Chapter 3: Air Chapter 3: Air TemperatureTemperature

Daily temperature variationsDaily temperature variations The controls of temperatureThe controls of temperature Air temperature dataAir temperature data Air temperature and human comfortAir temperature and human comfort Measuring air temperatureMeasuring air temperature

Daily Temperature Daily Temperature VariationsVariations

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

Daytime WarmingDaytime Warming Solar radiation heats ground; Solar radiation heats ground; heat is transferred to the atmosphere via conduction, thermal convection (under calm heat is transferred to the atmosphere via conduction, thermal convection (under calm

wind condition), and wind-forced convectionwind condition), and wind-forced convection Clouds reduce surface Clouds reduce surface

temperature by reducing temperature by reducing

solar heatingsolar heating

Nighttime CoolingNighttime Cooling radiational coolingradiational cooling nocturnal inversionsnocturnal inversions

• Inversions tend to occur on Inversions tend to occur on clear, calm, dry nights:clear, calm, dry nights:

strong winds would mixstrong winds would mix air;air;

clouds or water vapor would clouds or water vapor would absorb longwave radiationabsorb longwave radiation

Fig. 3-2, p. 57

Stepped Art

Fig. 3-5, p. 60

Tsfc reaches max atnoon;

Tair reaches max at3pm;

Both Tsfc and Tair

reach min at sunrise;

T decreases w/ heightduring the day;

T increases w/ heightat night

Cold Air Near the Cold Air Near the SurfaceSurface

inversionsinversions thermal beltsthermal 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 Tair, the min Tsfc is a) higher, b) lower, c) the same

Protecting Crops from the Cold Night Protecting Crops from the Cold Night AirAir

To protect crops from cold To protect crops from cold surface:surface:

orchard heaters for heating orchard heaters for heating and convection;and convection;

wind machines for mixingwind machines for mixing To protect smallTo protect small plants:plants:use straw, cloth,use straw, cloth,or plastic bagor plastic bagto coverto cover

The Controls of The Controls of TemperatureTemperature

Latitude:Latitude: control daylight period and solar control daylight period and solar radiationradiation

land and water distribution:land and water distribution: specific heat specific heat ocean currents:ocean currents: moving warm/cold water moving warm/cold water

aroundaround Elevation:Elevation: T decreases with height T decreases with height specific heat:specific heat: energy needed to raise T of 1 kg energy needed to raise T of 1 kg

air air

by 1 Kby 1 K • Average weather conditions in the interior of large continents Average weather conditions in the interior of large continents are much different than average conditions in coastal areas.are much different than average conditions in coastal areas.

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

Water has a higher specific heat and oceans have a deeper layerto absorb solar heating so that ocean T changes more slowly than soil T

Fig. 3-8, p. 64January 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 DataDaily Temperature Data diurnal temperatur range: Tdiurnal temperatur range: Tmaxmax – T – Tmin min

large range occurs for dry, clear sky over desertlarge range occurs for dry, clear sky over desert clouds and humidity effects: reduce daytime T but increase clouds and humidity effects: reduce daytime T but increase

nighttime T nighttime T proximity to large bodies of waterproximity to large bodies of water Urban heat island: Urban heat island: Fig. 3-11

Monthly and Yearly Temperature Monthly and Yearly Temperature DataData

annual temperature range: annual temperature range: max monthly T - min monthly Tmax 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 The Use of Temperature DataData

heating degree-days: daily mean T < 64heating degree-days: daily mean T < 64ooF (left panel)F (left panel) cooling degree-days: daily mean T > 65cooling degree-days: daily mean T > 65ooF (right panel)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 Air Temperature and Human ComfortHuman 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 ComfortAir 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 areasshaded 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 Measuring Air TemperatureTemperature

Q: T is as high as 500oCin the upper atmosphere of300 km. Without protection suit,astronauts would be dead due toa) too high T,b) too cold Tc) strong solar UV radiation

Measuring Air Measuring Air TemperatureTemperature

liquid-in-glass liquid-in-glass thermometers: thermometers: mercury mercury or alcohol or alcohol

maximum and maximum and minimum minimum thermometersthermometers

Electrical Electrical thermometersthermometers

instrument sheltersinstrument shelters Infrared sensor or Infrared sensor or

radiometer (for radiometer (for surface T)surface T)

Q: Even when Tair is 90F 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?Where to get T data?

UA:UA: http://www.atmo.arizona.edu/index.php?section=http://www.atmo.arizona.edu/index.php?section=weather&id=campusweather&id=campus

Tucson NWS: Tucson NWS:

http://www.wrh.noaa.gov/twc/http://www.wrh.noaa.gov/twc/ US: US:

http://www.ncdc.noaa.govhttp://www.ncdc.noaa.gov