GEU 0047: Meteorology
Lecture 3
Seasonal Cycle
Reasons of Seasonality and its Changes
• Earth’s Tilt (Seasonal)
– Tilt of rotational axis with respect to its orbit around sun
– Solar Radiation (Diurnal or Daily)
• More Direct Sun Light
• Longer Duration of Day Light Hours (Length of day)
• Orbit Variations (Very Long-Term, Small Effects)
– tilt
– Eccentricity (偏心率)
– Precession (進動)
Tilt
Earth’s rotational axis remains fixed with respect to
the stars.
Earth’s Circle of Illumination
50 % of the earth’s
surface is
illuminated
by the sun,
every hour
of every “day”.
Incident Angle
Changing Aspect Angle (方向角)
• Direct Sunlight is more intense and concentrated.
• Solar Incidence Angle is Latitude and Time Dependent
– Seasonal and Diurnal
Summer Solstice
• Sunlight is most direct at the Tropic of Cancer (23.5 N) on
~ June 21, less direct elsewhere.
SOLSTICE
Summer (June 21st-ish)
＝> Sun Located at the Tropic of Cancer
Winter (December 21st-ish)
＝> Sun Located at the Tropic of Capricorn
Tropic of Cancer
Equator
Tropic of Capricorn
Prime Meridian
June Solar Insolation
Winter Solstice
• Sunlight is most direct at the Tropic of Capricorn (23.5 S)
on ~ January 21.
Solar Insolation
• Energy received by the earth at various latitudes for
December 21 (solstice)
December Solar Insolation
Equinox
Equinoxes (Equal Day and Equal Night, 12 hours)
Spring (Vernal Equinox) March 21st
Fall (Autumnal Equinox) September 21st
Sun Located on Celestial Equator (Earth’s Equator)
Tropic of Cancer
Equator
Tropic of Capricorn
Prime Meridian
Seasonal Orientations
Day Light Hours (June Solstice)
Day Light Hours (Equinox)
Day Light Hours (c.f., Table 3.1)
Combined Annual Average Solar Insolation
Fictitious Extreme Seasons Tilt
Solar Altitude Angle (Solstice)
Seasonal Solar Latitude
Solar Altitude Angle
• Solar Altitude at Local Noon of Taipei
– What is your Latitude?
– What is the Date?
25N
2011 Oct 25
Solar Altitude Angle
• Solar Altitude at Local Noon of Taipei
– What is your Latitude?
– What is the Date?
25N
2011 Oct 25
• What is the apparent latitude (declination) of the Sun today? ~10oS
Solar Altitude Angle
• Solar Altitude at Local Noon of Taipei
– What is your Latitude?
– What is the Date?
25N
2011 Oct 25
• What is the apparent latitude (declination) of the Sun? ~10S
– Calculate the distance in degrees between your latitude and the
declination of the sun. 25 N + 10 S = 35 degrees
Solar Altitude Angle
• Solar Altitude at Local Noon of Taipei
– What is your Latitude?
– What is the Date?
25N
2011 Oct 25
• What is the apparent latitude (declination) of the Sun? ~10S
– Calculate the distance in degrees between your latitude and the
declination of the sun. 25 N + 10 S = 35 degrees
– 90 degrees minus the latitude declination difference will be the
solar altitude at local noon for your location on that date.
90 - 35 = 55
Altitude-Azimuth
Taipei 2011 Oct 25
Zenith
Path of the Sun
55 degrees
N
S
W
An environmental issue
Solar Homes Application
Position Angle of Solar Panels
Zenith
• There is much less atmosphere to travel through directly
overhead than on the horizon. The sun is more direct and
intense.
Diurnal Solar Intensity
• By the time the sun gets on the horizon, it is at a grazing
angle and heavily scattered, refracted and absorbed.
Diurnal Temperature
Thermal Inertia
For Any Given Northern Latitude
• Maximum Daily Solar Insolation Occurs At
• Maximum Yearly Solar Insolation Occurs On
• Hottest Hour of the Day (on average)
• Hottest Month of the Year (on average)
Thermal Inertia
For Any Given Northern Latitude
• Maximum Daily Solar Insolation Occurs At
• Maximum Yearly Solar Insolation Occurs On
• Hottest Hour of the Day (on average)
• Hottest Month of the Year (on average)
It takes a while to heat up and cool down!
Local Noon Shadow
Local Noon Shadow
Equinox Shadow
Sun rises
due East,
Sun sets
due West
8a.m.
sundial
Made by Egyptian 1500 BC
4p.m.
Sundial bridge (Redding California)
Summary
• Earth-Sun Geometry
• Seasons
• Solar Energy
– Seasonal (Latitude) Changes
– Diurnal (Daily) Changes
HW#2 (due date: TBA, because it depends on
weather)
Perform the field experiment (you may need a
partner)
• Calculate local (e.x. Taipei) solar altitude angle
someday in recent days
• Obtain the solar declination angle in the day you
perform the measurement