Chapter 3: Air Temperature

1. Chapter 3: Air Temperature • Daily temperature variations • The controls of temperature • Air temperature data • Air temperature and human comfort • Measuring air temperature

2. Daily Temperature Variations Record high T: 58 C or 136 F (Libya) Record low T: -89 C or -129 F (Antarctic)

3. Daytime Warming • Solar radiation heats ground; • heat is transferred to the atmosphere via conduction, thermal convection (under calm wind condition), and wind-forced convection • Clouds reduce surface temperature by reducing solar heating

4. Nighttime Cooling • radiational cooling • nocturnal inversions • Inversions tend to occur on clear, calm, dry nights: strong winds would mix air; clouds or water vapor would absorb longwave radiation

5. Stepped Art Fig. 3-2, p. 57

6. Tsfc reaches max at noon; Tair reaches max at 3pm; Both Tsfc and Tair reach min at sunrise; T decreases w/ height during the day; T increases w/ height at night Fig. 3-5, p. 60

7. Cold Air Near the Surface • inversions • thermal belts • Drainage winds: cold air that slides downhill. • Q: Where do you expect the minimum temperature in Tucson: • near river bed, • b) UA campus, • airport, • downtown

8. Q: Surface air temperature would keep increasing as long as the sun heats the surface. a) true, b) false Q: Surface air temperature reaches its maximum when a) the solar radiation is maximum, b) the difference of solar radiation minus longwave radiation becomes zero (from positive) Q: At night, compared with min Tair, the min Tsfc is a) higher, b) lower, c) the same

9. Protecting Crops from the Cold Night Air • To protect crops from cold surface: orchard heaters for heating and convection; wind machines for mixing • To protect small plants: use straw, cloth, or plastic bag to cover

10. The Controls of Temperature • Latitude: control daylight period and solar radiation • land and water distribution: specific heat • ocean currents: moving warm/cold water around • Elevation: T decreases with height • specific heat: energy needed to raise T of 1 kg air by 1 K • Average weather conditions in the interior of large continents are much different than average conditions in coastal areas.

11. Water has a higher specific heat and oceans have a deeper layer to absorb solar heating so that ocean T changes more slowly than soil T July sea level Ta Fig. 3-9, p. 64

12. Q: Winter T in UK is warmer than Russia over the same latitude, because of: a) warm ocean current, b) land-sea contrast, c) elevation difference January sea level Ta Fig. 3-8, p. 64

13. Daily Temperature Data • diurnal temperatur range: Tmax – Tmin large range occurs for dry, clear sky over desert • clouds and humidity effects: reduce daytime T but increase nighttime T • proximity to large bodies of water • Urban heat island: Fig. 3-11

14. Monthly and Yearly Temperature Data • annual temperature range: max monthly T - min monthly T • Q: Why does Richmond have a much larger annual range? • because Richmond is primarily affected by the ocean; • because Richmond is primarily affected by the American continent

15. What is normal T (30-yr average) for a particular day (March 15 over southwestern U.S.)? A: not a single value but a range of values (computed using statistical method) Figure 2, p. 69

16. The Use of Temperature Data • heating degree-days: daily mean T < 64oF (left panel) • cooling degree-days: daily mean T > 65oF (right panel) • growing degree-days: daily mean T > base T Q: Today’s average temperature is 85F. What is the cooling degree days? a) 5, b) 10, c) 20, d) 30

17. Air Temperature and Human Comfort • Human body stabilizes its T (i.e., prevents its T decrease) • primarily by converting food into heat (metabolism) • The stronger the wind, the faster the body’s heat loss • High winds in below-freezing air can remove heat from exposed • skin so quickly that the skin may actually freeze (called frostbite) • Dry heat: perspire to feel cool • Heat and humidity: feel hotter

18. Air Temperature and Human Comfort • wind chill index: frostbite could occur in 30 min or less in shaded areas Q: For the same wind speed of 5 mile/hr, the wind chill effect is stronger at: a) lower T, b) higher T, c) all T

19. Measuring Air Temperature • Q: T is as high as 500oC • in the upper atmosphere of • 300 km. Without protection suit, • astronauts would be dead due to • too high T, • too cold T • c) strong solar UV radiation

20. Measuring Air Temperature • liquid-in-glass thermometers: mercury or alcohol • maximum and minimum thermometers • Electrical thermometers • instrument shelters • Infrared sensor or radiometer (for surface T)

21. Q: Even when Tair is 90F in Tucson, you would feel hot as you go under the sun, because • Tair refers to T under the shade; • solar heating is too strong in Tucson • both a) and b)

22. Precip, wind, T/Td, rain type, lightning, cloud base, freezing rain, visibility

23. Where to get T data? • UA:http://www.atmo.arizona.edu/index.php?section=weather&id=campus • Tucson NWS: http://www.wrh.noaa.gov/twc/ • US: http://www.ncdc.noaa.gov