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

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

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

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

Fig. 3-2, p. 57 Stepped Art

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

Cold Air Near the Surface inversions inversions thermal belts thermal belts Drainage winds: cold air that slides downhill.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 T air, the min T sfc is a) higher, b) lower, c) the same

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

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

Fig. 3-9, p. 64 July sea level Ta 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

Fig. 3-8, p. 64 January sea level Ta 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

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

Monthly and Yearly Temperature Data annual temperature range: annual temperature range: max monthly T - min monthly T 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

Figure 2, p. 69 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)

The Use of Temperature Data heating degree-days: daily mean T < 64 o F (left panel) heating degree-days: daily mean T < 64 o F (left panel) cooling degree-days: daily mean T > 65 o F (right panel) cooling degree-days: daily mean T > 65 o F (right panel) growing degree-days: daily mean T > base T 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

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 wind chill index: frostbite could occur in 30 min or less in wind chill index: frostbite could occur in 30 min or less in shaded areas 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 liquid-in-glass thermometers: mercury or alcohol liquid-in-glass thermometers: mercury or alcohol maximum and minimum thermometers maximum and minimum thermometers Electrical thermometers Electrical thermometers instrument shelters instrument shelters Infrared sensor or radiometer (for surface T) Infrared sensor or radiometer (for surface T)

Q: Even when T air is 90F in Tucson, you would feel hot as you go under the sun, because a)T air 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? UA: &id=campus UA: &id=campus &id=campus &id=campus Tucson NWS: Tucson NWS: US: US: