This page is meant to clarify some of the answers to the problems. It's all about picking the right points from the book and not confusing longwave and shortwave radiation.
1. (a) most reflective? snow
lowest albedo and absorbs highest proportion of
incoming radiation? water
(b) snow (old and fresh), ice, water (sun very near horizon)
(c) In the winter, the trees don't have leaves and are snow-covered.
This results in very high albedos (60 - 90 %). In the spring the snow melts,
and the trees get leaves again. These are light green at first, and get
darker, as the year goes along. So, since the summer forest has an albedo
of 5 - 10 %, the spring foliage might be somewhere around 30 %. In the
fall, when leaves turn red, the albedo doesn't change much. When the foliage
wilts and falls off, albedo is likely to go up a little bit, but the most
pronounced change in the albedo will come when snow or frost turn the branches
white.
2 (a).
| 06 | 08 | 10 | 12 | 14 | 16 | 18 | |
| June 25 | 70 | 340 | 500 | 580 | 440 | 260 | 20 |
| August 14 | -30 | 140 | 170 | 170 | 170 | 125 | -50 |
| June 25 | 24 % | 20% | 19 % | 18 % | 18 % | 20 % | 25 % |
| August 14 | 33 % | 21 % | 18 % | 18 % | 18 % | 20 % | 33 % |
(e)
Shortwave radiation is being absorbed by the ground during the course
of each day. The ground got warmer. According to the Stefan-Boltzmann-law,
the warmer ground in the afternoon emits more longwave radiation than the
cooler ground in the morning.
(f)
A comparison of the incoming shortwave between the two days
tells you, that there was less shortwave radiation coming in on the surface
on August 14 than on June 25. That is an indication for a cloudy day. (In
our field measurements, we got about the following values for Sdown:
on the rainy Tuesday about 60 Watts per square meter, on the sunny Wednesday
around 500 Watt per square meter, and on the overcast Thursday somewhere
around 200 Watts per square meter.)
So, on this cloudy day, the humidity might also have been higher than
on June 25. The clouds and the water vapor counterradiate much more longwave
radiation (Idown) than an atmosphere without clouds
- which is the reason for Idown being larger on August
14 than June 25.
Some people suggested a smaller sun angle for August 14 than June 25.
This is also a justified point, in that the sun's rays have a longer way
to travel through the atmosphere. The atmosphere absorbs more shortwave
radiation and gets warmer. Therefore, it emits more longwave radiation.
However, without the clouds, this effect would not be so strong and, as
explained above, the clouds are obviously there.
3. All the questions in problem 3 deal with two nights. At night we do not assume to have any shortwave radiation (not from natural sources anyway!!!).
(a)
The overcast night has the higher "sky temperature". Think of the comparison
with the blanket (book, page 50).The outgoing (upwelling) longwave radiation
is absorbed by the clouds to a much higher degree than by an atmosphere
without clouds. They also emit more counterradiation.
(b)
The radiation balance will definitely be negative in night #1. In night
#2 it might be positive, if there is a really strong temperature inversion
so that the counterradiation.However, usually you would assume that the
radiation balance is negative in night #2, too. In any case, the radiation
balance of night #2 will be closer to zero than the extremely negative
radiation balance of night #1.
(c)
Since the surface looses energy by emitting radiation, and does not
"get much back" in night #1, it will become very cold. This process is
also called "radiative cooling".
Part II
The second part asked for calculating the shortwave balance (Sdown - Sup) and finding out the longwave balance as a lump difference: (Idown - Iup) = net radiation - (Sdown - Sup)
The groups that measured diffuse radiation found out that the diffuse radiation was only 50 Watts per square meter. This value is depending of the overall brightness of the day, and also changes throughout the day.
In order to interpret the results, you could have compared the albedo
values for grass (snow) with the values that are given on page 1. Albedo
should be a constant for the surface and fall into a certain range. Isn't
it also striking, that the net radiation is so much smaller than any of
the other components?