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Chapter 3: Air Temperature
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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.noaa.gov/twc/climate/all-time.php
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Daytime Warming
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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The Use of Temperature Data
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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
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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 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
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Air Temperature and Human Comfort
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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
a) too high T,
b) too cold T
c) 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
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liquid-in-glass
thermometers: mercury or
alcohol
maximum and minimum
thermometers
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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
a) Tair refers to T under
the shade;
b) solar heating is too
strong in Tucson
c) 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/
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