1. 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

2. 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

3. 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

4. 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

5. Figure 3.2: The daily variation in air temperature is controlled by incoming energy (primarily from the sun) and outgoing energy from the earth’s surface. Where incoming energy exceeds outgoing energy (orange shade), the air temperature rises. Where outgoing energy exceeds incoming energy (blue shade), the air temperature falls.
Stepped Art

6. temperature profile at night and during day.
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.
Figure 3.5: An Idealized distribution of air temperature above the ground during a 24-hour day. The temperature curves represent the variations in average air temperature above a grassy surface for a mid-latitude city during the summer under clear, calm conditions.

7. 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

8. 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?

9. 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

10. 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

11. Q: why is temperature over part of the Greenland ice greater than freezing point (32oF) in this figure?
Figure 3.9: Average air temperature near sea level in July (°F).
July sea level Ta

12. 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
Figure 3.8: Average air temperature near sea level in January (°F).
January sea level Ta

13. 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
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
Figure 3.8: Average air temperature near sea level in January (°F).

14. 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?

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

16. 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

17. 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

18. The Use of Temperature Data
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

19. The Use of Temperature Data
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

20. 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

21. 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

22. 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

23. 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

24. 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)

25. 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

26. 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: =weather
Tucson NWS:
US: