Weather Forecasting Handbook
FIFTH EDITION -- ANSWERS
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CHAPTER 1 -- FUNDAMENTALS
1. Outflow air is denser than the surrounding air. This is because
when pressure is constant, temperature is inversely proportional
to density.
2. The temperature of 300 deg K corresponds to 26.8 deg C or 80.3 deg F.
3. The potential temperature would be most useful for comparing
the air mass at different elevations.
4. 42 deg F. When you have entered a cloud, the relative humidity is for all
practical purposes 100%. Therefore the temperature equals the dewpoint.
5. A system with a scale of 1600 km is classified as synoptic scale.
This encompasses anything from 1000 to 10,000 km.
6. You can expect to find colder temperatures in eastern Canada, because
thickness is proportional to average layer temperature.
7. Assuming the hot air balloon is rising at a constant rate, the
thermometer would be most useful. You would watch for the temperature
to become steady or rise slowly with increasing height.
8. In January, the sun is shining almost directly on latitude
20 deg S. Therefore you can assume that you are in the equatorial
trough. The trade winds will be converging on this latitude. Therefore
if you go north you will encounter trade winds blowing from north to
south (or from the northeast or northwest). So the winds will not be
in a favorable direction and you will have to tack into them. Therefore
the mutiny is not recommended.
9. The barometer will read 500 millibars.
10. When the cabin pressure decreases, the air temperature cools. The
cabin air cools to its dewpoint, causing water droplets to condense.
This produces fog inside the plane.
CHAPTER 2 -- OBSERVATION
1. You would use synoptic reports. The METAR format only provides
an advantage in North America and in some cases Europe and Japan.
2. You would use METAR reports, since this format provides for an
observation once per hour. Synoptic observations only provide
data every 6 to 12 hours (sometimes three). METAR observations are
readily available in Japan.
3. United States aviation uses altimeter setting.
4. When dealing with a physical equation and in doubt, always use
station pressure. This is the only direct measure of pressure.
The other measures are extrapolations to an imaginary sea-level
pressure value.
5. The prevailing visibility is the greatest visibility equalled or
exceeded through at least half of the horizon circle. The visibility is
4 miles or greater throughout half of the horizon in the example,
therefore the prevailing visibility is 4 miles.
6. There is no ceiling. A ceiling requires cloud coverage of more
than half the sky.
7. The layer height is 3,000 ft. Layer height is defined as the
base of a cloud layer, not the top.
8. It is a cumulonimbus cloud. The upper portion only needs to be
smooth, fibrous, or striated to be classified as cumulonimbus.
9. The amorphous nature of the cloud indicates that it is either
nimbostratus or altostratus. Since no rain or snow is falling,
it is by convention altostratus.
10. Haloes are usually associated with cirrostratus, and this is
confirmed by the cloud being thin (and likely high). Altostratus
and in rare cases stratus can sometimes produce halo phenomena,
though they aren't the best answer for the question.
CHAPTER 3 -- TOOLS
1. The 500 mb chart typically contains plots, contours (height isopleths),
jet axes, troughs and ridges, and occasionally isotherms. Computer
diagnostics will usually include vorticity fields.
2. A thin cloud with light precipitation would have greater
reflectivity, since the radar is sensitive to precipitation
particles, not cloud droplets.
3. The best answer is A, a heavy reflectivity core near the edge of an
echo. This suggests that a circulation is present within the cell.
Answer B is not characteristic of any particular type of storm, and
Answer C may indicate a brief increase in rainfall rate but nothing
can be inferred.
4. A hook echo pattern is caused by rainfall from the thunderstorm
being wrapped into the circulation of a mesocyclone (or into a
tornadic circulation)
5. The tornado is made up of a convergent cyclonic rotation
couplet, with highest inbound (negative) velocities to the left
of and on the far side of the circulation center (looking along
the beam with our back to the radar), and the highest outbound
(positive) velocities would be to the right of and on the near
side of the circulation center.
6. The lakes are disappearing because a cool layer of stratus or fog which
has the same radiative temperature as the cool ground is beginning to
cover them. Fog or stratus should be in the forecast.
7. The area is more likely to represent a weak area of moisture in the
upper levels because moisture signatures in the lower and middle
levels of the troposphere are attenuated. The satellite is more
sensitive to high-level moisture patterns.
8. An "O" shaped hodograph centered on the graph's origin implies that
the wind speed is the same at all heights but the direction shifts
through all points of the compass.
9. Yes, a hodograph can be plotted from Doppler weather radars provided
that enough scatterers can reflect velocity information back to the
radar. NEXRAD radars provide a readily-available VAD/VWP product
that can be plotted up in the form of a hodograph.
10. A standard lightning detection network detects only cloud-to-ground
lightning. Therefore it would indicate that the storm with weak
cloud-to-ground lightning is more active.
CHAPTER 4 -- PHYSICS
1. Melting requires thermal energy to free molecular bonds.
2. The Petronas Towers are 452 meters tall (0.45 km), which implies
that the parcel in the elevator will cool by 5 degrees C on a dry
day and 3 deg C on a rainy day (assuming we start with saturated air).
Therefore the elevator will be warmer on a rainy day due to latent
heat being released as it ascends.
3. The temperature in the elevator will always increase by 5 deg C
when going from the top (0.45 km) to the surface. Upon returning
to the ground, the elevator air will be warmer than the surroundings
only in the case of the rainy day, when latent heat was released
into the elevator on the way up.
4. The condensation that occurred in the elevator on the way up
released latent heat into the atmosphere. As the elevator goes
down, the air dries and the condensation evaporates back into
the air, causing evaporative cooling. The total temperature change
from the last time the elevator left street level is zero (i.e.
the temperature is conserved).
5. The dry adiabatic lapse rate is about 9.8 deg C per kilometer
and the wet adiabatic lapse rate is about 6 deg C per kilometer.
Since the environmental lapse rate is greater than the wet adiabatic
lapse rate but smaller than the dry adiabatic lapse rate, the
layer is conditionally unstable.
6. The Coriolis force is strongest at the North Pole.
7. The wind is gaining kinetic energy in a subgeostrophic wind situation.
It is moving slowly and is not being acted upon significantly by the
Coriolis force, but it is entering an area of stronger contour gradient
force, which causes it to accelerate.
8. The air parcel is more likely to speed up as it rounds the ridge.
This is because centrifugal force acts together with the contour
gradient force, turning the parcel more towards lower pressure
and causing it to accelerate. This type of wind is called a
subgradient wind.
9. The Coriolis force is absent on the Equator. This causes the
pressure gradient force to become completely dominant, allowing
air to move wherever it prefers. A high pressure area would be
immediately drained before it could even build.
10. Low pressure areas fill more rapidly over land than over ocean because
the added friction slows down the air, reducing the effect of the
Coriolis force and allowing the air to turn more directly into the
low pressure area. This "fills the void" quicker.
CHAPTER 5 -- FRONTS AND JETS
1. A cold air mass moving over warm terrain (cPk) is more unstable and turbulent.
The lower layers warm rapidly, producing an unstable cold-over-warm
scenario.
2. A temperature difference is the primary measurable characteristic
of a front.
3. Surface convergence is a key process in strengthening a frontal
boundary. This draws temperature contrasts closer together.
4. If a frontal inversion is noted on the sounding, this indicates
that the station is in the cold air mass (behind, or north of the front)
and the front has already passed.
5. If thickness contours cross the front at a sharp angle, this indicates
that the cold front is a katafront (inactive) and any precipitation is
associated with convective lines.
6. The primary measurable characteristic of a dryline is a dewpoint gradient.
7. Mixing is a dominant dryline process during the daytime hours.
8. The plan is a dud. By early afternoon a sea breeze will become dominant,
driving the aromas inland and away from the beach.
9. Wintertime is the best time to launch the balloon to benefit from
strong winds aloft. The jet stream is mainly in Canada during the summer
and in the United States during the winter. It is stronger during the winter
and would probably favor a United States launch.
10. The low level jet is a key ingredient in severe thunderstorm formation.
CHAPTER 6 -- MOTION
1. The deflection in the path of the cirrus implies that a ridge is
building to the west. The cirrus takes a more northwest-to-southeast track
as a result. Therefore the weather will be good. Plunder the warehouse!
2. When the number of long waves decreases to 3, there is a tendency
for the pattern to retrogress. Therefore the ridge over the eastern
United States will tend to retrogress toward Chicago. Warmer, fair weather
can be expected.
3. The two dynamic causes of vertical motion are thermal advection and
vorticity advection.
4. The two components of vorticity are shear and curvature.
5. The clear area is associated with convergence aloft, which tends
to produce a cold dome in the stratosphere. Therefore this region
would have the coldest air. The captain should turn right towards
the unusually clear area.
6. Cloudy or rainy conditions assume that upward vertical motion is
occurring. Two factors can cause this: positive vorticity advection
(PVA) and/or warm air advection. Since the charts show strong PVA,
it can be assumed that cold air advection is dominant and is offsetting
the PVA. This is supported by the cold northwesterly winds that are
being observed. Of course other factors could be at play, but this
question is intended to demonstrate the basic relationship between
thermal and vorticity advection.
7. The warm temperatures on Capitol Hill would cause the isentropic
surfaces to bend downward (in the troposphere as a whole, potential
temperature always increases, and looking horizontally, potential
temperatures are increasing over Washington DC). Since winds are
blowing south to north, they descend south of the city and rise
north of the city. Therefore subsidence is being generated south
of the city and upward motion north of the city. Therefore the
picnic should be held south of the city.
8. The right-rear and left-front quadrants of a jet streak are associated
with bad weather (divergence aloft).
9. Q-vector convergence is an indicator of rising motion: bad weather.
10. Different isentropic surfaces should be used in different seasons
since the height of any surface varies with temperature. A summertime
surface may be too high in the wintertime, and a wintertime surface
may be underground in the summertime.
CHAPTER 7 -- BAROTROPIC SYSTEMS
1. Cold-core barotropic low (occlusion)
2. Warm-core barotropic low (tropical cyclone).
3. Warm-core barotropic high (cutoff high).
4. Cold-core barotropic high (polar source).
5. Warm-core barotropic high (cutoff high).
6. Cold-core barotropic low (cutoff low).
7. Warm-core barotropic low (heat low).
8. Warm-core barotropic high (subtropical ridge).
9. Cold-core barotropic high (polar source).
10. Warm-core barotropic high (subtropical ridge).
CHAPTER 8 -- BAROCLINIC SYSTEMS
1. A weak low pressure area in a baroclinic region is affected by an
upper-level disturbance. This causes convergence of the low-pressure
area, which draws the warm and cold air masses together and makes it
more baroclinic. This in turn strengthens the upper-level features,
which in turns produces stronger convergence at the surface.
2. Coastal regions are more favorable for baroclinic development
during the winter when cold land temperatures contrast sharply
with warm oceanic temperatures.
3. The rain above the warm sector will cause extensive evaporational
cooling in the dry warm sector air. This will reduce the thermal
contrasts (the baroclinicity) available to the low pressure area,
retarding its development.
4. The rapid deepening will cause stronger convergence of temperature
gradients, strengthening the upper-level features and possibly
causing further self-development.
5. Baroclinic low development is most likely in areas where thermal
gradients (thickness packing) is closest together.
6. The term for a baroclinic low that undergoes very rapid deepening
is a "bomb".
7. The final stage in the life cycle of a baroclinic low is known
as an occlusion (which by this time is largely barotropic).
8. A baroclinic high is different from a barotropic high in that it
is initially associated with temperature contrasts, which strengthen
upper level features which in turn strengthen the high.
9. A baroclinic high is most likely to dissipate quickly in mountainous
areas, where the increased friction reduces air velocity, and in turn
reduces the Coriolis force. This allows air to flow more directly
away from the high pressure area.
10. The dissipation of a baroclinic high can be evaluated by seeing
how rapidly the thermal contours (thickness lines) are spreading
apart with time in the high.
CHAPTER 9 -- CONVECTIVE WEATHER
1. The entrainment of dry air into the thunderstorm can allow massive
evaporational cooling in the downdraft, accelerating its motion and
producing damaging outflow wind.
2. The updraft is linear and is aligned along the leading edge of the
squall line.
3. Tornadoes are rare in most squall lines since updraft/downdraft cells
are very numerous and it is difficult for any particular cell to become
stronger than the others.
4. Tornadoes are more likely if a squall line breaks up into isolated cells.
5. The most dangerous weather is found at the updraft/downdraft interface.
6. The suspect section of the storm is a bow echo, and it indicates a
possibility of strong winds.
7. Storm splitting can cause a supercell to split into two mirror images,
the left split with anticyclonic circulation and the right split with
cyclonic circulation. This is often associated with unidirectional winds
with height.
8. The most significant danger of LP supercells is large hail. Lightning
is probably responsible for most of the deaths associated with LP supercells,
so this answer is also acceptable.
9. HP supercells are more dangerous and difficult to monitor because of
extensive rain that tends to obscure observation (both by radar and
visually) of tornadoes. Large hail and damaging winds are often
associated with HP supercells.
10. CAPE (convective availability of potential energy) gives the most
accurate reflection of instability through the troposphere.
CHAPTER 10 -- WINTER FORECASTING
1. Aggregation of multiple crystal types.
2. Warm precipitation falls into a layer less than 800 ft deep, so
freezing rain is likely.
3. The 1500 ft warm layer will cause the snow to change completely
to rain, and the cold layer is greater than 800 ft deep so ice pellets
will likely be the result.
4. The first cold layer will cause cold rain (which could possibly
be freezing rain if it contacted a surface), however it remains in
liquid form. Upon reaching the ground it is simply rain.
5. This suggests stronger cooling in the lower levels of the
atmosphere, which will lead to a transition to ice pellets.
6. Snow. The entire column is cold.
7. The presence of ice pellets indicates a warm layer above a
shallow cold layer. The warming of the atmosphere will lead to
freezing rain as the cold layer becomes shallower, then rain
will occur.
8. The temperature profile in a cold-core barotropic low
decreases strongly with height, with very cold air just above
the surface. Therefore smaller ice particles are likely to be
involved. On the other hand, the strong temperature contrasts
may produce a larger variety of particle sizes, leading to
aggregation.
9. Heavy snow is rare at frigid temperatures because the ice
particles are generaly too small.
10. Ice pellets indicate a warm layer above a cold layer
which is sufficiently deep and cold to freeze the rain drops.
Above the ground, the droplets have likely not been chilled
long enough to freeze, so freezing rain is most likely.
CHAPTER 11 -- TROPICAL WEATHER
1. Tropical weather is best described as barotropic (i.e. little or no
temperature advection).
2. The feature is the ITCZ (Intertropical Convergence Zone) or equatorial
trough. Since the solstice is some weeks away and the sun will reach
its northernmost position at that time, the ITCZ will continue moving
north.
3. Brisk trade winds are often associated with fair weather because they
transport cooler mid-latitude air southward, producing a warm-over-cold
(stable) situation.
4. Easterly waves affecting the Caribbean are caused by disturbances
generated along the southern west Africa coastline within an easterly
upper-level jet.
5. A typical easterly wave is a stable wave, whose axis slopes to the
east with height. It moves slower than the prevailing flow and most
precipitation is found to the east of the wave.
6. The wintertime Hawaiian rains are likely being caused by a
mid-tropospheric cyclone.
7. Tropical cyclones require a heat source, an unstable atmosphere,
a marginal amount of Coriolis force, weak vertical wind shear, and
a pre-existing low-level disturbance.
8. A hurricane and typhoon are identical, however tropical cyclones
that occur in the Atlantic are called hurricanes while west Pacific
tropical cyclones are called typhoons.
9. The outflow cirrus from a hurricane represents outflow moisture
diverging from the storm over a large area, usually a much larger
area than the strong surface winds.
10. Hurricane-spawned tornadoes should be forecast along the
outermost rain bands and to the right of the hurricane's track.
CHAPTER 12 -- NUMERICAL MODELS
1. Numerical modelling is a new science because adequate computational
power has not been available until recently.
2. If a model run is not properly initialized, the numerical fields
will not be balanced. This will cause the model to mathematically
"blow up" when it is executed.
3. The accurate rendering of Canadian weather is possible due to
incorporation of "first guess" fields from previous runs.
4. Limited domain models increasingly come under the influence of
weather systems outside the domain area. This degrades the
performance of the model as it predicts into later timeframes.
5. The GFS (previously known as the MRF, AVN, or spectral model)
is a global domain model.
6. The cold front was likely not forecast because of insufficient
data in Mexico. Without this data, the model likely did not anticipate
its presence.
7. The degraded performance may be because of inadequate parameterization
of the snow cover throughout the northern United States. This can
change the radiation balance in the region to a point where the
model does not accurately reflect it.
8. Thunderstorms may not be forecast because of any of the following
reasons: loss of mesoscale features, inadequate parameterization,
and limited observational data.
9. A model that makes forecasts based on comparisions with historical
data is a statistical model.
10. A spectral model analyzes the atmosphere as a series of waves
and makes calculations on these waves rather than on gridded data.