Download Atmospheric Sciences 101: Weather

Atmospheric Sciences 101: Weather
Review sheet for Midterm 2 (with partial solutions)
Here are some partial answers for the questions on the review sheet, given in
italics. EoM refers to the textbook Essentials of Meteorology.
1. What is a front?
A front is a zone between two air masses of different temperatures.
2. What are the four types of fronts? What symbols are used to represent each of these
on a surface weather map?
See EoM pp212—219.
3. Where in relation to the fronts is precipitation likely to occur?
Typically, along the front on the side facing the cooler/colder air.
4. What do the vertical cross sections of each type of front look like?
See EoM pp212—219, and think about how the different fronts slope upward.
5. What are the typical weather observations during frontal passage for the two basic
kinds of front?
Cold fronts have short periods of heavier, showery (i.e. intermittent) precipitation,
while warm fronts have longer periods of steady, light precipitation. This ties in
with the answer to #4 above.
6. What kinds of clouds are produced by each kind of front?
See the diagrams on EoM pages 214, 216.
7. What is a common relationship between troughs/ridges aloft and lows/highs at the
surface?
Note that lows are not directly below troughs (and similar for highs and ridges).
See EoM pg 224.
8. Why is it generally cloudy in the vicinity of a low pressure area and sunny in the
vicinity of high pressure?
Think about the vertical motions of the air above lows and highs, and what impact
those motions have on the saturation/drying of the air.
9. What are long and short waves on an upper level chart? How do they differ?
See EoM pg 226, but skip some of the more technical details.
10.Consider a large column of air (say 500 km across). What happens to the vorticity
(rate of spin) of this air if it is stretched vertically and is pulled inward like a figure
skater bringing in her arms?
Recall the demonstration in class with the spinning chair. Did the chair spin faster
or more slowly when the rider's arms were pulled in?
11.Give the complete definitions for the following variables, and their units (if they
have any), and what they represent:
humidity
A way of expressing the amount of water vapor in the air, by any of a
number of variables.
water vapor pressure
The partial pressure of the water vapor molecules in the air, given in mb.
saturation vapor pressure
The vapor pressure if the air were saturated, given in mb.
relative humidity
The ratio of the amount of water vapor currently in the air to the amount
which would saturate the air, given as a percentage.
dew-point temperature
The temperature of the air at which it would be saturated, without adding
or removing any moisture from the air; given in deg C (or K or F).
12.Describe the relationship between saturation vapor pressure and temperature.
See EoM pg 81.
13.Why is relative humidity not a very useful variable for specifying the amount of
moisture in the air?
See problem 4 on HW 4. Even though the sauna clearly had the highest dew point,
and thus the most moisture in the air, the cold garage had the highest relative
humidity.
14.What is the typical diurnal change in relative humidity, keeping in mind that the
amount of water vapor in the air does not change rapidly over a 24-hour period?
See EoM pg 82 for an example. Think about how relative humidty changes with
temperature.
15.What are condensation nuclei and ice nuclei?
Condensation nuclei are particles on which water can condense, and ice nuclei
are particles on which ice can form. What is special about ice nuclei compared to
condensation nuclei?
16.What is fog, and how is it defined?
A cloud at the ground.
17.List the four different types of fog, how they form, and an example of each.
See EoM pp 90—94. Note that the terms “evaporation fog”, “mixing fog”, and
“steam fog” are used interchangeably to denote the same phenomenon.
18.Why can you see your breath on cold mornings?
See EoM pg 93.
19.Clouds grab-bag:
a. Name the four basic cloud types
Cumulus, stratus, nimbus, and cirrus.
For each of these cloud types:
b. What are their visual characteristics?
See EoM pg 96. In particular, what is the difference between cumulus and stratus?
What is different about nimbus compared to the other types?
c. What is their typical composition (i.e. ice crystals or water droplets)?
d. What altitude are they typically observed at?
These two can become tricky as the different names are combined to yield up to 10
(or even more) sub-types of clouds. Cirrus always refers to a high ice cloud,
whereas nimbus is always some sort of water or mixed cloud (ice crystal clouds
cannot precipitate (see #29 below). When referring to just “stratus”, we are
talking about a low, warm cloud. Cumulus all by itself is a vertically developing
cloud usually found nearer to the surface; since the water is being continually
condensed out as the cloud top rises, it must be mostly water droplets.
20.What is the moist adiabatic lapse rate (give a definition and a value)?
The rate at which saturated air cools as it rises; typically 6 deg C per km.
21.Why is the moist adiabatic lapse rate less than the dry adiabatic lapse rate?
When saturated air is cooled further, water must condense out. What does
condensation release?
22.What are the three cases of atmospheric stability? Give a typical value of the lapse
rate for each. What types of clouds would we expect in each of these?
See EoM pp 113—117.
23.Why are absolutely unstable environmental lapse rates uncommon? Where might
we find an absolutely unstable environment?
If the air is absolutely unstable, then any lifting of the air, regardless of
saturation, will start convection almost immediately. Convection mixes the air to
yield a more uniform temperature, so the high lapse rate will be diminished.
However, if the atmosphere is heated from below strongly and continuously, the
atmosphere can maintain this lapse rate even with convection constantly
transporting heat upward. Can you think of such a place where the ground would
be so hot?
24.What are the common lifting processes which form clouds?
See EoM pg 118.
25.Someone reports seeing a “flying saucer” east of Mt Rainier. What is the most
likely explanation for what they saw?
Think back to lecture and remember what kind of cloud looks vaguely saucershaped.
26.What is the size difference between condensation nuclei, cloud droplets, and
raindrops?
You should know roughly how big a raindrop is and their relative size. EoM 121.
Each is about ______ times bigger than the next smaller?
27.How are 'cold' and 'warm' clouds defined with respect to growth of hydrometeors?
The Bergeron process describes hydrometeor growth in the ‘cold’ cloud case.
What simple difference is there? EoM 122
28.Describe briefly how droplets grow in warm clouds.
Condensation on nuclei, collision and coalescence. EoM 121-2
29.Why will ice particles grow more rapidly than super-cooled droplets in the mixedphase region of a cold cloud?
Difference in SVP. See EoM 81.
30.Name four types of precipitation.
EoM 127-132
31.What type of cloud and process is needed to make hail?
Ingredients: strong updraft, cold temps, lots of long falling and rising cycles,
means we need a _________ cloud. EoM 133.
32.Why do we not expect hail to fall from stratus clouds?
Check for the ingredients above.
33.Why would a nimbostratus cloud have steadier precipitation than a cumulonimbus
cloud? Think about how precipitation types form. Nimbostratus are much smaller
and shorter. The more convection, mixing, rising and sinking, the more varied
forms and shapes of precip will be generated.
34.What causes a sea breeze to form? How are sea breezes similar to and different
from monsoons? Surfaces are warmed at different rates. This changes
temperatures, densities, induces buoyancy, changes pressure thicknesses (column
heights), and induces an upper level PGF that keeps the cycle going. EoM 173-5
35.What causes a valley breeze to form? A mountain breeze?
Remember IR cooling? Mountains and valleys warm and cool at different rates.
Use same principles as for sea breeze. EoM 175-6
36.Which would be more strongly affected by Coriolis forces: a mid-latitude cyclone
or a severe thunderstorm? Why? Mid-latitude cyclone. At our latitude CF is small
and it takes a long time/requires a long distance or area for the effect to be
important. For winds the minimum is on the order of 1000 km. EoM 152
37.Which direction does the surface wind typically come from in the Tropics?
Think about how winds deflect as they head toward the equator from the North
and also from the South. What is the combined effect?
38.How do west winds along the equator in the central Pacific help promote El Nino?
Think about the sloshing of the warm water (thermocline). If warmer waters end
up somewhere, what effect does that tend to produce on the surface pressure?
39.What effect does El Nino have on the location of thunderstorms in the ITCZ,
upwelling off of Peru, and temperatures in the eastern equatorial Pacific? Move
eastward, reduces, warms. EoM 193.
40.Give one similarity and one difference between coastal and equatorial upwelling.
Similarities: Caused by winds, bring nutrient rich water which supports life,
affects local weather
Differences: costal requires flow parallel to coast, coastal not associated with
large scale wind changes such as El Nino.
41.What is the ITCZ? What pattern of surface winds causes it? InterTropical ______
____. EoM 183
42.In what season is the equator-to-pole temperature gradient the greatest? When
would the jet stream be the strongest? How are these two related?
Compare the heating at the equator and pole in summer vs winter? What causes
the jet and what affects its strength?EoM 186-7
43.At which latitudes are upward air motions dominant? Downward? Think about the
sea breeze problem on a global scale. EoM 186-8.
44.How can we identify where the jet stream is on an upper level chart (e.g. a contour
plot of 500 mb surface heights)? The jet is primarily geostrophic wind. What
causes this? And where do we find it? They’re both where the lines are….? EoM
156.
45.At what height above the surface is the jet stream usually found?
The tropopause. In midlatitudes ~ 10-13 km.
46.Why do airplanes try to avoid flying through clouds slightly below 0 deg C?
Below 0 deg. C the water is super-cooled and will form rime and ice on surfaces…
which is a bad thing if you are a plane or passenger. Clouds a little below 0 deg.
C typically have more liquid water content (per unit volume of air) than much
colder clouds. EoM 132.
47.A radar just north of Seattle does not pick up any precipitation southwest of the
Olympic mountains. However, observers there are reporting long periods of heavy
rain. What is going on? It’s in a radar shadow.
48.Describe some effects of the positive and negative phases of the Pacific Decadal
Oscillation. How is it related to snowpack in the mountains? See lecture slides and
EoM 196-7. Pay attention to sea-surface temps and ridges or trough over the
Northwest.
49.A long, wet winter brings lots of snow in the mountains. A weekend in March
suddenly sees highs in the mid 70s across much of western Washington up into the
Cascades and even some rain in the mountains. Great, right? What problem might
this create? More then just bad skiing. Warm air and rain do what to snow? Which
would cause a problem if you live beside a ________?
50.Is the number of weather observations that goes into each daily computer-based
global weather forecast on the order of 1000, 1,000,000, or 1,000,000,000? One
million. See lecture slides.
51.For each of the following stability situations, absolute instability, conditional
instability, and absolute stability, answer the following questions:
Give a typical value of the environmental lapse rate.
What would happen to a dry parcel that is lifted? A saturated one?
What kind of clouds and/or weather would we expect to find?
See lecture slides. Dry adiabatic is 10 C/km, moist is 6 C/km. The stability classes
are above, below, and between these two. Also see EoM 112-7. Clouds depend on
the depth of convection.