Chapter 3: Air Temperature

1. Chapter 3: Air Temperature • Warming and cooling the air near the ground • Application of temperature data • Air temperature and human comfort • Measuring air temperature Freezing rain

2. Warming and Cooling Air neat the Ground Record high T: 58oC or 136oF (Libya) Record low T: -89oC or -129oF (Antarctic) Q: What is the record high temperature in Tucson: a) 105oF; b) 111oF; c) 117oF; d) 123oF Q: What is the record low temperature in Tucson: a) 0oF; b) 6oF; c) 12oF; d) 18oF http://www.wrh.noaa.gov/twc/climate/all-time.php

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 Q: which surface is heated faster during the day? a) dry surface; b) wet surface; c) same

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 and emit longwave radiation to heat surface temperature difference between surface and 1.5m height is greater during the day than at night in magnitude

5. Stepped Art

6. Ts reaches max at noon; Ta reaches max at 3pm; Both Ts and Ta reach min at sunrise; T decreases w/ height during the day; T increases w/ height at night At quiz, you might be asked to plot temperature profile at night and during day.

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 Q: What weather conditions are most favorable for cold night and a strong inversion? Q: Why does a heavy jacket keep you warm in cold winter?

9. Protecting Crops from the Cold Night Air • To protect crops from cold surface: orchard heaters for radiative heating and convection; wind machines for mixing of colder surface with warmer air T; flooding for slower cooling and faster heat transfer from deeper soil; sprinkers for releasing latent heat and freezing to form a thin ice on buds Q: Small plants can be protected by covering them with a) straw; b) cloth; c) plastic bag; d) all of them

10. The Controls of Temperature • Latitude: control daylight period and sun elevation • land and water distribution: heat capacity • ocean currents: moving warm/cold water around • Elevation: T decreases with height • Specific heat: energy needed to raise T of 1 kg mass by 1 K; and specific heat for water is greater than that of soil • Heat capacity: total (relevant) mass times specific heat • Ocean has a deeper layer to absorb solar radiation; and hence has a much higher heat capacity than soil. • Therefore ocean temperature changes more slowly

11. Q: why is temperature over part of the Greenland ice greater than freezing point (32oF) in this figure? July sea level Ta

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

13. Using the January sea level Ta data below: Q: which has a higher temperature over the same midlatitude in northern hemisphere: a) land; b) ocean; c) same Q: which has a higher temperature over the same midlatitude in southern hemisphere: a) land; b) ocean; c) same Q: which hemisphere has a greater temperature change with latitude: a) northern hemisphere; b) southern hemisphere; c) same

14. Daily Temperature Data • diurnal temperatur range: Tmax – Tmin large range occurs for dry, clear sky over desert (dry soil) • clouds and humidity effects: reduce daytime T but increase nighttime T • dry soil has a smaller specific heat and lower evaporation Q: why does water surface have a smaller diurnal range?

15. Diurnal temperature range decreases with height Q: where is the diurnal temperature range higher: a) urban area; b) rural; c) same

16. 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

17. Normal T refers to the 30-year average T for that day Q: What is the “normal” temperature for a particular day (March 15 over southwestern U.S.) in the figure? A: not a single value but a range of values (computed using statistical method) Q: Is 85oF abnormal based on this figure: a) yes; b) no

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

19. The Use of Temperature Data • growing degree-days: daily mean T > base T Q: For an idealized situation: base T of 50oF, daily average T of 60oF, and GDD to maturity of 1200, how many months are needed (after planting) for harvest: a) 1 months; b) 2 months; c) 3 months; d) 4 months

20. 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

21. 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) same

22. Hypothermia: the rapid, progressive mental and physical collapse that accompanies the lowering of human body temperature. Favorable condition for hypothermia: cold, windy, and wet Q: For temperature of 0oF and wind speed of 20 mile/hr, the wind-chill temperature is: a) 0oF; b) -20oF; c) -22oF

23. 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 • strong solar UV radiation • Q: how do you measure T in such • an environment? • a) using a special thermometer; • b) using a special radiometer

24. 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)

25. 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 Q: what is the measurement height of surface air temperature? a) 0.5m; b) 1.5m; c) 2.5m

26. Over 900 Automated Surface Observing System (ASOS) Precip, wind, T/Td, rain type, lightning, cloud base, freezing rain, visibility Where to get T data? UA: http://www.atmo.arizona.edu/index.php?section =weather Tucson NWS: http://www.wrh.noaa.gov/twc/ US: http://www.ncdc.noaa.gov/