1. Class #2: Seasonal and daily variations in temperature
Chapter 3
Class #2 July 8, 2010

2. Seasonal and Daily temperatures
Chapter 3
Seasonal and Daily temperatures
Class #2 July 8, 2010

3. Why the Earth has seasons
Earth revolves in elliptical path around sun every 365 days.
Earth rotates counterclockwise or eastward every 24 hours.
Earth closest to Sun (147 million km) in January, farthest from Sun (152 million lm) in July.
Distance not the only factor impacting seasons.
Class #2 July 8, 2010

4. FIGURE 3.1 The elliptical path (highly exaggerated) of the earth
about the sun brings the earth slightly closer to the sun in January than
in July.
Class #2 July 8, 2010

5. ACTIVE FIGURE 3.2 Sunlight that
strikes a surface at an angle is spread over a
larger area than sunlight that strikes the surface
directly. Oblique sun rays deliver less energy
(are less intense) to a surface than direct
sun rays. Visit the Meterology Resource Center
to view this and other active figures at
academic.cengage.com/login
Class #2 July 8, 2010

6. FIGURE 3.1 The elliptical path (highly exaggerated) of the earth
about the sun brings the earth slightly closer to the sun in January than
in July.
Class #2 July 8, 2010

7. Why the Earth has seasons
The amount of energy that reaches the Earths surface is influence by the distance from the Sun, the solar angle, and the length of daylight.
When the Earth tilts toward the sun in summer, higher solar angles and longer days equate to high temperatures.
Class #2 July 8, 2010

8. Why the Earth has seasons
Seasons in the Northern Hemisphere
Summer solstice: June 21, Sun directly above Tropic of Cancer, Northern Hemisphere days greater than 12 hours
Winter solstice: December 21, Sun directly above Tropic of Capricorn, Northern Hemisphere days less than 12 hours
Autumnal and Spring Equinox: September 22, Marc 20, Sun directly above Equator, all locations have a 12 hour day
Class #2 July 8, 2010

9. FIGURE 3.5 The relative amount of radiant energy received
at the top of the earth’s atmosphere and at the earth’s surface on
June 21 — the summer solstice.
Class #2 July 8, 2010

10. FIGURE 3.6 During the Northern Hemisphere summer, sunlight
that reaches the earth’s surface in far northern latitudes has passed
through a thicker layer of absorbing, scattering, and reflecting atmosphere
than sunlight that reaches the earth’s surface farther south. Sunlight
is lost through both the thickness of the pure atmosphere and by
impurities in the atmosphere. As the sun’s rays become more oblique,
these effects become more pronounced.
Class #2 July 8, 2010

11. FIGURE 3.8 The apparent path of the sun across the sky as observed at different latitudes on the June solstice (June 21),
the December solstice (December 21), and the equinox (March 20 and September 22).
Class #2 July 8, 2010

12. Figure 2.24: The apparent path of the sun across the sky as observed at different latitudes on the June solstice (June 21), the December solstice (December 21), and the equinox (March 20 and September 22).
Stepped Art
Class #2 July 8, 2010
Fig. 3-8, p. 63

13. Figure 3.4
Land of the Midnight Sun. A series of exposures of the sun taken before, during, and after midnight in northern Alaska during July.
Class #2 July 8, 2010
Fig. 3-4, p. 60

14. TABLE 3.1 Length of Time from Sunrise to Sunset for
Various Latitudes on Different Dates in the Northern Hemisphere
Class #2 July 8, 2010

15. Why the Earth has seasons
Special Topic: First day of winter
December 21 is the astronomical first day of winter, sun passes over the Tropic of Capricorn; not based on temperature.
Class #2 July 8, 2010

16. Why the Earth has seasons
Seasons in the Southern Hemisphere
Opposite timing of Northern Hemisphere
Closer to sun in summer but not significant difference from north due to:
Greater amount of water absorbing heat
Shorter season
Class #2 July 8, 2010

17. Local temperature variations
Southern exposure: warmer, drier locations facing south. Implications for
Vegetation
Viniculture
Ski slopes
Landscaping
Architecture
Class #2 July 8, 2010

18. FIGURE 3.10 In areas where small temperature changes can
cause major changes in soil moisture, sparse vegetation on the southfacing
slopes will often contrast with lush vegetation on the northfacing
slopes.
Class #2 July 8, 2010

19. Local temperature variations
Environmental Issues: Solar Heating
In order to collect enough energy from solar power to heat a house, the roof should be perpendicular to the winter sun.
For the mid-latitudes the roof slant should be 45°- 50°
Class #2 July 8, 2010

20. Daily temperature variations
Each day like a tiny season with a cycle of heating and cooling
Daytime heating
Air poor conductor so initial heating only effects air next to ground
As energy builds convection begins and heats higher portions of the atmosphere
After atmosphere heats from convection high temperature 3-5PM; lag in temperature
Class #2 July 8, 2010

21. FIGURE 3.11 On a sunny, calm day, the air near the surface can
be substantially warmer than the air a meter or so above the surface.
Class #2 July 8, 2010

22. FIGURE 3.12 Vertical temperature profiles above an asphalt
surface for a windy and a calm summer afternoon.
Class #2 July 8, 2010

23. Figure 3.2: The daily variation in air temperature is controlled by incoming energy (primarily from the sun) and outgoing energy from the earth’s surface. Where incoming energy exceeds outgoing energy (orange shade), the air temperature rises. Where outgoing energy exceeds incoming energy (blue shade), the air temperature falls.
Stepped Art
Class #2 July 8, 2010
Fig. 3-13, p. 67

24. Daily temperature variations
Properties of soil affect the rate of conduction from Earth to atmosphere
Wind mixes energy into air column and can force convection.
Class #2 July 8, 2010

25. Daily temperature variations
Nighttime cooling
As sun lowers, the lower solar angle causes insolation to be spread across a larger area
Radiational cooling as infrared energy is emitted by the Earth’s surface
Radiation inversion: air near ground much cooler than air above
Thermal belt
Class #2 July 8, 2010

26. FIGURE 3.14 On a clear, calm night, the air near the surface
can be much colder than the air above. The increase in air temperature
with increasing height above the surface is called a radiation temperature
inversion.
Class #2 July 8, 2010

27. FIGURE 3.15 Vertical temperature profiles just above the ground
on a windy night and on a calm night. Notice that the radiation inversion
develops better on the calm night.
Class #2 July 8, 2010

28. Daily temperature variations
Protecting crops from cold
Cover
Smudge pots
Fans
Sprinklers
Class #2 July 8, 2010

29. FIGURE 3.19 Wind machines mix cooler surface air with
warmer air above.
Class #2 July 8, 2010

30. FIGURE 3.20 Average air temperature near sea level in January (oF).
Class #2 July 8, 2010

31. The controls of temperature
Latitude: solar angle and day length
Land & water: specific heat
Ocean currents: warm and cold currents
Elevation: cooling and increase range
Class #2 July 8, 2010

32. FIGURE 3.20 Average air temperature near sea level in January (oF).
Class #2 July 8, 2010

33. FIGURE 3.21 Average air temperature near sea level in July (oF).
Class #2 July 8, 2010

34. Air temperature data Daily, monthly, yearly temperature
Range: maximum minus minimum
Mean: average of temperature observations
Maximum: highest temperature of time period
Minimum: lowest temperature of time period
Class #2 July 8, 2010

35. FIGURE 3.25 Temperature data for San Francisco, California
(37oN), and Richmond, Virginia (37oN) — two cities with the same
mean annual temperature.
Class #2 July 8, 2010

36. Air temperature data Special topic: What’s normal?
Climate normal is the 30 year average for a given temperature variable.
Class #2 July 8, 2010

37. Air temperature data The use of temperature data
Heating degree-day: people heat when temperature below 65°F
Cooling degree-day: people cool when temperature above 65°F
Growing degree-day: temperature above of below base temperature for specific crop
Class #2 July 8, 2010

38. FIGURE 3.25 Temperature data for San Francisco, California
(37oN), and Richmond, Virginia (37oN) — two cities with the same
mean annual temperature.
Class #2 July 8, 2010

39. FIGURE 3.26
Mean annual total heating degree-days
across the United States (base 65oF).
Class #2 July 8, 2010

40. TABLE 3.2 Estimated Growing Degree-Days for Certain
Naturally Grown Agricultural Crops to Reach Maturity
Class #2 July 8, 2010

41. Air temperature and human comfort
Body heats through metabolism
wind-chill index
Hypothermia
Body cools through emitting infrared energy and evaporation of perspiration
Class #2 July 8, 2010

42. Air temperature and human comfort
Observation: 1000 degrees
Thin air at the top of the atmosphere does not have enough molecules to create a high temperature as measured by a thermometer.
Class #2 July 8, 2010

43. Measuring air temperature
Thermometers: liquid-in-glass, maximum, minimum, electrical resistance, bimetallic
ASOS
Thermistors
Infrared sensors
Class #2 July 8, 2010

44. Measuring air temperature
Observation: Thermometers in the shade
Radiant energy from the Sun in direct sunlight increases the temperature recorded by a sensor.
True air temperature measured in the shade.
Class #2 July 8, 2010