NAS 125 TCC Winds Fronts and Weather Forecasting Lab

Earth and Beyond – An Introduction to Earth-Space Science Lab ManualEXERCISE 6: WINDS, FRONTS AND WEATHER FORECASTING
WIND
The difference in atmospheric pressure from place to place (the pressure gradient) is the sole cause of horizontal air
movement (or wind). Air will always attempt to move from areas of high pressure to areas of low pressure (i.e., along the
pressure gradient). We can think of this gradient as being the force which gives the initial “push” to the air. In order for air to
move there must be a net force acting upon the air. Rarely are there periods of no wind. Therefore some kind of horizontal
force or forces must be acting upon the air. It turns out that there are three significant forces which act to cause and/or
modify horizontal air movements: the horizontal pressure gradient force; the Coriolis force; and friction.
The Horizontal Pressure Gradient
The horizontal pressure gradient always acts to initially “push” air from areas of higher pressure to areas of lower pressure.
The greater the pressure gradient force, the stronger this “push” and hence, the stronger the wind. The isobars you
constructed in Exercise 5 depict the variations in pressure across the United States during a particular day. The areas where
the isobars are closest together are the areas with the strongest pressure gradients. Where the isobars are farthest apart, the
pressure gradient is weakest (see Figure 1 for another example).
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30
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29
The Coriolis force is an apparent force which exists as a
result of the spinning of the earth about its axis. It is an
apparent force that acts to deflect the wind (or any object free
to move across the earth’s surface for that matter) away from
a straight line. Although a detailed explanation of the
Coriolis force is beyond the scope of this lab there are
several important effects of this force which we must
consider here:
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Forces Modifying the Wind: Coriolis Force
30
30
The actual direction and strength of the wind varies from what
would be expected based on the horizontal pressure gradient
force alone. This is because of the effects of the other two
forces alluded to above: The Coriolis force and friction. Both
of these forces act to modify the horizontal movement of air.
Figure 1: Diagram showing how the spacing of isobars
is related to wind speed.
a. The Coriolis force deflects the air to the RIGHT in the Northern Hemisphere and to the LEFT in the Southern Hemisphere.
b. The Coriolis force acts at a right angle (i.e. 90°) to the initial direction of motion (see Figure 2).
c. At a given latitude, the magnitude of the Coriolis force is proportional to the speed of the air.
d. There is no Coriolis force at the Equator and the Coriolis force increases poleward, reaching a maximum at each pole.
.4
30
30
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.
30
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Air motio
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Friction is a force which acts to reduce the velocity of the
wind at the surface. This acts to also indirectly affect the
direction of the surface wind. This is because of the fact that
the magnitude of the Coriolis force depends largely upon the
velocity of the wind. By acting to reduce the wind velocity,
friction also serves to reduce the magnitude of the Coriolis
force acting on the surface winds. Therefore, the surface
winds are not deflected by 90° but rather by some lesser
amount (typically about 45°). This results in the wind
traveling across the isobars from higher to lower pressure but
at an angle (Figure 2). Thus, to determine the wind direction at
any location, you need to determine the pressure gradient first!
.2
Forces Modifying the Wind: Friction
30
00
.
30
10
.
30
e. The Coriolis force only affects wind direction, not speed.
Figure 2: Diagram showing how the pressure gradient force,
the Coriolis force and friction control wind direction in the
Northern Hemisphere.
© 2008 Kendall Hunt Publishing Company and Brent Zaprowski
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Earth and Beyond – An Introduction to Earth-Space Science Lab Manual
Use Figure 3 to answer questions 1-7:
1) Using an arrow, show the pressure gradient, the Coriolis force deflection, and the wind direction at stations B – E. Station
A has been done for you. (Hint: think about the effects of the pressure gradient force, the Coriolis force and friction).
2) Which station has the strongest horizontal pressure gradient?
3) Which station has the weakest horizontal pressure gradient?
4) Which station is experiencing the strongest winds?
5) Which station is experiencing the lightest winds?
6) What is the general flow of air around the low pressure system? ( clockwise / counterclockwise )
7) What is the general flow of air around the high pressure system? ( clockwise / counterclockwise )
iolis
Cor
d dir
e
ectio
n
ent
radi
g
sure
s
Pre
B
forc
Win
A
H
D
30.40
30.30
30.20
30.10
30.00
C
E
L
29.50
29.60
29.70
29.80
29.90
Figure 3: Map of the United States showing a typical high and a low pressure system.
STATION MODELS
Most of the weather data collected must be compiled into a form that can be
readily depicted on a weather map. Due to the great volume of data which
must be displayed, a set of “shorthand” codes and symbols have been devised
to allow for the most effective data depiction (Figure 4). The data are plotted
around a small circle used to represent the location of the station. The circle
and its data are collectively referred to as a station model. The circle is used
to depict cloud cover over the site. The current temperature is found to the
upper left of the circle while the current dew point temperature is found to the
lower left. The current barometric pressure reading is found to the upper right
of the circle. Wind direction is shown by a line pointed into the direction from
which the wind is coming. Thus, a wind line pointing towards the north
would represent a northerly wind. Barbs are placed on the wind line to
indicate the current wind speed. Additional symbols are placed around the
circle to depict precipitation, pressure tendency, cloud types, etc.
Wind speed (mph)
Wind direction
Barometric
pressure (inches)
Temperature (°F)
31
**
Present
weather
29.76
29
Cloud cover
Dew Point (°F)
Figure 4: The parts of a station model. See
Figure 5 for a description of the symbols.
© 2008 Kendall Hunt Publishing Company and Brent Zaprowski
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8) What are the current weather conditions for the three weather stations below?
Enter your answers in Table 1. Assume up is north!
WIND SPEED
PRESENT WEATHER
Calm
Fog or ice fog
1-2
’’
3-8
56
9-14
30
30.05
36
48
29.76
21-25
Heavy rain
26-31
27
32-37
**
***
****
38-43
44-49
43
50-54
55-60
Station A
Station B
61-66
Station C
Station B
*
78-83
Station C
Moderate snow
Heavy snow
Snow showers
Thunderstorm, with
or without precipitation
84-89
Temperature (°F)
Light snow
Rain showers
67-71
72-77
Station A
Light rain
Moderate rain
15-20
29.36
****
Drizzle
CLOUD COVER
Dew point (°F)
Cloud cover (tenths)
Wind speed (mph)
No clouds
One-tenth or less, not zero
Two-tenths to three-tenths
Wind direction
Pressure (inches)
Present weather
Four-tenths
Five-tenths
Table 1: Weather station model data for question 8.
Six-tenths
9) Which station has the highest relative humidity? How can you tell?
Seven-tenths to eight-tenths
Nine-tenths or overcast with openings
Completely overcast
10) Which station is experiencing the heaviest precipitation at this time?
Figure 5: Some of the symbols used in a
typical station model. See Appendix 2 for a
more complete listing.
FRONTS
Largely because of density differences (due to temperature and moisture differences), adjacent air masses do not readily mix
with one another. Rather, they remain separated by boundaries known as fronts. Since they represent the boundaries between
differing air masses, fronts are zones of rapid transition of temperature and/or moisture. This is part of the reason why fronts
are such an important element of the weather. The next time you watch the weather on television count how many times the
weather broadcaster says the word front!
Warmer air
Cold front
Colder air
Colder air
Warm front
Warmer air
Direction of motion
The two most common fronts are cold fronts and
warm fronts. A cold front is depicted on a
weather map by a line containing a series of
triangles pointing in the direction toward which
the front is moving (Figure 6). A warm front is
shown by a line containing a series of semicircles,
also pointing into the direction that the front is
moving. Another type of front is the stationary
front. A stationary front represents the boundary
between two different air masses when neither air
mass is advancing. A stationary front is shown on
a map as a line with a series of alternating
triangles and semicircles. The triangles point
toward the warmer air mass and the semicircles
point toward the cooler air mass. A stationary
front will typically become either a warm front or
a cold front once one of the air masses begins to
advance.
Warmer air
Stationary front
Colder air
Figure 6: Frontal symbols used on weather maps.
© 2008 Kendall Hunt Publishing Company and Brent Zaprowski
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Earth and Beyond – An Introduction to Earth-Space Science Lab Manual
FRONTAL ANALYSIS
A great deal of active weather occurs in the vicinity associated fronts. Because of this active weather, it is especially
important that fronts are located accurately on surface weather maps. There are many clues that a meteorologist uses to find
fronts on a weather map.
Clues To Locating Fronts:
a. Relatively sharp temperature changes: Colder temperatures behind a cold front and
warmer temperatures behind a warm front.
b. Changes in dew point:
*Much lower dew points (relative to air temperature) behind the cold front.
*Dew point and temperature should be very similar to one another ahead of a warm front.
c. Shifting winds:
*SE winds ahead of warm front, SW winds behind warm front
*SW winds ahead of cold front, NW winds behind cold front.
d. Clouds: Clouds ahead of fronts, clearing skies behind fronts.
e. Precipitation: Precipitation ahead of fronts, little if any precipitation behind fronts
*Narrow band of heavy precipitation ahead of cold front
*Large area of light to moderate precipitation ahead of warm front.
f. Pressure tendency: Pressure falling ahead of warm front, pressure nearly steady behind
warm front, pressure rising (often rapidly) behind cold front.
**WARNING: Not all of these clues are always found near every front**
11) Figure 7a contains a cold front and Figure 7b contains a warm front. Based on the station data, draw in the front using the
proper symbols. Label the cold air mass and the warm air mass on each figure.
(a)
(b)
67
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30
29.91
23
78
30.15
63
43
76
71
65
29.53
29.42
46
41
59
29.89
39
57
29.66
49
77
64
79
66
79
29.67
29.92
29.41
29.52
45
29.52
81
29.74
46
29.73
57
29.39
29.66
48
62
29.73
77
50
42
43
74
49
74
29.91
42
79
81
23
37
73
85
29.88
41
45
30
29.63
29.64
29.42
46
51
29.62
62
51
57
29.86
77
29.62
29.66
49
50
87
29.76
79
Figure 7: Typical weather maps displaying (a) a cold front and (b) a warm front.
© 2008 Kendall Hunt Publishing Company and Brent Zaprowski
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Earth and Beyond – An Introduction to Earth-Space Science Lab Manual
CYCLONES AND ANTICYCLONES
The low pressure system located over the eastern United States on Figure 8a is
also known as a cyclone. The surface pressure is lowest in the center of the
cyclone and increases outward in all directions. As a result, the surface winds
blow inward around the cyclone and counterclockwise around a cyclone in the
Northern Hemisphere. The most important aspect of the air movement associated
with a cyclonic system in terms of affecting the weather in its vicinity is the
vertical component of air movement. Because the surface winds blow towards
the center of the cyclone from all directions this air converges and is forced to
rise since it must go somewhere. Recall that as air rises it cools adiabatically.
Oftentimes the air will cool to its dew point (i.e., become saturated). As air rises,
there is usually a great amount of cloud cover and often significant precipitation
around the center of cyclonic systems. The rotation of air around a cyclone
often initiates the movement of air masses relative to each other thereby
creating fronts.
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29.40
29.50
W
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29.60
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29.70
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The high pressure system located over the Great Plains of the United States on
Figure 8b is also known as an anticyclone. The surface pressure is highest in the
center of the anticyclone and decreases outward in all directions. Surface winds
blow out away from the anticyclone and clockwise around anticyclones in the
Northern Hemisphere and outward and counterclockwise around anticyclones in
the Southern Hemisphere. What is most important in terms of affecting the
weather in the vicinity of an anticyclone is that in addition to the horizontal air
movement (wind) surrounding the anticyclone the air from above is subsiding.
As the air subsides it warms adiabatically and gets farther from being saturated.
High pressure systems do not include fronts because a single airmass forms
the anticyclone. Therefore, there is usually no significant cloud cover and very
little chance of any precipitation in the vicinity of an anticyclone.
fro
29.80
H
30.40
30.30
30.20
Figure 8: (a) A typical cyclonic system.
(b) A typical anticyclonic system.
THE IMPORTANCE OF THE MID-LATITUDE CYCLONE
29.86
23
LD
29.51
35
29.
31
5
29
40
25
32
38
41
29.39
38
42
29.48
39
30.0
43
29.71
66
29.7
29.8
29.9
25
29.43
51
59
29.43
50
69
72
29.47
29.92
27
66
29.5
29.46
W
ar
62
m
70
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29.54
76
63
66
74
29.6
72
29.58
35
30.05
45
29.52
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29.85
23
29.81
23
29.6
29.53
33
Co
The path and speed of a mid-latitude cyclone as
it moves across the United States is controlled
by the Jet Stream. As a rule of thumb, a cyclone
moves at a speed which is half of the jet stream
speed. For example, if the Jet Stream is flowing
at a speed of 50 km/hr, then we would expect a
cyclone to move at about 25 km/hr, following
the path of the Jet Stream.
29.7
36
38
41
30
29.8
AIR
CO
ld
The exact location of anticyclonic systems (high
pressure systems) is not quite as important since
they do not have fronts associated with them
and are usually surrounded by a large area of
generally fair conditions.
30
fro
Cyclonic systems (low pressure systems) are the
most important features affecting the day-to-day
variations of weather in the mid-latitudes during
the fall, winter and spring seasons. During these
three seasons, cyclonic systems and their
associated fronts are responsible for almost all
of the precipitation that falls in the mid-latitudes.
The passage of these systems also causes most
of the day-to-day temperature variations across
the mid-latitudes during these three seasons.
Although no two mid-latitude cyclones are
exactly the same, there are fairly similar weather
patterns associated with many of these cyclonic
systems (Figure 9).
29.55
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29.57
62
29.66
65
77
29.65
69
IR
MA
79
73
29.64
R
WA
29.
7
Figure 9: An example of a typical mid-latitude cyclone with its
associated fronts.
© 2008 Kendall Hunt Publishing Company and Brent Zaprowski
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Earth and Beyond – An Introduction to Earth-Space Science Lab Manual
12) Figure 10 below shows two typical weather maps for the eastern U.S. Using the station data, draw in the fronts
associated with the cyclone using the proper symbols. Label the cold air mass and the warm air mass on each Figure.
(a)
(b)
30
23
29.91
28
76
30.18
63
46
29.63
37
29.52
32
77
65
78
58
30.05
29.41
51
L
30.15
63
56
54
29.83
29.53
65
29.89
76
29.39
52
58
29.52
57
67
77
64
52
29.76
79
29.67
29.58
36
29.66
29.48
50
73
29.68
49
62
29.53
59
29.92
32
27
39
29.67
47
29.42
59
80
64
36
35
55
73
29.49
49
52
29.81
23
32
33
22
28
34
29.73
23
29.61
23
L
31
35
30
29.80
19
29.66
62
30.08 66
53
30.13
29.86
51
62
67
55
85
29.74
30.09
H
79
Figure 10: Two typical weather maps of the eastern United States for question 12.
UPPER LEVEL WIND ANALYSIS
The 500 millibar winds are said to “steer” mid-latitude cyclones. These upper level winds produce upper-level divergence
that strengthens and maintains a surface low pressure system. As a simple rule, surface low pressure systems move at
about 50% of the 500 millibar wind speeds. Use the following formula to determine how far the surface low will move
over time:
500 mb wind speed
x time = distance traveled
2
13) Figure 11a indicates a surface low pressure in eastern Colorado and a deep trough in the western United States. The 500
millibar winds in this example are 100 mi/hr. Assuming the upper-level winds remain unchanged, indicate the new location of
the surface low on the map after 24 hours and 48 hours.
14) Figure 11b indicates a surface low pressure in northern Montana and a relatively zonal flow aloft. The 500 millibar
winds in this example are 90 mi/hr. Assuming the upper-level winds remain unchanged, indicate the new location of the
surface low on the map after 36 hours and 72 hours.
(a)
(b)
L
L
500 miles
500 miles
Figure 11: 500 mb pressure surface charts for questions 13 and 14.
© 2008 Kendall Hunt Publishing Company and Brent Zaprowski
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Earth and Beyond – An Introduction to Earth-Space Science Lab Manual
Use Figure 12a below and the skills you learned in previous sections of this lab to answer questions 15-23:
15) Which two cities probably have the highest temperature?
16) Which city should be colder than it was the day before?
17) Recall that winds are named for the direction from which they blow. Which two cities should be experiencing
southwesterly winds?
18) Which city should be experiencing northwesterly winds?
19) Which three cities are most likely receiving precipitation?
20) Of the cities likely receiving precipitation, which should be having the heaviest precipitation?
21) Of the cities likely receiving precipitation, which should be having the lightest precipitation?
22) Which cities should be experiencing rising barometric pressure?
23) Which cities should be experiencing falling barometric pressure?
(b)
(a)
L
Des Moines
Des Moines
Columbus
Columbus
Salisbury
Denver
Salisbury
Denver
Nashville
Nashville
Oklahoma City
Oklahoma City
500 miles
500 miles
Figure 12: a. Map of a mid-latitude cyclone for questions 15-23. b. 500 mb pressure chart for question 24.
24) Using the skills you learned in the previous sections of this lab and the data in Figure 12b, forecast the meteorological
conditions for Denver, Oklahoma City, Nashville, and Salisbury for the next day and enter your answers in Table 3 below.
Use an upper level wind speed of 80 mi/hr to determine the location of the cyclone. Assume the fronts maintain a similar
pattern over the 24 hour time period. Relative temperatures include: hot, warm, cool and cold.
Denver
Oklahoma City
Nashville
Salisbury
Relative
temperature
Precipitation
Cloud cover
Wind direction
Pressure trend
(rising or falling)
Table 2: Current meteorological conditions for select cities based the new location of the cyclone in figure 12b.
© 2008 Kendall Hunt Publishing Company and Brent Zaprowski
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HUMAN COMFORT INDICES
Our level of comfort or discomfort is largely influenced by the atmospheric conditions. Virtually everyone living in the midlatitudes experiences both uncomfortably hot and uncomfortably cold conditions numerous times during the year. However,
the level of discomfort associated with both high and low temperatures can be amplified by other meteorological variables.
When it is hot, high humidity can greatly increase our level of discomfort. With cold temperatures, wind speed has the
greatest influence on increasing our degree of discomfort.
The Heat Index
Recall that evaporation is a cooling process. When we are hot, our bodies respond by perspiring so that this perspiration may
evaporate, resulting in evaporative cooling. High humidity enhances the degree of discomfort associated with high
temperatures because evaporation will not take place as readily in humid air as in drier air. As a result, there is much less
evaporative cooling whenever the air is humid.
The National Weather Service uses an index called the Heat Index to give us a better indication of how uncomfortably hot we
will feel on a summer day than that provided by temperature alone. The heat index is often reported and even forecasted on
the weather broadcasts during the summer. The advantage of the heat index is that its value represents a “feel like”
temperature. That is, it is the temperature our body senses based on the combination of the temperature and humidity, as
opposed to the actual temperature read from a normal thermometer.
Temperature (°F)
Relative humidity (%)
104
102
100
98
96
94
92
90
88
86
84
82
80
78
76
74
10
98
97
97
93
91
89
87
85
82
80
78
77
75
72
70
68
20
104
101
99
91
95
93
90
88
86
84
81
79
77
75
72
70
30
110
108
105
101
98
95
92
90
87
85
83
80
78
77
75
73
40
120
117
110
106
104
100
96
92
89
87
85
81
79
78
76
74
50
132
125
120
110
108
105
100
96
93
90
86
84
81
79
77
75
60
70
80
90
132
125
120
111
106
100
95
92
89
86
83
80
77
75
128
122
115
106
100
96
91
89
85
81
77
75
122
114
106
100
95
91
86
83
78
76
122
115
109
99
95
89
85
79
77
Table 3: The heat index.
The formula for computing the Heat Index is complicated, therefore, rather than compute it directly, we will determine it
using Table 3. During the summer, the higher the Heat Index value, the more uncomfortable we feel. For extremely hot and
humid conditions, the following Heat Index danger categories have been established (Table 4).
Heat Index (°F)
Danger Category
80-90
I. Caution
91-105
II. Extreme caution
106-130
III. Danger
131+
IV. Extreme Danger
Table 4: Heat index danger categories.
© 2008 Kendall Hunt Publishing Company and Brent Zaprowski
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25) Determine the Heat Index and Danger Category for the three examples below:
Temperature
Relative
humidity
Case A
100°F
20%
Case B
80°F
60%
Case C
88°F
80%
Category
Heat Index
26) In which of the three cases above are the conditions most uncomfortable? What is the danger category for this case?
27) In which case is the temperature higher?
28) Explain why the case with the highest temperature is not the most uncomfortable according to the Heat Index.
Wind Chill
During the winter, our level of discomfort associated with the weather is primarily a function of the combination of the
temperature and the wind speed. When it is cold, strong winds will make us feel colder because the wind acts to carry heat
away from our skin surface. The wind chill equivalent temperature (usually just called the wind chill) has been developed to
give us an idea of what the temperature actually feels like to us based on the combination of the temperature and wind speed.
Interestingly, if the air temperature is significantly higher than your body temperature, strong winds can actually make you
feel hotter. The formula for computing the wind chill is also rather complicated, therefore, we will determine it using Table 6.
Temperature (°F)
Wind speed (mph)
Calm
5
10
15
20
25
30
35
40
30
30
25
21
19
17
16
15
14
13
20
20
13
9
6
4
3
1
0
-1
10
10
1
-4
-7
-9
-11
-12
-14
-15
0
0
-11
-16
-19
-22
-24
-26
-27
-29
-10
-10
-22
-28
-32
-35
-37
-39
-39
-20
-20
-34
-41
-45
-48
-51
-30
-30
-46
-53
-58
-61
-40
-40
-57
-66
-71
Table 5: Wind chill equivalent temperature.
29) Use Table 5 to determine the wind chill equivalent temperature for the following examples:
Temperature
Wind speed
Case A
10°F
40 mph
Case B
-10°F
5 mph
Case C
0°F
35 mph
Wind Chill (°F)
30) Which case in question 29 has the lowest actual temperature?
31) In which case should we feel most uncomfortable? Why?
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