Answer
the following questions on a separate sheet of paper. DO NOT write answers on this page. If you need to calculate an answer, you must show your work. Tables of saturation mixing ratios for both
Fahrenheit and Celsius temperature are provided below. Make
sure you read and answer all the parts to each question! Answers for each question may not be
weighted equally.
|
Temperature (ºF) |
Sat. Mixing Ratio (g/kg) |
|
Temperature (ºF) |
Sat. Mixing Ratio (g/kg) |
|
Temperature (°F) |
Sat. Mixing Ratio (g/kg) |
|
10 |
1.52 |
|
40 |
5.28 |
|
70 |
15.95 |
|
15 |
1.89 |
|
45 |
6.40 |
|
75 |
18.94 |
|
20 |
2.34 |
|
50 |
7.74 |
|
80 |
22.43 |
|
25 |
2.88 |
|
55 |
9.32 |
|
85 |
26.48 |
|
30 |
3.54 |
|
60 |
11.19 |
|
90 |
31.16 |
|
35 |
4.33 |
|
65 |
13.38 |
|
95 |
36.56 |
|
Temperature (ºC) |
Sat. Mixing Ratio (g/kg) |
|
Temperature (ºC) |
Sat. Mixing Ratio (g/kg) |
|
Temperature (ºC) |
Sat. Mixing Ratio (g/kg) |
|
-30 |
0.3 |
|
5 |
5.4 |
|
25 |
20.1 |
|
-20 |
0.8 |
|
10 |
7.6 |
|
30 |
27.2 |
|
-10 |
1.8 |
|
15 |
10.6 |
|
35 |
36.6 |
|
0 |
3.8 |
|
20 |
14.7 |
|
40 |
49.0 |
1.
Clouds
most often form when air is lifted upward and cools below its dew point
temperature, but this is not the only way clouds can form. Clouds can also form when warm, moist air is
mixed with cold air. For example,
“steam fog” sometimes occurs when cold air moves over a warmer water
surface. Suppose a cold wind blows over
a warm lake. The temperature and
relative humidity are given below for an air parcel just above the lake surface
and an air parcel embedded in the cold wind.
|
Air Parcel above Lake |
|
Air Parcel in Cold Wind |
||
|
Air Temperature |
65° F |
|
Air Temperature |
35° F |
|
Relative Humidity |
97 % |
|
Relative Humidity |
80 % |
(a)
Compute
the mixing ratio for each air parcel.
You will need to use the table of saturation mixing ratios in
Fahrenheit.
(b)
Assume
that the parcels mix equally, such that the temperature of the mixed parcels is
the average of the two parcel temperatures (i.e., [65+35]/2 = 50° F) and the mixing ratio is the average of
the two mixing ratios computed in part (a).
Will a fog form? Explain your
answer.
2. “Advection fog” is common along the northern California
coast in summer. The main reason that
fog forms in this region is that the surface ocean water near the coast is much
colder than the surface ocean water farther offshore. When surface winds are westerly, warm, moist air from the Pacific
Ocean is carried over the cold, coastal waters, forming fog. This fog is often carried inland by the
westerly winds (e.g., San Francisco fog).
(a) Explain why fog forms when the warm, moist air
contacts the much colder coastal waters.
Don’t worry about mixing parcels in this case. The function of the cold coastal water is to cool the warm, moist
air coming from well offshore.
(b) Over land, this fog often persists through the morning
hours, but “burns off” as the afternoon wears on. This occurs because some sunlight is able to penetrate through
the fog and warm the ground. Explain
how this would act to dissipate the fog (of course, the fog doesn’t actually
“burn”). Would you expect the fog to
dissipate from the bottom up or from the top down? Explain.
3.
Answer the following questions and fill in tables for each part
below. Create your own tables (using
WORD perhaps) or re-write tables on your own paper. Do not squeeze answers into the tables below. You will need to use the table of saturation mixing ratios
provided in Celsius for this problem.
You are going to follow a parcel of air that is forced to rise up a
mountain, then back down the other side.
(a)
Fill
in the table below for an air parcel forced to rise from 0 meters up to the top
of a 4,000 m mountain. At what
altitude, if at all, will a cloud start to form?
|
Altitude |
Parcel Temperature |
Parcel Dew point
Temperature |
Saturated? |
|
4,000 m |
|
|
|
|
3,000 m |
|
|
|
|
2,000 m |
|
|
|
|
1,000 m |
|
|
|
|
0m |
25º C |
5º C |
No |
(b)
Bring
the parcel back down the other side of the mountain. If a cloud developed on the way up, assume that the cloud
evaporates on the way down. For this
part assume that the total water content in the parcel remains constant. Fill in the table below. Don't worry about stability. (HINT: when parcels move down in the atmosphere,
they warm by compression. Unsaturated
parcels warm by 10º C for every 1000 m drop in elevation. Parcels that contain clouds warm more slowly
because energy is required to evaporate the cloud, therefore a parcel which
contains a cloud warms by only 6º C for every 1000 m drop in elevation until
the entire cloud evaporates. Keep in
mind that as a cloud evaporates, the dew point temperature will change.)
|
Altitude |
Parcel Temperature |
Parcel Dew point
Temperature |
Saturated? |
|
4,000 m |
* (from part a) |
* (from part a) |
* (from part a) |
|
3,000 m |
|
|
|
|
2,000 m |
|
|
|
|
1,000 m |
|
|
|
|
0 m |
|
|
|
* to get started you need to
copy the values you computed at 4000 m from the table in Part (a)
(c)
Repeat
part (b), but this time assume that any and all liquid or ice that condensed
into a cloud as the parcel went up the mountain fell out of the parcel as
precipitation. Fill out the same table
as for part (b). Remember: this time
there is no cloud to evaporate on the way down.
(d)
Compute
the relative humidity for each of the parcels at 0 m elevation only for
parts (a), (b), and (c). If the exact
values are not found in the saturation mixing ratio table, you will have to
make estimates based on the nearest values.
Explain why the parcel in part (c) arrives back on the ground warmer than
it was before it went up and over the mountain.
4. Answer the following questions or fill in tables for
each part below. Create your own tables
(using WORD perhaps) or re-write tables
on your own paper. Do not squeeze
answers into the tables below.
The lifted index (LI) is
defined as the difference between the environmental air temperature at 500 mb
(T500) and the air temperature inside an air parcel after it has
been lifted from the surface up to 500 mb (TParcel). Meteorologists
use the lifted index to access the stability of the atmosphere.
LI = T500 - TParcel
(a) Explain why the atmosphere is said to be stable when
the lifted index is positive and unstable when the lifted index is negative.
(b) The following
information is available for Asheville, NC (elevation ~500 m above sea level)
at 8:00 AM. Fill in the table by
lifting an air parcel from the surface up to 5500 m, where air pressure is 500
mb. At what altitude does a cloud start to form? What is the lifted index at
8:00 AM? Is the atmosphere unstable for parcels lifted to 500 mb?
|
Air Pressure |
Altitude (m) |
Atmospheric Temperature (°C) |
Parcel Temperature (°C) |
Parcel Dew Point (°C) |
|
500 mb |
5500 |
-20 (this is T500) |
(this will be TParcel) |
|
|
---------- |
4500 |
-13 |
|
|
|
---------- |
3500 |
-6 |
|
|
|
---------- |
2500 |
1 |
|
|
|
---------- |
1500 |
8 |
|
|
|
---------- |
500 |
10 |
10 |
0 |
(c) Later that day at 3:00 PM, the following conditions
were measured in Asheville, NC. Fill in
the table below by lifting an air parcel from the surface up to 5500 m, where
air pressure is 500 mb. At what altitude does a cloud start to form? What is
the lifted index at 3:00 PM? Is the atmosphere unstable for parcels lifted to
500 mb?
|
Air Pressure |
Altitude (m) |
Atmospheric Temperature (°C) |
Parcel Temperature (°C) |
Parcel Dew Point (°C) |
|
500 mb |
5500 |
-20 |
|
|
|
---------- |
4500 |
-13 |
|
|
|
---------- |
3500 |
-5 |
|
|
|
---------- |
2500 |
3 |
|
|
|
---------- |
1500 |
11 |
|
|
|
---------- |
500 |
20 |
20 |
0 |
(d)
What change took place in the atmosphere between
8:00 AM and 3:00 PM that caused the stability of the atmosphere to change? Explain why this change tends to make the
atmosphere more unstable.