Science Earth Science Interpreting the Surface Weather Map Lab Exercise

exercise9
Interpreting the Surface Weather Map
Everyday meteorologists around the world measure the current weather conditions just like we did in the last
exercise. These scientists compile this information into weather maps to represent the current weather conditions
globally. They use abbreviations and symbols on these weather maps for ease of interpretation. The goal of these
maps is to not only illustrate the current weather conditions, but to predict the weather for the coming days or even
weeks. In this exercise we will learn how to read and interpret such weather maps.
I. Representing Weather Station
Data on a Weather Map Using
Standard Symbols
Wind Speed/Direction
Current Temp
Current Conditions
Dew Point Temp
74
124
Barometric Pressure
Sky Coverage
Pressure Change
In the last lab we learned how to
measure weather elements, but in
this lab we will learn how to
50
+25
represent the data we have collected
onto a standard weather map. In
order to show weather data from Figure 1: Weather station data represented using symbols. In this example:
many stations on one map, Temp = 74°F, Dew Point Temp = 50°F, Barometric Pressure = 1012.4 mb,
meteorologists must use symbols and Pressure Change = 2.5 mb, Sky Coverage = 6/10, Wind Speed = 26-31 mph,
Wind Direction = NE, and Current Weather Conditions = Rain Showers
abbreviations.
These
standard
created to illustrate nearly every
symbols and abbreviations can show all the
Current
condition possible. A short list of
current weather conditions in one location in
Symbol
Conditions
the most common symbols is
Rain:
very limited space. Consult figure 1 for proper
intermittent,
shown in the table 2. Simply find
placement of each weather element
light, moderate,
the correct symbol and place it in
and heavy
Sky
the proper location.
Sky Coverage
Snow:
Symbol Coverage
intermittent,
Converting Barometric Pressure
The
circle
at
the
center
light,
moderate,
No Clouds
and heavy
of the weather station
Drifting or
There is very limited space on a
data symbol indicates
≤ 1/10
blowing snow
weather map containing data from
how much cloud cover
Thunderstorms
many weather stations. Therefore,
the weather station
2/10-3/10
meteorologists abbreviate the
has. Simply shade the
Drizzle
barometric pressure and the
circle according to the
4/10
Rain Showers
barometric pressure change to
symbols in table 1.
Snow Showers
alleviate space. If a barometric
Hail
Showers
5/10
pressure is over 1000 mb, drop the
Current Weather
“10” and remove the decimal. If
Conditions
Sand Storms
6/10
the barometric pressure is below
Freezing Rain
1000 mb, drop the “9” and remove
There
are
many
the decimal place.
7/10–8/10
possible
weather
Fog
Examples:1012.5 mb = 125
conditions a location
9/10 or
1005.8 mb = 058
could be experiencing
Haze
overcast
996.5 mb = 965
including;
various
w/openings
Smoke
989.2 mb = 892
degrees of rain and
Completely snow,
To abbreviate the barometric
hail,
Hurricane
Overcast
pressure change, simply remove
thunderstorms,
Table
2:
Current
Weather
Sky
the decimal place.
blowing snow, drizzle,
Obscured
Conditions Symbols
Examples: 2.3 mb decrease = -23
showers, and so on.
Table 1: Sky Coverage Symbols have been
4.2 mb increase = +42
Symbols
67
Wind Speed
Symbol
Miles
Per Knots per
Hour
Hour
Wind Speed and
Direction
21-25
18-22
26-31
23-27
32-37
28-32
38-43
33-37
44-49
38-42
50-54
43-47
55-60
48-52
61-66
53-57
67-71
58-62
In order to represent the
speed and direction of
the wind, meteorologist
use flag symbols (Table
3). Each flag symbol
represents a range of
wind
speed.
The
orientation of the flag
indicates
the
wind
direction. It is important
here to note that winds
are always labeled from
the
direction
they
originate. For example: a
northerly wind blows out
of the north and towards
the south. To indicate
wind direction, place the
tail of the flag in the
direction the wind is
labeled.
72-77
63-67
II. Air Masses
78-83
68-72
84-89
73-77
Calm
Calm
1-2
3-8
1-2
3-7
9-14
8-12
15-20
13-17
One
of
the
most
important factors in
interpreting
and
Table3: Wind Flag
predicting
weather
is
Symbols
understanding air masses
and how the characteristics of these air masses can
influence the weather. An air mass is a large body of
air with similar characteristics throughout; namely
temperature
and
mosture
content.
These
characteristics are dictated by the source region or
location the air mass originated. There are two
classifications that dictate the type of air mass. The
first dictates moisture content. If an air mass
originates in a continental location it will produce a
dry air mass, while marine regions will produce
moist air masses. The second dictates the temperature.
If an air mass originates in a tropical location it will
produce a warm air mass, while polar regions will
produce cold air masses.This classification scheme
produces 4 distinctive air mass types. When
abbreviated the first letter is lower case and the
second is upper case.
68
Air Mass Type
Abb. Temp. Moisture
Continental Polar
cP
Cold
Dry
Continental Tropical cT
Warm
Dry
Marine Polar
mP
Cold
Moist
Marine Tropical
mT
Warm
Moist
Table 4: Air Mass Types and Characteristics
2
1
4
3
5
6
Figure 2: Air Masses Affecting the US
There are several source regions that produce air
masses of each type that effect weather in the US.
Certain US regions are affected by similar air masses
throughout the year leading to the general
characteristics of the local climate.
III. Weather Fronts
When two air masses collide they often produce
similar weather conditions depending on the types of
air masses interacting. The boundary between two air
masses where they are interacting is termed a front.
When a cold air mass is overtaking a warm air mass
this is termed a cold front. Cold air is very dense and
tends to stay close to Earth’s surface, while warm air
is less dense and tends to rise. When a cold air mass
encounters a warm air mass the warm air can do one
of two things; it may just move out of the way of the
cold air (producing a warm front see below) or be
forced to rise above the cold air. As the warm air rises
quickly along this steep front it will loss of pressure
causing it to cool. This cooling causes the relative
humidity to rise. Eventually, it will reach the dew
point and condensation will produce cloud droplets.
With continued condensation, precipitation will
eventually form. For this reason cold fronts tend to
produce towering clouds and severe storms.
clouds, and prolonged precipitation. When one of the
air masses begins to move and overtake the other
either a cold or warm front will eventually be
produced.
Symbols are used on a weather map to denote the
front type and its direction. The point of the symbols
indicates the direction the front is moving. This
information is very useful in predicting the weather
over the coming days.
Symbol
Front Type
Cold Front
Warm Front
Occluded Front
Stationary Front
Figure 3: Diagram of a Cold Front
When a warm air mass is overtaking a cold air mass
this is termed a warm front. When warm air
encounters cold dense air it will tend to rise above it.
It does so slowly and at a low angle. This will
produce clouds in the same manner mentioned above,
but in this case the rising of the air is slow along this
low angled front. For this reason warm fronts may
produce cloud cover, but precipitation is typically low
and widespread instead of producing storms.
Figure 4: Diagram of a Warm Front
When a cold air mass overtakes yet another cold air
mass an occluded front is produced. This type of
front is typically associated with mid-latitude cyclone
(discussed below) and can produce severe storms
similar to a normal cold front.
When two are masses are interacting with one
another, but neither is overtaking each other it is
termed a stationary front. Conditions along a
stationary front can vary, but they typically produce
Table 5: Weather Map Symbols for Fronts
IV. Mid-Latitude Cyclones
In the middle-latitude, cold and warm air masses are
continuously interacting
with
one
another
producing
weather
systems.
The
characteristics of these
weather systems are
dictated
by
the
barometric
pressure,
which
effects
the
circulation of air and
wind patterns.
Low pressure systems
contain lower pressures
towards their interiors.
Wind will always flow
towards a lower pressure
causing winds to blow
towards the center of
low pressure. These
winds, however, are
turned by the Coriolis
Effect; the apparent Figure 5: Wind Circulation
bending of winds due to
in a Cyclone
the rotation of Earth on
its axis. Winds are turned to the right in the northern
hemisphere and to the left in the southern hemisphere.
As the winds blow toward the center of the low
pressure the bending due to the Coriolis Effect
produces counterclockwise rotation in the northern
69
hemisphere
and
clockwise rotation in
the
southern
hemisphere.
This
rotation is termed
cyclonic, and a low
pressure system is
named a cyclone. Air
converging at the
center of the cyclone
is forced to rise. This
rising
air
produces Figure 6: Rising and sinking air
clouds, precipitation, and
in a cyclone (left) and an
severe
weather
as
anticyclone (right)
described above in the
section on cold fronts. For this reason low pressure
systems (cyclones) are associated with severe
weather.
High pressure systems contain higher pressures
towards their interiors. Again, winds blow towards a
lower pressure; therefore, winds in a high pressure
system will blow away from the center. The Coriolis
Effect
will
produce
cyclonic rotation, but in
this case it is clockwise in
the northern hemisphere
and counterclockwise in
the southern hemisphere.
These
high
pressure
systems
are
termed
anticyclones.
The
diverging winds in the
center of an anticyclone
cause air to sink. This
sinking air does not
produce
clouds
or
precipitation. Therefore,
high pressure systems
(anticyclones)
are
associated
with
fair
weather conditions.
These
cyclones
and
anticyclones
typically
circulate in pairs next to
one another. As you can
see in the diagram below Figure 7: Wind Circulation in
an anticyclone
the winds blowing out of
the high pressure (bar maximum) then blow into the
low pressure (bar minimum).
70
Figure 8: Circulation of a paired Cyclone and
Anticyclone
When a cyclone rotates it tends to move air masses
developing fronts as these air masses collide with one
another. The development of a cyclone can vary, but
it typically takes on several stages. The result in the
US is a southerly migrating cold front, and a northerly
migrating warm front. Where the cold front overtakes
the warm front and encounters the cold air mass
ahead of the warm front an occluded front is
produced.
Figure 9: Weather Map Showing a Cyclone
Circulating in the US Producing Various Weather
Fronts
V. Isobars
An isobar is a line on a weather map that connects
points of equal barometric pressure (see figure 9).
These lines are very similar to the isotherms we
learned how to draw in the previous exercise except
they show were the high and low pressures are instead
of temperatures. The process for drawing isobars is
the same. Image that your pencil equals the pressure
of the isobar you are drawing. Keep all higher
pressures on one side and all lower pressures on the
other. Remember, barometric pressures have been
abbreviated on the weather map (see the section on
converting barometric pressure). It may be easier to
convert them back to standard millibars prior to
drawing
your
lines.
Figure 10: NOAA Weather Map of the US (9-3-2010). Temp and Dew Point Temp in °F, Barometric Pressure and Pressure Change in millibars
(mb), Wind Flags are to the right of stations for ease of reading.
71
72
EX9
Interpreting the Surface Weather Map
Summary Exercise
Name: __________________
Date: ___________________
1. Using the data found in the charts below and tables 1-3, fill in the weather station data symbols on the circles
below. Consult figure 1 for an example.
Dew Barometric
Sky
Wind
Wind
Temp Point Pressure Pressure Change Coverage Speed Direction Current Weather
Station 1 75°F 50°F 1016.2 mb Increase 2.4 mb
3/10
3 mph
SW
none
Station 2
Heavy Snow,
24°F 16°F 992.6 mb Decrease 4.8 mb
9/10
25 mph
NE
Blowing Snow
Station 3
Completely
59°F 59°F 1002.7 mb Increase 3.2 mb
2 mph
E
Fog and Drizzle
Overcast
Station 4 95°F 60°F 998.9 mb Decrease 1.0 mb Obscured 30 mph
S
Smoke
2. Using the weather station symbols below and tables 1-3; fill in the correct data in the chart below. Consult
figure 1 for an example.
Dew Barometric
Sky
Wind
Wind
Temp Point Pressure Pressure Change Coverage Speed Direction Current Weather
Station 1
Station 2
Station 3
Station 4
95
865
82
985
32
165
20
001
72
-65
59
-20
16
+45
0
-12
3. Fill the chart below for the air masses affecting the US from figure 2.
#
Air Mass Type
Abbr.
Moisture
1
Temp
2
3
4
5
6
73
4. Cyclones are (high/low) pressure systems with winds blowing (inward/outward) and
(clockwise/counterclockwise) in the Northern Hemisphere leading to (convergence/divergence),
(rising/sinking) air, and (clear/stormy) conditions. Circle your answers for each.
5. Anticyclones are (high/low) pressure systems with winds blowing (inward/outward) and
(clockwise/counterclockwise) in the Northern Hemisphere leading to (convergence/divergence),
(rising/sinking) air, and (clear/stormy) conditions. Circle your answers for each.
Use the weather map provided (figure 10) to answer the next 5 questions.
6. What type of pressure system is centered over
the Great Lakes.? What type of weather
conditions should they be experiencing?
8. What type of front is about to overtake the
Southeastern US? What type of weather should
they expect overt the next 24 hours?
7. What type of pressure system is centered over
the mid-western states? What type of weather
should they be experiencing?
9. What type of front is migrating north into
Canada from New England? What type of
weather should they expect overt the next 24
hours?
10. A hurricane, like Hurricane
Earl on the map, is a (low/high)
pressure system and is therefore a form
of (cyclone/anticyclone). Note how
close the isobars are together and the
direction and speed of winds around
the center of the storm. Do your
observations match your answers
above (yes/no)? Circle your answers
for each.
11. Draw isobars on the map to the
left at 4 mb intervals starting at 1004
mb. Consult the section on Isobars for
help. Remember, the pressures have
been abbreviated on the map; you may
want to convert to the actual pressure
for each weather station first.
74