Download Global Circulations

Global Circulations
Air flow for no rotation and no water on a planet.
Global Circulations explain how air and storm systems travel over the Earth's
surface. The global circulation would be simple (and the weather boring) if the
Earth did not rotate, the rotation was not tilted relative to the sun, and had no
water.
In a situation such as this, the sun heats the entire surface, but where the sun is
more directly overhead it heats the ground and atmosphere more. The result
would be the equator becomes very hot with the hot air rising into the upper
atmosphere.
That air would then move toward the poles where it would become very cold and
sink, then return to the equator (above right). One large area of high pressure
would be at each of the poles with a large belt of low pressure around the
equator.
Three main circulations exist between the equator and poles due to earth's
rotation.
However, since the earth rotates, the axis is tilted, and there is more land mass in
the northern hemisphere than in the southern hemisphere, the actual global
pattern is much more complicated.
Instead of one large circulation between the poles and the equator, there are
three circulations...
1. Hadley cell - Low latitude air movement toward the equator that with
heating, rises vertically, with pole ward movement in the upper
atmosphere. This forms a convection cell that dominates tropical and subtropical climates.
2. Ferrel cell - A mid-latitude mean atmospheric circulation cell for weather
named by Ferrel in the 19th century. In this cell the air flows pole ward
and eastward near the surface and equator ward and westward at higher
levels.
3. Polar cell - Air rises, diverges, and travels toward the poles. Once over the
poles, the air sinks, forming the polar highs. At the surface air diverges
outward from the polar highs. Surface winds in the polar cell are easterly
(polar easterlies).
Between each of these circulation cells are bands of high and low pressure at the
surface. The high pressure band is located about 30° N/S latitude and at each
pole. Low pressure bands are found at the equator and 50°-60° N/S.
Usually, fair and dry/hot weather is associated with high pressure, with rainy and
stormy weather associated with low pressure. You can see the results of these
circulations on a globe. Look at the number of deserts located along the 30°N/S
latitude around the world. Now, look at the region between 50°-60° N/S latitude.
These areas, especially the west coast of continents, tend to have more
precipitation due to more storms moving around the earth at these latitudes.
The Jet Stream
How the earth's rotation the effects the west to east direction of the jet stream.
Jet streams are relatively narrow bands of strong wind in the upper levels of the
atmosphere. The winds blows from west to east in jet streams but the flow often
shifts to the north and south. Jet streams follow the boundaries between hot and
cold air.
Since these hot and cold air boundaries are most pronounced in winter, jet
streams are the strongest for both the northern and southern hemisphere
winters.
Why does the jet stream winds blow from west to east? Recall from the previous
section what the global wind patterns would be like if the earth was not rotating.
(The warm air rising at the equator will move toward both poles.)
We saw that the earth's rotation divided this circulation into three cells. The
earth's rotation is responsible for the jet stream as well.
The motion of the air is not directly north and south but is affected by the
momentum the air has as it moves away from the equator. The reason has to do
with momentum and how fast a location on or above the Earth moves relative to
the Earth's axis.
Your speed relative to the Earth's axis depends on your location. Someone
standing on the equator is moving much faster than someone standing on a 45°
latitude line. In the graphic (above right) the person at the position on the
equator arrives at the yellow line sooner than the other two.
Someone standing on a pole is not moving at all (except that he or she would be
slowly spinning). The speed of the rotation is great enough to cause you to
weigh one pound less at the equator than you would at the north or south pole.
The momentum the air has as it travels around the earth is conserved, which
means as the air that's over the equator starts moving toward one of the poles, it
keeps its eastward motion constant. The Earth below the air, however, moves
slower as that air travels toward the poles. The result is that the air moves faster
and faster in an easterly direction (relative to the Earth's surface below) the
farther it moves from the equator.
North hemisphere cross section showing jet streams and tropopause elevations.
In addition, with the three-cell circulations mentioned previously, the regions
around 30° N/S and 50°-60° N/S are areas where temperature changes are the
greatest. As the difference in temperature between the two locations increase,
the strength of the wind increases. Therefore, the regions around 30° N/S and
50°-60° N/S are also regions where the wind, in the upper atmosphere, is the
strongest.
The 50°-60° N/S region is where
the polar jet located with the subtropical jet located around 30°N. Jet streams
vary in height of four to eight miles and can reach speeds of more than
275 mph (239 kts / 442 km/h).
The actual appearance of jet streams result from the complex interaction between
many variables - such as the location of high and low pressure systems, warm
and cold air, and seasonal changes. They meander around the globe, dipping and
rising in altitude/latitude, splitting at times and forming eddies, and even
disappearing altogether to appear somewhere else.
Jet streams also "follow the sun" in that as the sun's elevation increases each day
in the spring, the average latitude of the jet stream shifts poleward. (By Summer
in the Northern Hemisphere, it is typically found near the U.S. Canadian border.)
As Autumn approaches and the sun's elevation decreases, the jet stream's
average latitude moves toward the equator.
Also, the jet stream is often indicated by a line on maps and by television
meteorologist. The line generally points to the location of the strongest wind. Jet
streams are typically wider and not as distinct but a region where the wind
increase toward a core of strongest wind.One way of visualizing this is to
consider a river. The river's current is generally the strongest in the center with
decreasing strength as one approaches the river's bank. It can be said that jet
streams are "rivers of air".
One way of visualizing this is to consider a river. The river's current is generally
the strongest in the center with decreasing strength as one approaches the river's
bank. It can be said that jet streams are "rivers of air".
The strength of the wind increases toward the core of the jet stream. It also does not reside at any one
particular height but can extend across hundreds of mile wide and 1,000s of feet in height.
Climate vs. Weather
Time is the basic difference between climate and weather. When one averages
the weather (maximum temperature, minimum temperature, wind speed and
direction, rainfall, etc.) for any place, for any day, over a fixed number of years,
that determines 2015 was an extraordinary year for rainfall in North Texas. This
view is looking north into Oklahoma, from Texas, across the Red River in a
"normal" year (bottom) and after very heavy rains that was part of the
phenomenally wet year weather-wise.
the average weather experienced, for that day, at that location.
Those averaged weather values then become to represent theclimatic
normal weather for that day. From the National Centers for Environmental
Information (NCEI, formally NCDC), "the climatic normal is simply the arithmetic
average of the values over a 30-year period (generally, three consecutive
decades)."
The current set of climate normals is based upon observed weather in the years
of 1981 to 2010. in 2021, a new set of climate normals will be generated based
upon the observed weather between 1991 and 2020.
Climatic normals (or averages) are most commonly seen on local weather
broadcasts. The daily observed maximum and minimum temperatures is often
compared to the "normal" temperatures based upon the 30-year average.
Also, these climatic normals help provide context if you hear something like "this
winter will be wetter (or drier, or colder, or warmer, etc.) than normal. Other
phrases such as "unseasonably warm (or cool)" weather is a comparison of the
current weather conditions as related to the "climatic normal" for that time.
It has been said "Climate is what you expect. Weather is what you get." In part,
that is true but for the vast majority of time, the observed weather rarely
"normal".
A good example is the all-time record rainfall for the Dallas/Fort worth Airport in
2015. Climate normal for rainfall is 36.14" (918 mm). The actual rainfall for the
year was 62.61" (1,590 mm). In only three months (May, October and November)
nearly a "normal" year's worth of rain fell.
The annual rainfall for 2015 broke the old all-time record by over 9" (229 mm).
This was truly an extraordinary rain record that will stand for tens of decades if
not centuries.
2015 was an extraordinary year for rainfall in North Texas. This view is looking
north into Oklahoma, from Texas, across the Red River in a "normal" year
(bottom) and after very heavy rains that was part of the phenomenally wet year
weather-wise.
2015 was a "roller coaster" year for the occurrence of rainfall in the DFW area as
well. That same year, there was a stretch of 41 consecutive days
with NO precipitation, which was the third longest number of rain-free days on
record.
Even individual days can have a wide variety of weather yet appear to be near
climatologically normal. Again at the Dallas/Fort worth Airport on November 27,
2015, the average of the maximum and minimum temperature was 55°F (13°C).
The normal for that day is 52°F (11°C). So at first glance it would have appeared
to be a "near normal" day temperature-wise.
The maximum temperature was 70°F (21°C) but occurred around 3 AM in the
morning. The a strong cold front moved past Fort Worth and Dallas early that
morning and the temperature began to fall.
The minimum temperature on was 39°F (4°C) and occurred just prior to midnight.
So, the average temperature was near normal climate-wise when that day was
quite different weather-wise.
So, large swings in day-to-day, month-to month and even year-to year weather
does not necessarily imply large, rapid changes in climate. Weather, over time,
will become part of the 30-year normal.
Origin of Wind
Wind is simply air in motion. Usually in meteorology, when we are talking about
the wind it is the horizontal speed and direction we are concerned about. For
example, if you hear a report of a west wind at 15 mph (24 km/h) that means the
horizontal winds will be coming FROM the west at that speed.
Although we cannot actually see the air moving we can measure its motion by
the force that it applies on objects. We use a wind vane to indicate the wind's
direction and an anemometer to measure the wind's speed. But even without
those instruments we can determine the direction.
For example, a flag points in the opposite direction of the wind. The wind blows
leaves opposite the direction from which the wind is blowing. Airplanes taking off
and landing at airports will be into the direction of the wind.
The vertical direction of wind motion 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 weather maps marked with H's and L's which indicate
high and low pressure centers. Usually 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". So everywhere along each line is the pressure has
the same value.
High and low pressure indicated by lines of equal pressure called isobars.
Although we cannot actually see the air moving we can measure its motion by
the force that it applies on objects. We use a wind vane to indicate the wind's
direction and an anemometer to measure the wind's speed. But even without
those instruments we can determine the direction.
Press
ure gradient force extends from high pressure to low pressure
With high pressure systems, the value of air pressure along each isobar increases
toward the center with each concentric line. The opposite is true for low pressure
systems in that with each concentric line toward the center represents lower
pressure. Isobars maybe be close together or far apart.
The closer the isobars are drawn together the quicker the air pressure changes.
This change in air pressure is called the "pressure gradient". Pressure gradient is
just the difference in pressure between high and low pressure areas.
The speed of the wind is directly proportional to the pressure gradient meaning
that as the change in pressure increases (i.e. pressure gradient increases) the
speed of the wind also increases at that location.
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.
How the coriolis force works on a rotating disk.
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.
How the corilois force works on the earth.
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.
Airflow around highs and 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,
condensation begins to exceed evaporation so the invisible vapor condenses,
forming clouds and and then 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,
evaporation begins to exceed condensation which means that clouds will tend to
evaporate. That is why fair weather is often associated with high pressure.
Air Masses
North American air masses
An air mass is a large body of air with generally uniform temperature and
humidity. The area over which an air mass originates is what provides it's
characteristics. The longer the air mass stays over its source region, the more
likely it will acquire the properties of the surface below. As such, air masses are
associated with high pressure systems.
There are two broad overarching divisions of air masses based upon the moisture
content. Continental air masses, designated by the lowercase letter 'c', originate
over continents are therefore dry air masses. Maritime air masses, designated by
the letter 'm', originate over the oceans and are therefore moistair masses.
Each of the two divisions are then divided based upon the temperature content
of the surface over which they originate.

Arctic air masses, designated by the letter 'A', are very cold as they
originate over the Arctic or Antarctic regions.

Polar air masses, designated by the letter 'P', are not as cold as Arctic air
masses as they originate over the higher latitudes of both land and sea.

Tropical air masses, designated by the letter 'T', are warm/hot as they
originate over the lower latitudes of both land and sea.
Putting both designations together, we have, for example, a "continental arctic"
air mass designated by 'cA', which source is over the poles and therefore very
cold and dry. Continental polar (cP) is not as cold as the Arctic air mass but is
also very dry. Martime polar (mP) is also cold but moist due to its origination
over the oceans. The desert region air masses (hot and dry) are designated by
'cT' for 'continental tropical'.
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).
Air mass boundaries
The motion of air mass motion is usually based upon the air flow in the upper
atmosphere. As the jet stream changes intensity and position, it affects the
motion and strength of air masses. Where air masses converge, they form
boundaries called "fronts".
3-D view of a cold front
.Fronts are identified by change of temperature based upon their motion. With
acold front, a colder air mass is replacing a warmer air mass. A warm front is the
opposite affect in that warm air replaces cold air. There is also a stationary front,
which, as the name implies, means the boundary between two air masses does
not move.
The motion of air masses also affects where a good portion of precipitation
occurs. The air of cold air masses is more dense than warmer air masses.
Therefore, as these cold air masses move, the dense air undercuts the warmer air
masses forcing the warm air up and over the colder air causing it to rise into the
atmosphere.
So fronts just don't appear at the surface of the earth, they have a vertical
structure or slope to them as well. Warm fronts typically have a gentle slope so
the air rising along the frontal surface is gradual.
3-D view of a warm
front.With warm fronts, the gentle slope favors a broad area of rising air so there
is typically widespread layered or stratiform cloudiness and precipitation along
and to the north of the front. The slope of cold fronts, being much more steep
forces air upward more abruptly. This can lead to a fairly narrow band of showers
and thunderstorms along or just ahead of the front.
There is another boundary that exists except this boundary divides moist air from
dry air. Called a dry line this boundary will separate moist air from the Gulf of
Mexico (to the east) and dry desert air from the southwestern states (to the
west).
It typically lies north-south across the central and southern high Plains states
during the spring and early summer. The dry line typically advances eastward
during the afternoon and retreats westward at night.
How to read 'Surface' weather
maps
Weather maps come in myriads of styles with each providing different levels of
information. However, there are some common features typically found of these
images.
In the section about the Origin of Wind we have already seen the source of the
"highs" and "lows". But how are the boundaries between air masses depicted?
We draws lines, called "fronts".
Fronts are usually detectable at the surface in a number of ways. Winds often
"converge" or come together at the fronts. Also, temperature differences can be
quite noticeable from one side of a front to the other side. Finally, the pressure
on either side of a front can vary significantly.
Fronts
Cold Front
Cold fronts are depicted by blue line with triangles
pointing in the direction of motion. Cold fronts demarcate the leading edge of a
cold air mass displacing a warmer air mass.
Phrases like "ahead of the front"and "behind of the front" refer to its motion. So
being "ahead of the cold front" is being in the "warm" air mass and "behind of
the cold front" is in the cold air mass.
Also remember however, the terms "cold" and "warm" are relative. So, it is still
called a cold front even in summer if the temperature only lowers from, for
example, 95°F (35°C) ahead of the front to near 90°F (32°C) behind the front.
Cold fronts nearly always extend anywhere from a south direction to a west
direction from the center of low pressure areas and never from the center of high
pressure systems.
Warm Front
A warm front is the leading edge of a relatively
warmer air mass replacing a colder air mass. A warm front are depicted by a red
line with half-moons located on the side of the direction of its motion.
Like cold front, warm fronts also extend from the center of low pressure areas
but on nearly always on the east side of the low.
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.
Stationary Front
If the front is essentially not moving (i.e. the two air
masses on either side are not moving perpendicular to the front) it is called a
stationary front. Stationary front are depicted by an alternating red and blue line
with a triangle on the blue portion and half moon on the opposite side of the
red portion of the line.
A cold front (or warm front) that stops moving becomes a stationary front. The
difference in temperature and wind direction from one side of a stationary front
to the other is generally not large but there can be times where the difference is
stark.
Occluded Front
The
cold air mass is moving faster than the cool air mass. As the two fronts converge
the cold air undercuts the cooler air mass.
Cold fronts typically move faster than warm fronts, so in time they can "catch up"
to warm fronts. As they do the warm air mass is forced up forming an occlusion.
The surface location of the occluded front is directly below the convergence
point of the warm, cool and cold air masses. Occluded fronts points to a
decrease in intensity of the parent weather system and are indicated by a purple
line with alternating triangles and half-moons on the side of its motion.
While there is no difference in how they are depicted on a weather map, there
are two types of occlusions; cold and warm.
The
cool air mass is out running the cold air mass. But the because thecold air mass
is more dense, the cool (less dense) air is forced up.
Cold occlusions are the most common where the cold front over takes the warm
front and also undercuts the cooler air mass ahead of the warm front.
Warm occlusions occur when the air associated with the "cold" front is actually
not a cold as the air mass associated with the warm front. The warm air is forced
up as before but the colder, more dense, air mass ahead of the warm
front remains at the surface forcing the air mass associated with the cold front up
as well.
Other Boundaries
Dry Line
Dry air, being more dense undercuts the light moist air forcing it up.
A dryline marks the boundary between a moist air mass and dry air mass. It
typically lies north-south across the central and southern high Plains states
during the spring and early summer, where it separates moist air from the Gulf of
Mexico (to the east) and dry desert air from the southwestern states (to the
west).
The dry line typically advances eastward during the afternoon and retreats
westward at night. However, a strong storm system can sweep the dry line
eastward into the Mississippi Valley, or even further east, regardless of the time
of day.
A typical dry line passage results in a sharp drop in humidity, a rise in
temperatures, clearing skies, and a wind shift from south or southeasterly to west
or southwesterly. (Blowing dust and rising temperatures also may follow,
especially if the dry line passes during the daytime.) These changes occur in
reverse order when the dry line retreats westward.
Since drier air is more dense than moist air, as the dryline moves east it forces
moist air up into the atmosphere. Therefore, severe and sometimes tornadic
thunderstorms can develop along a dry line or in the moist air just to the east of
it.
Squall Line
This is a line of thunderstorms that generally form along a
front but the storms move ahead of the front. As the rain cooled air under the
thunderstorms begins to surge forward new thunderstorms form on the leading
edge of the outflow.
The outflow acts like a cold front with an increase of forward speed and therefore
an increase in forward speed of the line of thunderstorms. Squall lines are most
notably seen in derechos.
Other Symbols
Trough
A trough is not a boundary but an elongated area of lower air
pressure. There are changes in wind direction across a trough but there is no
change in air mass.
While not specificity a surface boundary, troughs reflect the change in
atmospheric conditions in the upper atmosphere. As such, troughs can be areas
where showers and thunderstorms can form.
Precipitation
Historically, areas of precipitation have been shaded green regardless if it the
precipitation is frozen or not. The type of precipitation on weather maps itself
also comes in numerous forms. Sometimes the precipitation type is spelled out
or, as more often the case, use a wide variety of graphics to indicate type.
Below are some of the more traditional meteorological symbols used on maps to
indicate precipitation types.
Norwegian Cyclone Model
If you track low pressure areas and fronts you will often notice a particular cycle
these systems undergo. 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.
Initial Condition
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.
Norwegian cyclone model initial stage - weather map view
Norwegian cyclone model initial stage - 3D view
Beginning Stage
A wave develops on the front as an upper level low pressure system, embedded
in the jet stream moves, over the front. The front develops a "kink" where the
wave is developing. The stationary front changes into a cold front and warm front
as the air masses begin to move. Precipitation will begin to develop with the
heaviest occurrence along the front (dark green).
Wave forms on front - weather map view
Wave forms on front - 3D view
Intensification
As the wave intensifies, both cold and warm fronts become better organized.
Wave intensifies - overhead view
Wave
intensifies - 3D view
Mature Stage
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.
A mature low pressure system - overhead view
A mature low pressure system - 3D view
Dissipation
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.
Dissipating stage of cyclone - overhead view
Dissipating stage of cyclone - 3D view
Name:_____________________
Class & Roll #_____________________
Date:______________________
Synoptic Meteorology Quiz
Question 1
Sleet and freezing rain are caused by a cold layer aloft, with temperatures at or
below freezing.
(a) True
(b) False
Question 2
Sleet will typically occur to the north of a warm front.
(a) True
(b) False
Question 3
Which one of these clouds can produce moderate to heavy precipitation?
(a) Cumulonimbus
(b) Nimbostratus
(c) Altostratus
(d) Stratocumulus
Question 4
The force that results from the rotation of the earth is called the
____________________ force.
(a) pressure gradient
(b) frictional
(c) Coriolis
(d) convergence
Question 5
An east wind means that the air is moving from west to east.
(a) True
(b) False
Question 6
The force that results from roughness of the earth's surface is called the
____________________ force.
(a) pressure gradient
(b) frictional
(c) divergence
(d) Coriolis
Question 7
A mid-level cloud deck that has a heap-like appearance would be called?
(a) cumulus
(b) altocumulus
(c) altostratus
(d) stratocumulus
Question 8
The temperature at the station in the weather plot below is
(a) 78°C
(b) 124°F
(c) 78°F
(d) 98°F
Question 9
The wind direction and speed at the station in the weather plot below is
(a) northwest at 10 knots
(b) southeast at 20 knots
(c) northwest at 15 knots
(d) southeast at 15 knots
Question 10
The dewpoint temperature at the station in the weather plot below is
(a) 78°F
(b) 124°F
(c) 98°F
(d) 78°C