Chapter 3 Air Temperature Daily temperature variations n

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

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

Daytime Warming n n n 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

Nighttime Cooling n n 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

Stepped Art Fig. 3 -2, p. 57

Tsfc reaches max at noon; Tair reaches max at 3 pm; 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

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

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

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

The Controls of Temperature n n n Latitude: control daylight period and solar radiation land 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.

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

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

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

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

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

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

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

Air Temperature and Human Comfort n 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

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

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

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

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

Where to get T data? n n n 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