Transcript Chapter 3: Air Temperature

Chapter 3: Air
Temperature





Daily temperature variations
The controls of temperature
Air temperature data
Air temperature and human comfort
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;
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
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
Stepped Art
Fig. 3-2, p. 57
Tsfc reaches max at
noon;
Tair reaches max at
3pm;
Both Tsfc and Tair
reach min at sunrise;
T decreases w/ height
during the day;
T increases w/ height
at night
Fig. 3-5, p. 60
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:
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 Air
To protect crops from cold
surface:
orchard heaters for heating
and convection;
wind machines for mixing
 To protect small
plants:
use straw, cloth,
or plastic bag
to cover

The Controls of Temperature





Latitude: control daylight period and solar radiation
land and water distribution: specific heat
ocean currents: moving warm/cold water around
Elevation: T decreases with height
specific heat: energy needed to raise T of 1 kg air
by 1 K
• Average weather conditions in the interior of large continents
are much different than average conditions in coastal areas.
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
July sea level Ta
Fig. 3-9, p. 64
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
Fig. 3-8, p. 64
Daily Temperature Data




diurnal temperatur range: Tmax – Tmin
large range occurs for dry, clear sky over desert
clouds and humidity effects: reduce daytime T but increase
nighttime T
proximity to large bodies of water
Urban heat island:
Fig. 3-11
Monthly and Yearly Temperature Data

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
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)
Figure 2, p. 69
The Use of Temperature Data



heating degree-days: daily mean T < 64oF (left panel)
cooling degree-days: daily mean T > 65oF (right panel)
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
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 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
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)
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?



UA:
http://www.atmo.arizona.edu/index.php?section=weather
&id=campus
Tucson NWS:
http://www.wrh.noaa.gov/twc/
US:
http://www.ncdc.noaa.gov