail later in the course.
HEAT TRANSPORT BY OCEAN
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Elevation and Temperature
Temperatures decrease as elevation
increases. Lapse rate 6.5C per km Note the
uniform decrease in temperature with latitude in
the eastern U.S. Temperature pattern in the
western U.S. reflects the topography.
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Surface Temperature Tucson vs. Humphreys Peak
Humphreys Peak Elevation 3850 m Highest Point
in AZ About 20C cooler than Tucson, on average!
Tucson Elevation 728 m
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Summary of Lecture 8
We looked at how daytime and nighttime
temperatures are affected by the three modes of
heat transfer. During a clear, calm day the
ground warms because the incoming solar radiation
exceeds the outgoing terrestrial radiation and
conduction from ground to air, and convection.
The time of maximum temperature occurs in
mid-afternoon, several hours after the maximum in
incoming solar energy. During a clear, calm
night, the ground cools because of outgoing
terrestrial radiation and conduction from air to
ground. An inversion forms when the temperature
near the surface is colder than above. Minimum
temperature occurs around sunrise after the
surface has cooled the entire night. Winds,
clouds, and evaporation and transpiration are
three factors that would change how temperature
evolves during the day. Temperature variations
over space are controlled by latitude, land and
water distribution, ocean currents, and
elevation. Oceans in particular moderate climate
because of the high heat capacity of water and
heat transported by currents (like the Gulf
Stream).
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Reading Assignment and Review Questions
All of Chapter 4 Appendix D Chapter 3
questions Questions for Review
7,8,9,10,11,12,13,16,17,18,19,20,25,26,27 Questio
ns for Thought 6,7,9,11,12,13