Radiation and Climate
Thermal Properties at Earth’s Surface
1
Colors
• If you have visited the South or Southwest,
you may have noticed that many cars are light
colored, both inside and out. Do you know
why?
• If you had to walk barefoot across an asphalt
parking lot on a hot, sunny day, you would
probably find that carefully walking on the
white, painted stripes is more comfortable
than walking on the dark asphalt. Why?
2
Reflectivity
• When solar radiation strikes a surface, some photons
are absorbed, which increases the surface
temperature, and other photons are reflected.
– The proportion of radiation that is reflected, expressed as
the material’s reflectivity, does not raise the object’s
temperature.
– Light-colored surfaces reflect more radiation, and
therefore remain cooler, than dark-colored surfaces.
• For example, clean snow reflects almost 95% of solar radiation,
whereas forests reflect very little
– Variations in the reflectivity of materials at Earth’s surface
help determine local surface temperatures.
3
Asphalt absorbs almost all solar radiation that
strikes it, which causes asphalt roads to become
quite hot and radiate heat.
http://www.travelblog.org/Oceania/Australia/Wester
n-Australia/Perth/blog-363577.html
4
Clean snow has reflectivity, but trees by
contrast, possess relatively low reflectivity—
when they’re not snow-covered as depicted
here.
http://www.deskpicture.com/dps/sports
/skier_1.html
5
Specific Heat Capacity
• Every material has a characteristic reflectivity
and heat capacity, which together determine
how much and how fast the material warms.
• Specific heat capacity is the quantity of
thermal energy (heat) needed to raise the
temperature of one gram of a substance by
one degree Celsius.
6
“Storage Capacity”
• In effect, a specific heat capacity value for a
material can be considered as its “storage
capacity” for thermal energy.
– The lower a material’s specific heat capacity, the
higher its temperature will rise if a certain
quantity of thermal energy is added.
– The higher the specific heat capacity of the
material, the less its temperature will rise for a
given quantity of added thermal energy.
7
Examples
• Materials with higher specific heat capacities can
store more thermal energy.
– For instance, copper metal, with a specific heat
capacity of 0.387 J/g°C will have a larger temperature
change in response to the same quantity of thermal
energy added than will water, with a specific heat
capacity of 4.18 J/g°C.
– In particular, 10.0 J of thermal energy will raise the
temperature of 1.00 g of copper by 25.8°C; however, it
will increase the temperature of an equal mass of
water by only 2.4°C, which is less than 10% of
copper’s temperature increase.
8
Specific Heat Capacities for Common Materials at 20°C
Material
Specific Heat Capacity, J/g°C
Air
1.00
Aluminum
0.895
Asphalt
0.92
Brass
0.380
Carbon dioxide
0.832
Copper
0.387
Ethyl alcohol
2.45
Gold
0.129
Granite
0.803
Iron
0.448
Lead
0.128
Sand
0.29
Silver
0.233
Stainless steel
0.51
Water (liquid)
4.18
Zinc
0.386
9
Water
• Water’s unique properties make it influential in
the world’s climates.
• Water’s specific heat capacity is higher than that
of most other common materials.
• Because of this, bodies of liquid water can store
large quantities of thermal energy and are thus
slow to heat up or cool down.
• By contrast, land surfaces have much lower
specific heat capacities; they heat up and cool
down much more readily.
10
HOMEWORK
1) a) Compare lake water and asphalt in terms of how
readily each warms up when exposed to sunlight.
b) What properties of these two materials account for
differences in their behavior?
2) From a scientific viewpoint, why do many desert
dwellers elect to wear white or light-colored
garments?
3) Given their respective functions, compare the specific
heat capacities of the material from which the handle
of a frying pan is made and the material from which
the frying pan itself is made.
11
HOMEWORK
4) Given the same energy input, which would you
expect to have the greater temperature increase,
equal masses of aluminum or iron? Use the table
in this PowerPoint to explain your answer in
terms of the concept of specific heat capacity.
5) Lake Erie has a water volume of about 484 km3.
How much energy would be required to raise its
temperature 1°C? Show your calculations.
12
HOMEWORK
6) Normal human body temperature is about 37°C. Room
temperature is usually about 25°C:
a) How much energy is required to raise the
temperature of 1.0 L of aire (density = 0.0012 g/mL)
from room temperature to body temperature?
b) How much energy is required to raise the
temperature of 1.0 L of liquid water (density = 1.0
g/mL) from room temperature to body temperature?
c) Using your answers to a) and b), explain why cold
water, or even room temperature water, feels colder on
the skin than does air at the same temperature.
13