class webpage is available and this week’s
lectures are available as PDF files
www.geog.ucsb.edu/~siegel/GEOG3A/
Honor’s Section People
Let’s set time after class
Jory’s Office Hours
Monday 12-1
Chapter 2: Solar energy to earth and
the seasons (continued)
Earth’s orbit around the sun
Solar energy: composition and amount
reaching Earth
March of the seasons
The reasons for seasons
note the
difference in
< 90 degrees
area to equa
90 degrees
the total
radiation
received
subsolar point = the point on Earth’s surface
where the sun is 90 degrees overhead
note the
difference in
area to equal
the total
radiation
received
Relative Intensity = sin(90-latitude)
recall that sin(90)=1
sin(45) = 0.71 (or 71% of the intensity when
the sun is directly overhead)
at the Arctic Circle (66.5 N)
intensity = sin(90-66.5) = 0.4
at the Arctic Circle (66.5 N)
intensity = sin(90-66.5) = 0.4
1/0.4 = 2.5
northern
hemisphere winter
TOA insolation
ranges from 100
to 500 W m-2
Summertime values
are higher in which
hemisphere?
northern
hemisphere
summer
Credit:
http://langley.atmos.colostate.edu
/srb/toasw2dc.jpg
Why are these TOA fluxes so much lower than
the solar constant (1370 W/m2)?
Insolation is determined by …
• Length of daylight
• Sun’s height in the sky
Seasonality is due to changes in
• the sun’s elevation
• daylength
Seasonal insolation by latitude
Some definitions
the sun’s position above the horizon is called its
altitude (or elevation or angle)
zenith - when the sun is directly overhead
(altitude is 90˚ above horizon)
Do we ever see this at UCSB?
subsolar point - a zenith sun
graphically...
zenith
z = solar zenith angle
h = solar altitude
90˚
Credit: http://www.srrb.noaa.gov/highlights/sunrise/azelzen.gif
March of the Seasons
solstice (literally “sun stance”) is the time when the sun is directly (90
degrees) overhead at the latitude of 23.5 N (Tropic of Cancer) or
23.5 S (tropic of Capricorn)
equinox - the period in spring and fall when the poles are
not tilted toward or away from the sun - the declination is
the equator
Credit: www.physicalgeography.net
declination - latitude of the subsolar point
annually migrates between 23.5˚S and 23.5˚N
Declination through the year
Variations in daylength
how long are the days on the equator?
12 hours, year-round
we get a little over
4 hours of
daylength
variation in Santa
Barbara
Credit: http://www.geog.ouc.bc.ca/physgeog/contents/images/daylength.GIF
Approximate daylength at key seasonal dates for
different latitudes
Latitude
Vernal Summer Autumna Winter
equinox solstice l equinox solstice
0˚
12 hrs
12 hrs
12 hrs
12 hrs
30˚
12 hrs
14 hrs
12 hrs
10 hrs
60˚
12 hrs
18.5 hrs 12 hrs
5.5 hrs
90˚
12 hrs
24 hrs
12 hrs
0 hrs
all locations on earth away from the equator
experience continuous changes in solar altitude and
daylength
changes in solar altitude at the equator (left) and
at 50˚ N for three key dates
Credit: www.physicalgeography.net
formula to calculate noontime solar altitude at
any extratropical latitude
altitude = 90 - latitude + declination
path of sun through the sky
would you have long sunsets in the tropics?
observer at 40˚ N
what do you think the declination is right now
(rough answer - hint: we are not too far from the
winter solstice)?
22.7˚ S (-22.7˚)
what do you think the sun’s current altitude (elevation)
is at our latitude (≈34.5˚ N) at solar noon?
about 33˚
to calculate the sun’s position in the sky and
sunrise/sunset for any point on earth, visit this online
calculator
http://www.srrb.noaa.gov/highlights/sunrise/gen.html
Reasons for seasons
Sphericity
Axial parallelism
Revolution (around the sun)
Rotation (on its axis)
Tilt (relative to the perpendicular)
earthyear.mov
Sphericity
parallel rays
intercepted by
a curved
surface
Axial
Parallelism
the central axis through
the poles points at the
north star - this
alignment is maintained
throughout the year
Revolution about Sun
the average speed of earth in its orbit
is 107,280 kph (~66,000 mph)
Credit: UN FAO
Rotation about the Earth’s Axis
the rotation and revolution
direction are both
counterclockwise
the sun rotates on its
axis (~28 days)
Earth’s Tilt