Chapter 3:
January
July
perihelion
aphelion
147 x 106 km
152 x 106 km
A few % difference
Does this effect cause seasons?
No - too small effect
- Doesn’t explain North/South hemisphere differences
What determines seasons?
1. The angle sunlight strikes the surface of earth
2. Number of daylight hours
60°
At 60° from
perpendicular,
area is double and
intensity is half
In addition to this, more atmosphere must be penetrated at a large angle.
Therefore, more light is scattered or absorbed before reaching earth.
23.5° tilt of earth on axis effects both sun angle and length of day.
June 21
Summer solstice
Tropic of Cancer
September 22
Atumnal equinox
Equator
December 21
Winter solstice
Tropic of Capricorn
March 20
Vernal equinox
Equator
•
On June 21 at arctic circle (66.5° N) daylight lasts 24 hrs
•
If axis were not tilted, there would be 12 hours of day and 12 hours of night at each
latitude every day of the year
This is the case on September 22 and March 20 !
What is the highest angle of the sun from the horizon
at the Arctic Circle (Northern Hemisphere)
on June 21 ?
Arctic Circle
66.5° – 23.5° = 43°
90° – 43° = 47° = θ
66.5° - 23.5°
θ
Sun rays
66.5°
23.5°
Equator
What is the lowest angle of the sun from the horizon (at solar noon) in Gainesville (30°N)
on December 21 ?
53.5°
θ
Sun rays
30°
23.5°
30° + 23.5° = 53.5°
90° – 53.5° = 36.5°
θ = 36.5°
equator
Each day past June 21, the noon sun is slightly lower in the sky in the NH. (p. 57)
Wrong! Why?
On December 21, no sunlight at latitudes > 66.5° in the NH
During winter (in NH) sun rises in SE and sets in SW
During summer (in NH) sun rises in NE and sets in NW
Figure 3.9
W
set
winter
set
summer
S
N
rise
winter
E
rise
summer
A solar house
June 21
Dec 21
36.5°
On June 21, highest sun angle from horizontal at noon?
30° - 23.5° = 6.5°
90° - 6.5° = 83.5°
(in Gainesville)
Note: If your roof slope (down from horizontal) is equal to your latitude (facing south),
then the sun is most perpendicular to your roof averaged over entire year.
Mar 20
Sept 21
June 21
Middle line
perpendicular to roof
other 2 lines are ±
23.5° from middle line.
Dec 21
23.5°
30°
Often, a solar collector will be
tilted at a slightly greater
angle, so as to be more
effective in winter than
summer. See focus on p. 63
Controls of Temperature
What determines variations in temperature from one location to another?
1.
2.
3.
4.
Latitude – amount of daily solar radiation reaching surface (sun angle, length of day)
Land and water
Ocean currents
Elevation (fig 1.9)
Look at Figures 3.21 and 3.22 :
Isotherms bend from land to water
Put line along 20° isotherm in mid land on Jan. map (fig 3.21)
•
Oceans are warmer
Put line along 70° isotherm on land in July
•
Oceans are cooler
Why? Think about aluminum / glass rod experiment and reasons why something gets
hot :
1. Ability to absorb heat – circulation, albedo
2. Latent heat – some heat used to evaporate water
3. Specific heat or heat capacity – same amount of heat absorbed ⇒ smaller temperature
rise in water
Use of temperature data
People require heat when mean daily temperature (MDT) drops below 65°
MDT is average of highest and lowest temperature in 24 hrs.
Example
MDT =
For a spring day in Gainesville:
66° + 89°
= 77.5°
2
Heating degree-days for 1 day
65° - MDT
below 0 → set to 0
low 66°, high 89°
Cooling degree-days for 1 day
below 0 → set to 0
MDT - 65°
Add these up for 1 year to get maps like figure 3.25 (3.26) :
Mean annual total heating (cooling) degree-days
For Gainesville, in our example, we had 12.5 cooling degree-days
For year on average
~ 1000 heating degree-days
~ 2900 cooling degree-days
See growing degree-days, p 72
Measuring Air Temperature
Thermocouple:
Iron
Reference
Junction
constantan
Millivolt
meter
Sensing
Junction
+
_
Constantan
(55% Cu, 45% Ni)
A junction between 2 metals gives rise to a small voltage, ∆V, that varies with
temperature.
Calibrate millivolt meter to read in temperature.
Electrical thermometer :
Apply voltage, V
Measure current, i
V
R varies with temperature
R
i
Calibrate ammeter
i = V/R
Liquid in glass thermometers and bimetallic thermometers are based in the fact the most
materials expand when heated. (For the same reason gases do)
NOTE: Rubber shrinks when heated (heat shrink tubing or plastic wrap)
Dial moves as coil is heated,
calibrate to read temperature.