Chapter%203:%20Air%20Temperature

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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
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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.noa
a.gov/twc/climate/all-time.php
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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
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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

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Stepped Art
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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.
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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

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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?
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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

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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

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

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

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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
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The Use of Temperature Data

• heating degree-days daily mean T lt 64oF (left
  panel)
• cooling degree-days daily mean T gt 65oF (right
  panel)

Q Todays average temperature is 85oF. What is
the cooling degree days? a) 5 b) 10 c)
20 d) 30
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The Use of Temperature Data

• growing degree-days daily mean T gt 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
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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 bodys
  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

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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
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• 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

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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

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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)

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• 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
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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/