Seasonal & Daily Temperatures

1. Seasonal & Daily Temperatures • This chapter discusses: • The role of Earth's tilt, revolution, & rotatation in causing locational, seasonal, & daily temperature variations • Methods & tools for measuring temperature

2. Seasons & Sun's Distance Figure 3.1 Earth's surface is 5 million kilometers further from the sun in summer than in winter, indicating that seasonal warmth is controlled by more than solar proximity.

3. Seasons & Solar Intensity Figure 3.2 Solar intensity, defined as the energy per area, governs earth's seasonal changes. A sunlight beam that strikes at an angle is spread across a greater surface area, and is a less intense heat source than a beam impinging directly.

4. Solstice & Equinox Figure 3.3 Earth's tilt of 23.5° and revolution around the sun creates seasonal solar exposure and heating patterns. A solstice tilt keeps a polar region with either 24 hours of light or darkness. A equinox tilt perfectly provides 12 hours of night and 12 hours of day for all non-polar regions.

5. 24 Hours of Daylight Figure 3.4 Summer north of the artic circle will reveal a period of 24 hour sunlight, where the earth's surface does not rotate out of solar exposure, but instead experiences a midnight sun.

6. Earth's Tilt & Atmosphere Figure 3.5 Figure 3.6 Earth's atmosphere reduces the amount of insolation striking earth's surface. Earth's atmosphere and tilt combine to explain variation in received solar radiation.

7. Earth's Unequal Heating Figure 3.7 Incoming solar radiation is not evenly distributed across all lines of latitude, creating a heating imbalance.

8. Earth's Energy Balance Earth's annual energy balance between solar insolation and terrestrial infrared radiation is achieved locally at only two lines of latitude. A global balance is maintained by excess heat from the equatorial region transferring toward the poles. Figure 3.8

9. Longer Northern Spring & Summer Figure 3.9 Earth reaches its greatest distance from the sun during a northern summer, and this slows its speed of revolution. The outcome is a spring and summer season 7 days longer than that experienced by the southern hemisphere.

10. Local Solar Changes Northern hemisphere sunrises are in the southeast during winter, but in the northeast in summer. Summer noon time sun is also higher above the horizon than the winter sun. Figure 3.10

11. Landscape Solar Response Figure 3.11 South facing slopes receive greater insolation, providing energy to melt snow sooner and evaporate more soil moisture. North and south slope terrain exposure often trigger differences in plant types and abundance.

12. Daytime Warming Solar radiation heats the atmosphere from below by soil conduction and gas convection. Figure 3.12 Winds create a forced convection of vertical mixing that diminishes steep temperature gradients. Figure 3.13

13. Temperature Lags Earth's surface temperature is a balance between incoming solar radiation and outgoing terrestrial radiation. Peak temperature lags after peak insolation because earth continues to warm until infrared radiation exceeds insolation. Figure 3.14

14. Nighttime Cooling Figure 3.15 Figure 3.16 Earth's surface has efficient radiational cooling, which creates a temperature inversion that may be diminished by winds. Evening length, water vapor, clouds, and vegetation affect earth's nighttime cooling.

15. Cold Dense Air Figure 3.17 Nighttime radiational cooling increases air density. On hill slopes, denser air settles to the valley bottom, creating a thermal belt of warmer air between lower and upper cooler air.

16. Protecting Crops from Below Figure 3.18 Figure 3.19 Impacts of radiational cooling can be diminished by orchard heaters creating convection currents to warm from below and by wind machines mixing warmer air from above.

17. Protecting Crops from Above Crops subjected to below freezing air are not helped by convection or mixing, but by spraying water. The cold air uses much of its energy to freeze the water, leaving less to take temperatures below 0° C that damage the crop. Figure 3.20

18. Controls of Temperature • Earth's air temperature is governed by length of day and intensity of insolation, which are a function of: • latitude • Additional controls are: • 2) land and water • 3) ocean currents • 4) elevation

19. January Isotherms Latitude determines that earth's air temperatures are warmer at the equator than at the poles, but land and water, ocean currents, and elevation create additional variations. Figure 3.21

20. July Global Isotherms The southern hemisphere has fewer land masses and ocean currents that encircle the globe, creating isotherms that are more regular than those in the northern hemisphere. Figure 3.22

21. Daily Temperature Range Earth's surface efficiently absorbs solar energy and efficiently radiates infrared energy, creating a large diurnal temperature range (max - min) in the lower atmosphere. Figure 3.23

22. Regional Temperatures Regional differences in temperature, from annual or daily, are influenced by geography, such as latitude, altitude, and nearby water or ocean currents, as well as heat generated in the urban area. Figure 3.24

23. Heating Degree Day Figure 3.25 Temperature data are analyzed to determine when living space will likely be heated (e.g. when below 65° F) and how much fuel is required for that region.

24. Cooling & Growing Degree Days Figure 3.26 Daily temperature data are also used to determine cooling loads for living space above 65° F, as well as growing hours for specific crops above a base temperature.

25. Recording Thermometer Figure 3.27 Figure 3.28 Non-digital thermometers recorded maximum and minimum temperature using simple designs to temporarily trap the mercury or a marker along the thermometer scale.

26. Technological Upgrades Pen and lever recording drums required regular calibration for accurate data. Figure 3.29 Modern weather stations predominantly use digital data recording techniques that are less likely to introduce data error and generate data more readily analyzed by computers. Figure 3.30