Introduction
The
word synoptic mea
ns "view together" or
"view at a common
point". Therefore,
synoptic
meteorology is
primarily concerned
with viewing the
weather at a
common point -time.
Also known as large
scale or cyclonic
scale, the size of
weather patterns we
are looking at range
upwards from about
620 miles (1,000 kilometers) across to about 1,500 miles (2,500 kilometers).
When different parameters of the earth's atmosphere are viewed together at the synoptic scale then large-scale
weather patterns emerge, such as extratropical cyclones and their associated fronts.
The forecast weather map (right) is an example of the use of synoptic meteorology. It shows the positions of high and
low pressure systems as well as locations of weather fronts.
But before this map will provide any relevant information, one of the primary things a good meteorologist will check is
the "time" these various weather elements were observed. We will begin by learning about synoptic times displayed
on weather maps and text products issued by the National Weather Service.
Z-time (Coordinated Universal Time)
All aspects of meteorology are based upon a world-wide 24-hour clock called Zulu time (Z), more commonly called
Coordinated Universal Time (UTC). You will notice all weather maps, radar, and satellite images all have their time
expressed in "Z". The Zulu term stems from military usage while Coordinated Universal Time is the civilian term for
this 24-hour clock.
The origin of these time zones began in the mid 1800's in England. In 1852 the Royal Observatory in Greenwich,
England began transmitting time telegraphically and just three years later became the standard time in most of Great
Britain. Also, charts and maps were made with reference to the observatory as the "zero" or prime meridian.
Over the next few decades a number of locations around the world had developed their own meridians as reference
points. The Greenwich meridian was the most popular of these in part due to the reputation for reliability and
correctness of the Greenwich Observatory's publications of navigational data.
In 1884, a conference in Washington, D.C. settled the matter where it was agreed to establish a single "prime"
meridian (0° longitude), passing through Greenwich. This was for both for longitude and timekeeping. This 0°
meridian divides the Eastern from the Western Hemisphere. Today, one can visit the Royal Observatory and straddle
this 0° meridian with one foot in each hemisphere.
With the 360° daily rotation of the earth, the sun is moving 15° each hour which leads to the formation of 24 time
zones. However, today the individual zone boundaries are no longer straight, nor are they always continuous, as they
have been modified for convenience and to satisfy the desires of sovereign nations.
"Time zone offsets are identified as -12 UTC through 0 to +12 UTC with the minus values signifying time "before" or
ahead of prime meridian (which is the Western Hemisphere). At the meeting of the -12 and +12 time zones is the
International Date Line. Moving west across the International Date Line one moves from the -12 UTC time zone to the
+12 UTC time zone. This means you advance the clock 24 hours; in essence you advance one day on the calendar.
Moving east across the International Date Line means subtracting 24 hours from the clock thereby reversing one day
on the calendar.
The 24-hour clock (Z time) begins at midnight (00Z) at this prime meridian. 00Z for the United States begins in the
evening local time. Z-time does not change with the change for daylight saving time but the local time will change. For
example, 00Z in the Central STANDARD Time is at 6:00 p.m. but 00Z occurs at 7:00 p.m. in Central DAYLIGHT
SAVING Time.
U. S. Standard Time vs. Z-time (Coordinated Universal Time)
off
UTC 0
Time se
t
2
3
4
5
6
7
8
9
10
1
12 13
1
14
15 16 17 18 19 20 21 22 23
9p 10p 11p 12
7
12
1a 2a 3a 4a 5a 6a
8a 9a 10a 11a
1p 2p
*
*
* M
a
N
8p
10p 11p 12
6
12
EST -5
9p*
1a 2a 3a 4a 5a
7a 8a 9a 10a 11a
1p
*
*
* M
a
N
7p
10p 11p 12
5
12
CST -6
8p* 9p*
1a 2a 3a 4a
6a 7a 8a 9a 10a 11a
*
*
* M
a
N
6p
10p 11p 12
4
MST -7
7p* 8p* 9p*
1a 2a 3a
5a 6a 7a 8a 9a 10a 11a
*
*
* M
a
5p
10p 11p 12
3
PST -8
6p* 7p* 8p* 9p*
1a 2a
4a 5a 6a 7a 8a 9a 10a
*
*
* M
a
4p
10p 11p 12
2
AK -9
5p* 6p* 7p* 8p* 9p*
1a
3a 4a 5a 6a 7a 8a 9a
*
*
* M
a
3p
10p 11p 12 1
Hi
2p
4p* 5p* 6p* 7p* 8p* 9p*
2a 3a 4a 5a 6a 7a 8a
10
*
*
* M a
Gua +1 10 11 12
9 10 11 12M 1a 2a 3a 4a
1p 2p 3p 4p 5p 6p 7p 8p
m
0 a a N
p p p
% % % % %
Note: AST - Atlantic AK - Alaska time HI - Hawaii time *The previous day %The next day
AST -4
8p
*
7p
*
6p
*
5p
*
4p
*
3p
*
1
3p 4p 5p 6p 7p
2p 3p 4p 5p 6p
1p 2p 3p 4p 5p
12
1p 2p 3p
N
12
11a
1p 2p
N
12
10a 11a
1p
N
12
9a 10a 11a
N
5a 6a 7a 8a
% % % %
4p
3p
2p
1p
9a
%
The following tables show z-times for each time zone in the United States and U.S. Territories.
Daylight Saving Time vs. Z-time (Coordinated Universal Time)
UTC
Time
EDT
CDT
MDT
PDT
off0
set
-4
-5
-6
-7
8p*
7p*
6p*
5p*
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15
9p*
8p*
7p*
6p*
10p*
9p*
8p*
7p*
11p*
10p*
9p*
8p*
12M
11p*
10p*
9p*
1a
12M
11p*
10p*
2a
1a
12M
11p*
3a
2a
1a
12M
4a
3a
2a
1a
5a
4a
3a
2a
6a
5a
4a
3a
7a
6a
5a
4a
8a
7a
6a
5a
9a
8a
7a
6a
10a
9a
8a
7a
11a
10a
9a
8a
16
17
18
12N
11a
10a
9a
1p
12N
11a
10a
2p
1p
12N
11a
19 20 21 22 23
3p
2p
1p
12N
4p
3p
2p
1p
5p
4p
3p
2p
6p
5p
4p
3p
7p
6p
5p
4p
Regardless which National Weather Service weather map, radar, or satellite picture you view, be sure to first check
the time of the image as it will be reported in 24-hour "Z" time.
Clouds
When looking for synoptic scale weather patterns
from space much can be learned looking at clouds.
Clouds form when air is cooled to its dewpoint or
the temperature, if the air is cooled, it reaches
saturation. Air can reach saturation in a number of
ways. The most common way is through lifting.
As a bubble or parcel of air raises it moves into an
area of lower pressure (pressure decreases with
height). As this occurs the parcel expands. This
requires energy, or work, which takes heat away
from the parcel. So as air rises it cools. This is
called an adiabatic process.
The rate at which the parcel cools with increasing
elevation is called the "lapse rate". The lapse rate of
unsaturated air (air with relative humidity <100%) is
5.4°F per 1000 feet (9.8°Cper kilometer). This is
called the dry lapse rate. This means for each 1000
feet increase in elevation, the air temperature will decrease 5.4°F.
Since cold air can hold less water vapor than warm air, some of the vapor will condense onto tiny clay and salt
particles called condensation nuclei. The reverse is also true. As a parcel of air sinks it encounters increasing
pressure so it is squeezed inward.
This adds heat to the parcel so it warms as it sinks. Warm air can hold more water vapor than cold air, so clouds tend
to evaporate as air sinks.
Types of Clouds
There are four basic cloud categories observed in our atmosphere:
Cirro-form
High-level clouds which form above 20,000 feet (6,000 meters) and are usually composed of
ice crystals. High-level clouds are typically thin and white in appearance, but can create an
array of colors when the sun is low on the horizon. Cirrus generally occur in fair weather and
point in the direction of air movement at their elevation.
Nimbus comes from the Latin word meaning "rain". These clouds typically form between
7,000 and 15,000 feet (2,100 to 4,600 meters) and bring steady precipitation. As the clouds
thicken and precipitation begins to fall, the bases of the clouds tend to lower toward the
ground.
Nimbo-form
Clouds look like white fluffy cotton balls or heaps and show the vertical motion or thermal
uplift of air taking place in the atmosphere. The level at which condensation and cloud
formation begins is indicated by a flat cloud base, and its height will depend upon the
humidity of the rising air. The more humid the air, the lower the cloud base. The tops of these
clouds can reach over 60,000 feet (18,000 meters).
Cumulo-form
Strato-form
"Stratus" is Latin for layer or blanket. The clouds consist of a feature-less low layer that can
cover the entire sky like a blanket, bringing generally gray and dull weather. The cloud bases
are usually only a few hundred feet above the ground. Over hills and mountains they can
reach ground level when they may be called fog. Also, as fog "lifts" off the ground due to
daytime heating, the fog forms a layer of low stratus clouds.
Origin of Wind
Wind is simply the air in motion. Usually when we are talking about the wind it is the horizontal motion we are
concerned about. If you hear a forecast of west winds of 10 to 20 mph that means the horizontal winds will be 10 to
20 mph FROM the west.
Although we cannot actually see the air moving we can measure its motion by
the force that it applies on objects. For example, on a windy day leaves rustling
or trees swaying indicate that the wind is blowing. Officially, a wind vane measures the wind direction and an
anemometer measures the wind speed.
The vertical component of the wind is typically very small (except in thunderstorm updrafts) compared to the
horizontal component, but is very important for determining the day to day weather. Rising air will cool, often to
saturation, and can lead to clouds and precipitation. Sinking air warms causing evaporation of clouds and thus fair
weather.
You have probably seen a surface map marked with H's and L's which indicate high and low pressure centers.
Surrounding these "highs" and "lows" are lines called isobars. "Iso" means "equal" and a "bar" is a unit of pressure so
an isobar means equal pressure. We connect these areas of equal pressure with a line. Everywhere along each line
is constant pressure. The closer the isobars are packed together the stronger the pressure gradient is.
Pressure gradient is the difference in pressure between high and low pressure areas. Wind speed is directly
proportional to the pressure gradient. This means the strongest winds are in the areas where the pressure gradient is
the greatest.
Also, notice that the wind direction (yellow arrows) is clockwise around the high
pressure system and counter-clockwise around the low pressure system. In
addition, the direction of the wind is across the isobars slightly, away from the
center of the high pressure system and toward the center of the low pressure
system. Why does this happen? To understand we need to examine the forces
that govern the wind.
There are three forces that cause the wind to move as it does. All three forces
work together at the same time.
The pressure gradient force (Pgf) is a force that tries to equalize pressure differences. This is the force that causes
high pressure to push air toward low pressure. Thus air would flow from high to low pressure if the pressure gradient
force was the only force acting on it.
However, because of the earth's rotation, there is second force, the Coriolis force that affects the direction of wind
flow. Named after Gustav-Gaspard Coriolis, the French scientist who described it mathematically in 1835, this force is
what causes objects in the northern hemisphere to turn to the right and objects in the southern hemisphere to turn to
the left.
One way to see this force in action is to see what happens when a straight line becomes a curve. Picture the Earth as
a turntable (see number 1) spinning counter-clockwise. A ruler is placed over the turntable (see number 2) and a
pencil will move in a straight line from the center to the edge while the turntable spins underneath. The result is a
curved line on the turntable (see number 3).
When viewed from space, wind travels in a straight line. However, when
viewed from the Earth, air (as well as other things in flight such as planes and
birds) is deflected to the right in the northern hemisphere (red arrow on image
at right). The combination of the two forces would cause the wind to blow
parallel to straight isobars with high pressure on the right.
So why does air spiral out from highs and into lows? There is one other force,
called Friction, which is the final component to determining the flow of wind.
The surface of the earth is rough and it not only slows the wind down but it
also causes the diverging winds from highs and converging winds near lows.
What happens to the converging winds near a
low? A property called mass continuity states
that mass cannot be created or destroyed in a
given area. So air cannot "pile up" at a given spot.
It has to go somewhere so it is forced to rise. As it
rises it cools. When air cools it can hold less water
vapor so some of the invisible vapor condenses,
forming clouds and precipitation. That is why there
is often inclement weather near low pressure
areas.
What about the diverging air near a high? As the air spreads away from the high, air from above must sink to replace
it. Sinking air warms. As air warms it can hold more water vapor, which means that clouds will tend to evaporate. That
is why fair weather is often associated with high pressure.
Air Masses
An air mass is a large body of air with generally uniform
temperature and humidity. The area from which an air
mass originates is called a "source region."
Air mass source regions range from extensive snow
covered polar areas to deserts to tropical oceans. The
United States is not a favorable source region because of
the relatively frequent passage of weather disturbances
that disrupt any opportunity for an air mass to stagnate
and take on the properties of the underlying region. The
longer the air mass stays over its source region, the more
likely it will acquire the properties of the surface below.
The four principal air mass classifications that influence the continental United States according to their source region
are:




Polar latitudes - Located poleward of 60° north and south.
Continental - Located over large land masses between 25°N/S and 60°N/S.
Maritime - Located over the oceans between 25°N/S and 60°N/S
Tropical latitudes - Located within about 25° of the equator.
As these air masses move around the earth they can begin to acquire additional attributes. For example, in winter an
arctic air mass (very cold and dry air) can move over the ocean, picking up some warmth and moisture from the
warmer ocean and becoming a maritime polar air mass (mP) - one that is still fairly cold but contains moisture. If that
same polar air mass moves south from Canada into the southern U.S. it will pick up some of the warmth of the
ground, but due to lack of moisture it remains very dry. This is called a continental polar air mass (cP).
The Gulf Coast states and the eastern third of the country commonly experience the tropical air mass in the summer.
Continental tropical (cT) air is dry air pumped north, off of the Mexican Plateau. If it becomes stagnant over the
Midwest, a drought may result. Maritime tropical (mT) air is air from the tropics which has moved north over cooler
water.
Air masses can control the weather for a relatively long time period: from a period of days, to months. Most weather
occurs along the periphery of these air masses at boundaries called fronts.
Fronts
Fronts are the boundaries between two air masses. Fronts are
classified as to which type of air mass (cold or warm) is replacing the other. For example, a cold front demarcates the
leading edge of a cold air mass displacing a warmer air mass. A warm front is the leading edge of a warmer air mass
replacing a colder air mass. If the front is essentially not moving (i.e. the air masses are not moving) it is called a
stationary front.
Fronts don't just exist at the surface of the earth; they have a vertical
structure or slope as well. Warm fronts typically have a gentle slope so the air rising along the frontal surface is
gradual. This usually favors the development of widespread layered or stratiform cloudiness and precipitation along
and to the north of the front. The slopes of cold fronts are steeper and air is forced upward more abruptly. This
usually leads to a narrow band of showers and thunderstorms along or just ahead of the front, especially if the rising
air is unstable.
Cold fronts typically move faster than warm fronts, so in time
they "catch up" to warm fronts. As the two fronts merge, an occluded front forms. In the occluded front, the cold air
undercuts the cooler air mass associated with the warm front, further lifting the already rising warm air.
Fronts are usually detectable at the surface in a number of ways. Winds usually "converge" or come together at the
fronts. Also, temperature differences can be quite noticeable from one side of the front to another. Finally, the
pressure on either side of a front can vary significantly.
Here is an example of a location that experiences typical warm frontal passage followed by a cold frontal passage:
Clouds lower and thicken as the warm front approaches with several hours of light to moderate rain. Temperatures
are in the 50s with winds from the east.
As the warm front passes, the rain ends, skies become partly cloudy and temperatures warm into the mid 70s. Winds
become gusty from the south. A few hours later, a line of thunderstorms sweeps across the area just ahead of the
cold front. After the rain ends and the front passes, winds shift to the northwest and temperatures fall into the 40s and
skies clear.
Norwegian Cyclone Model
The Norwegian cyclone model, so named to honor the Norwegian meteorologists who first conceptualized the typical
life cycle of cyclones in the 1910s and 1920s, presents the evolution of a cyclone. In this model, there will initially be a
boundary, or front, separating warm air to the south from cold air to the north. The front is often stationary. (Left Overhead view; Right - 3D view)
A wave on the front will form as an upper level disturbance embedded in the jet stream moves over the front. The
front develops a "kink" where the wave is developing. Precipitation will begin to develop with the heaviest occurrence
along the front (dark green).
As the wave intensifies, both cold and warm fronts become better organized.
The wave becomes a mature low pressure system, while the cold front, moving faster than the warm front, "catches
up" with the warm front. As the cold front overtakes the warm front, an occluded front forms.
As the cold front continues advancing on the warm front, the occlusion increases and eventually cuts off the supply of
warm moist air, causing the low pressure system to gradually dissipate.
Precipitation
In order for precipitation to form,
particularly over a large area, several
ingredients are necessary. First there
must be a source of moisture. The primary
moisture sources in the U.S. are the
Atlantic and Pacific Oceans as well as the
Gulf of Mexico. Winds around high and
low pressure systems (a subject of
another lesson) transport this moisture
inland.
Once the moisture is in place, clouds still
need to form. The most effective way to
do this is by lifting the air. This can be accomplished by forcing the air up and over mountains or, more commonly, by
forcing air to rise near fronts and low pressure areas.
Cloud droplets and/or ice crystals are too small and too light to fall to the ground as precipitation. So there must be a
process(es) for the cloud water, or ice, to grow large enough to fall as precipitation. One process is called the collision
and coalescence or warm rain process. In this process, collisions occur between cloud droplets of varying size, with
their different fall speeds, sticking together or coalescing, forming larger drops.
Finally the drops become too large to be suspended in the air and they fall to the ground as rain. The other process is
the ice crystal process. This occurs in colder clouds when both ice crystals and water droplets are present. In this
situation it is "easier" for water vapor to deposit directly onto the ice crystals so the ice crystals grow at the expense of
the water droplets. The crystals eventually become heavy enough to fall. If it is cold near the surface it may snow,
otherwise the snowflakes may melt to rain.
Winter precipitation types and their environments
The vertical distribution of temperature will often determine the type of precipitation (rain vs. snow vs. sleet vs.
freezing rain) that occurs at the surface during the wintertime. More often than not, the temperature does not
decrease with height but increases, many times by several degrees, before decreasing. This increase, then decrease
is called an inversion. In winter, an inversion can be critical in determining the type or types of weather.
In the image (left) the green dashed line is the temperature in
respect to elevation. The surface temperature is 25°F (-4°C)
and increases with height before decreasing. However, since
the temperature remains below freezing any precipitation that
falls will remain as snow.
In this image the surface temperature is higher, 27°F (-3°C).
Also as elevation increases, the temperature increases to a
point where some of the atmosphere is above freezing
before the temperature lowers again below freezing.
As snow falls into the layer of air where the temperature is
above freezing, the snowflakes partially melt. As the
precipitation reenters the air that is below freezing, the
precipitation will re-freeze into ice pellets that bounce off the
ground, commonly called sleet. The most likely place for
freezing rain and sleet is to the north of warm fronts. The
cause of the wintertime mess is a layer of air above freezing
aloft.
Freezing rain will occur if the warm layer in the atmosphere is
deep with only a shallow layer of below freezing air at the
surface. The precipitation can begin as either rain and/or
snow but becomes all rain in the warm layer. The rain falls
back into the air that is below freezing but since the depth is
shallow, the rain does not have time to freeze into sleet.
Upon hitting the ground or objects such as bridges and
vehicles, the rain freezes on contact. Some of the most
disastrous winter weather storms are due primarily to
freezing rain.
How to read 'Surface' weather maps
On surface maps you will often see station weather plots. Since
meteorologists must convey a lot of information without using a lot of words,
plots are used to describe the weather at a station for a specific time. When
all stations are plotted on a map, a "picture" of where the high and low
pressure areas are located, as well as the location of fronts, can be obtained.
There are a large number of weather symbols used for station plotting. Some
are used for weather elements such as rain, snow, and lightning. Others
represent the speed of the wind, types of clouds, air temperature, and air
pressure. All of these symbols help meteorologists depict the weather
occurring at a weather observing station.
This sample plot represents the maximum amount of information about the current weather at an observing station.
Hand plotted maps usually contain the full weather information. However, most computer generated surface weather
maps omit some data such as clouds types and heights.
Before computers, the plotting of weather maps was considered an art. In fact, Aerographers (weathermen) in the
U.S. Navy continue to plots maps by hand. A skilled plotter can easily fit the above information under the space
covered by a dime.
In the upper left, the temperature is plotted in Fahrenheit. In this example, the temperature is 77°F.
Along the center, the cloud types are indicated. These cloud types use the same cloud codes as
found in the cloud chart section. The top symbol is the high-level cloud type followed by the midlevel cloud type. The lowest symbol represents low-level cloud over a number which tells the height
of the base of that cloud (in hundreds of feet) In this example, the high level cloud is Cirrus, the midlevel cloud is Altocumulus and the low-level clouds is a cumulonimbus with a base height of 2000
feet.
At the upper right is the atmospheric pressure reduced to mean sea level in millibars (mb) to the
nearest tenth with the leading 9 or 10 omitted. In this case the pressure would be 999.8 mb. If the
pressure was plotted as 024 it would be 1002.4 mb. When trying to determine whether to add a 9 or
10 use the number that will give you a value closest to 1000 mb.
On the second row, the far left number is the visibility in miles. In this example, the visibility is 5
miles.
Next to the visibility is the present weather symbol. There 95 symbols which represent the weather
that is either presently occurring or has ended within the previous hour. In this example, a light rain
shower was occurring at the time of the observation.
The circle symbol in the center represents the amount of total cloud cover reported in eighths. This
cloud cover includes all low, middle, and high level clouds. In this example, 7/8th of the sky was
covered with clouds.
This number and symbol tell how much the pressure has changed (in tenths of millibars) in the past
three hours and the trend in the change of the pressure during that same period. In this example,
the pressure was steady then fell (lowered) becoming 0.3 millibars LOWER than it was three hours
ago.
These lines indicate wind direction and speed rounded to the nearest 5 knots. The longest line,
extending from the sky cover plot, points in the direction that the wind is blowing FROM. Thus, in
this case, the wind is blowing FROM the southwest. The shorter lines, called barbs, indicate the
wind speed in knots (kts). The speed of the wind is determined by the barbs. Each long barb
represents 10 kts with short barbs representing 5 kts. In this example, the station plot contains two
long barbs so the wind speed is 20 kts, or about 24 mph.
The 71 at the lower left is the dewpoint temperature. The dewpoint temperature is the temperature
the air would have to cool to become saturated, or in other words reach a relative humidity of 100%.
The lower right area is reserved for the past weather, which is the most significant weather that has
occurred within the past six hours excluding the most recent hour.
Analyzing Weather Maps
Once you can read a station plot you can begin to perform map analyses. Meteorologists use the station plots to draw
lines of constant pressure (isobars), temperature (isotherms), and dewpoint (isodrosotherms) to achieve an
understanding of the current state of the atmosphere. This knowledge ultimately leads to better weather forecasts
and warnings.