Chapter 7 Summary and Review
The difference in pressure between
two places is the basic force that drives winds. But first we have to define pressure, as the amount of force per
unit of surface area. Gas molecules in
the atmosphere exert a certain force on the objects around them. Pressure can increase by adding more
molecules (increasing density) or by making molecules move faster (increasing
temperature). The relationship between
pressure, density and temperature is expressed in the Ideal Gas Law. Pressure follows Dalton’s law, which states
that each gas (nitrogen, oxygen, etc) exerts is own partial pressure, the sum
of which makes up the total atmospheric pressure. Pressure tries to equalize itself, moving from high pressure
areas to low pressure areas, creating a pressure gradient force (PGF). Pressure is exerted equally in all
directions and is measured by barometers.
Pressure also decreases rapidly with altitude. A station such as Miami, FL would have a higher pressure reading
than Denver, CO since there is a thicker layer of atmosphere (more molecules)
above Miami. In order to ignore the
differences in pressure due to altitude and focus on the pressure differences
due to weather, meteorologists reduce surface pressure measurements to sea level
pressure.
The pressure gradient force, since
it tries to equalize pressure differences, gets air moving. We could assume that since pressure
decreases rapidly with altitude, the PGF force would always be directed
upwards. However, air doesn’t usually
flow upwards very rapidly. This is due
to hydrostatic equilibrium, which is the balance between the pressure gradient
force and gravity. There is also a
pressure gradient force between a column of cold air and a column of warm
air. This is because the cold air is
more dense, and thus vertical decrease in pressure is much faster than the warm
column. The different columns will have
a different pressure reading at the same altitude. Thus in the upper atmosphere, the PGF starts moving air from
higher to lower pressure, or from the warm column to the cold column. The stronger the PGF is, the faster the air
starts moving.
The PGF is what starts air moving,
but it isn’t the only force affecting wind.
The Coriolis “force” is an apparent force that changes the direction of
wind. Due to the rotation of the earth,
the Coriolis force deflects moving air to the right in the Northern Hemisphere
and to the left in the Southern Hemisphere.
The faster the air is flowing, the more deflection occurs. The Coriolis force is zero at the equator
and is at its maximum at the poles. The
Coriolis force has no effect on small movements, such as water flowing down the
drain, but has a large effect on movements over a large area, such as wind
moving across a continent. We’ll begin
by assuming the isobars are straight and air starts out moving from high to low
pressure due to the PGF. The Coriolis
force then turns this moving air to the right (in the Northern Hemisphere), so
the air in the upper atmosphere flows parallel to the isobars. This is known as geostrophic wind. If the isobars are curved, then centripetal
force becomes a factor, and the wind is known as gradient wind. Around a curved high, the PGF and Coriolis
are out of balance due to the centripetal acceleration. The Coriolis is greater, so thus the winds
must be faster. The gradient winds in
this case are supergeostrophic. Around
a curved low pressure system, the two forces are again out of balance, this
time with the Coriolis being less, so the wind slows. The gradient winds in this case are subgeostrophic. If the effect of friction is included, as it
must be at the surface, the winds get more complex. Friction slows the winds down, the slower winds result in less
Coriolis force, and the PGF and Coriolis are again knocked out of balance. The net effect of friction is that winds
blow across isobars towards lower pressure.
The friction effect is usually limited to the lowest 1.5 km of the
atmosphere, known as the planetary boundary layer.
The net effect of all the forces
causes air around high and low pressure systems to flow a certain way. Around upper level highs (anticyclones) in
the Northern Hemisphere, air flows clockwise parallel to the isobars, while a
surface high has air flowing clockwise and outwards from the center. The Southern Hemisphere highs are the same,
except the flow is counterclockwise.
Around lows (cyclones) in the upper levels the flow is counterclockwise
and parallel to the isobars in the Northern Hemisphere. N.H. surface lows have flow counterclockwise
and inward towards the low center.
Southern Hemisphere lows behave the same, but flow clockwise. The convergence of air from a surface low
causes it to rise, while air above a surface high sinks. If the systems are not closed, the low
pressure is called a trough and high pressure is called a ridge.
Know:
·
What
pressure is and its relationship to density and temperature
·
The
basic properties of pressure
·
What
hydrostatic equilibrium is
·
What
the pressure gradient force is and how it behaves
·
What
the coriolis effect is and how it behaves
·
The
differences between geostrophic and gradient wind
·
How
friction effects surface winds
·
How
winds flow around highs and lows in both hemispheres
Chapter 8 Summary and Review
Back in 1735, George Hadley proposed that the
difference in heating between the Earth’s equator and poles was what drives the
atmosphere’s circulation. Since the
equator received more heat, air there rose and moved high in the atmosphere
towards the poles. At the poles the air
would cool and sink, moving back to the equator along the surface. The Coriolis force then turned the winds in
each hemisphere, so the Northern Hemisphere surface winds flowed from the
northeast. Hadley’s model wasn’t very
realistic, so scientists in more recent times have proposed the three-cell
model. The cells nearest the equator
were most like Hadley’s original model and were named after him. In the Hadley cell, the air rises at the
equator, creating an area of clouds and precipitation known as the
Intertropical Convergence Zone (ITCZ).
Air in the upper part of the Hadley cell flows towards the poles, but
begins sinking at about 30o latitude. This creates an area of subtropical highs, with little rainfall
and light winds. The surface winds flow
back to the equator, but are turned by the Coriolis force, making the
easterlies, a prevalent trade wind seen in the tropics. In the middle latitudes there is the Ferrel
cell, where air comes down at the subtropical highs and moves poleward along
the surface, rising again at around 60o latitude. This rising air creates another area of
convergence and precipitation, the subpolar lows. The final cell is the polar cell, where air rising from the
subpolar lows moves in the upper atmosphere towards the poles, where it sinks
and creates the polar highs.
While the
three-cell model describes the ITCZ and the subtropical highs well, it doesn’t
do so well with upper level wind flow in the mid-latitudes. The temperature, and thus the height of a
column of air decreases as you go poleward.
Thus there is a pressure gradient force acting from the equator to the
poles. The Coriolis turns these winds
to the right in the Northern Hemisphere so air flows from west to east parallel
to the isobars. Certain parts of the
upper level flow have a sharp boundary in temperature known as the polar
front. The difference in temperatures
creates a strong PGF, making the winds along this boundary unusually strong,
known as the polar jet. The polar jet
is usually stronger in winter due to the greater temperature differences and
exists about 9-12 km above sea level. A
similar jet occurs in the subtropics and is important for bringing moisture. The flow at upper levels often distorts into
a system of troughs and ridges known as Rossby waves, which circle the globe
and bring cold air south and warm air north.
I skimmed over local wind systems,
but you should know the basics. A
monsoon is a thermally driven circulation seen in Asia, with winter winds
flowing off of the continent and summer winds flowing onto the continent,
bringing rain from the ocean. A Chinook
is air flowing down from the mountains that is warmed through compression. Foehn, Santa Ana, and katabatic winds are
all also mountain driven winds seen at different places around the globe. A sea breeze begins when land heats faster
than water, creating a pressure difference which causes winds to blow towards
land during the day. At night, the land
cools quickly, reversing the pressure gradient and winds so the air flow from
the land out to sea.
There is a complex relationship
between the ocean and atmosphere.
Normally in the tropical Pacific, trade winds blow the surface ocean
water from the east to the west, causing warm water to pool in the western
Pacific and cold water to upwell in the eastern Pacific. This Walker Circulation normally keeps
Indonesia fairly wet and the west coast of South America fairly dry. However, this situation changes when there
is an El Nino Southern Oscillation.
When El Nino occurs the trade winds reverse, causing warm water to move
eastward. This disrupts the Walker
circulation, bringing drought to Indonesia and floods to South America. El Nino can even have an effect on U.S.
weather though teleconnections, where Rossby waves shift, bringing a unusual
pattern of troughs and ridges. However,
El Nino doesn’t create storms, it just moves them to different places. With La Nina, the opposite circulation
occurs. The trade winds strengthen,
bringing abnormally cold water to the eastern Pacific. La Nina usually brings wet conditions to the
western Pacific and dry to South America.
The reasons for why the oscillation occurs are not well know, but may be
due to an imbalance in ocean heat transfer.
General pattern can be forecasted from El Nino, but variations can cause
difficulty in specific forecasts.
Know:
Chapter 9 Summary and Review
Air that rests over certain parts of
the globe for several days begins to take on the characteristics of that
area. The air masses move from these
source regions, usually in the tropics and poles, into the mid-latitudes. The air masses are classified by two properties: temperature (tropical, polar, or arctic) and
moisture (continental or maritime).
Continental Polar (cP) air usually comes to Oklahoma from central
Canada, bringing cold, dry, stable air.
Continental Arctic (cA) air comes off of the polar ice caps, bringing
bitterly cold air. Maritime Polar (mP)
usually forms over the Northern Pacific and brings cold, humid air that is
stable in the summer and unstable in the winter. Continental Tropical (cT) comes off of central Mexico, with hot,
dry, and unstable air. Maritime
Tropical (mT) usually arrives in OK from the Gulf of Mexico, bringing warm,
moist, unstable air. Air masses are
often modified as they move away from their source regions, for example cP air
masses become slightly warmer as they move south.
The boundaries that separate air
masses are known as fronts. A cold
front, normally designated on weather maps with blue triangles, brings dropping
temperatures, dropping dew points and a wind shift to a northerly direction. Since there is a density difference between
the warm and advancing cold air, there is a sharp boundary which causes the
warm air to rise along it. This usually
forms cumulus clouds, showers, or thunderstorms. With a warm front (the red half circles) the temperatures rise,
the dew points rise, and the winds shift from easterly to south or
southwesterly. The warm front boundary
is not as steep as the cold front. Warm
air flows up and over the cold air, a process known as overrunning. The gradual rise of warm air brings stratus clouds
and light precipitation. Stationary
fronts are simply a boundary between two air masses that doesn’t move. With an occluded front, two fronts combine
in some way. With a cold-occlusion, a
cold front with continental polar air behind it catches up with a warm front
with cool air in front of it. The warm
air that was between the two fronts is displaced upwards in the
atmosphere. With a warm occlusion, the
process is the same, but there is mP air behind the cold front and cP air ahead
of the warm front. A final type of
boundary is a dryline, which is a difference in density between humid and dry
air. This density difference causes
convergence, which in turn can fire thunderstorms.
Finally a bit from Chapter 10. An ideal low pressure system goes through a
certain life cycle. First a stationary
front separates two air masses. A kink
develops in the front, with warm air moving north and cold air beginning to
move south. A cyclonic circulation
develops (cyclogenesis) and a low pressure system is born, with a cold front
and a warm front. Eventually the fronts
catch up and occlude. The occlusion
cuts off the low from its source of warm, moist air, so the low pressure system
begins to die. This process is
idealized, since upper level winds have a large effect on surface lows, but
that’s not something we’re going to cover in this class.
Know:
Chapter 11 Summary and
Review
In order for thunderstorms to form
you need: conditionally unstable air,
some lifting mechanism or free convection, and abundant moisture. For severe thunderstorms you also need wind
shear. Air mass thunderstorms, however,
are short lived and not severe. The storm
begins when unstable air rises, cools adiabatically, and forms a cumulus
cloud. Warm, moist air feeds the storm in
an updraft, and if conditions are right, the storms continues to grow. Eventually the ice crystals high in the
storm grow large enough to fall. Once
precipitation begins, the storm moves from the cumulus stage to the mature
stage. The falling rain drags air down
with it and also cools the air, creating a downdraft. At this stage the storm is at its most intense. The turbulent air around the storm causes
entrainment, bringing dry air into the storm, which evaporates drops and
increases the downdraft. Eventually the
downdraft takes over the entire storm and it enters the dissipative stage. The cold air from the downdraft cuts off the
warm, moist air feeding the storm and the cloud droplets begin to evaporate.
For a storm to be considered severe,
it needs to produce winds greater than 58 mph, hail greater than 0.75 inches,
or a tornado. Severe thunderstorms are
usually self-propagating and can occur in may different forms. Severe thunderstorms that form in clusters
of multiple cells are known as Mesoscale Convective Complexes. Different cells in the complex are often at
different stages of development, with new cells being formed from the outflow
boundaries of old cells. A squall line
is thunderstorms formed in a linear band, capable of traveling large distances
and creating high winds. In squall
lines, the cold air from the downdraft forms a gust front, which pushes up the
warm, moist air ahead of it and feeds it into the storm. Supercell thunderstorms are small, isolated
storms with a rotating updraft called the mesocyclone. Supercells are the thunderstorms most likely
to produce strong tornadoes. Strong
wind shear creates the rotating updraft which feeds humid air into the storm
and keeps it going. Rain and hail often
wrap around the mesocyclone, producing a hook shape that can be seen on
radar.
Lightning, one of the many hazards
produced by thunderstorms, is a charge of electricity between objects. Most lightning occurs within the cloud or to
another cloud, but about 20% occurs from cloud to ground. In a thunderstorm a thin coating of liquid
water often exists on ice pieces within the cloud. When the ice collides with each other the water coating mixes so,
positive charge gets transferred to the small ice crystals and negative charge
is transferred to the larger graupel pieces.
The graupel is heavier and falls to the bottom of the cloud, creating a
concentration of negative charge. The
ice crystals stay in the top of the cloud, producing a concentration of
positive charge. At times, an object on
the ground such as a tree becomes positively charged. Negative charges begin moving towards the tree in a
stepped-leader, which is invisible to the naked eye. Positive charges begin moving up from the tree in an
upward-leader. Once the two leaders
meet, electrons flow through the connected channel in a return stroke. Another leader, called a dart leader, sets
up another channel and forms another return stroke. Several return strokes make up the flash of lightning we
see. The lightning heats the air
intensely, causing the air to expand and create thunder. The lightning flash is seen before the
rumble of thunder due to light traveling faster than sound. Lightning usually strikes the tallest object
available, and can travel through phone lines and pluming. Cars and airplanes are usually safe from
lightning, since the charge is transferred around the occupants through the
metal shell of the vehicle. Other, rare
types of lightning observed include ball lightning, St. Elmo’s fire, sprites,
and blue jets.
A thunderstorm hazard that is
important for aviation is downbursts.
These are strong downdrafts that hit the ground and create very high
winds. If a downburst is less than 4 km
in diameter, it is known as a microburst.
The wind shear that occurs as an aircraft flies through a downburst can
cause it to crash.
Another danger of severe
thunderstorms is flash flooding. This
can occur when heavy rain falls on already saturated ground. Often there is training, when one
thunderstorm goes over the path of another in rapid succession. Flash floods kill more people than any other
weather event, since drivers are often think they can drive though rushing
water. The force of the water can
easily knock over a person when it is just six inches deep, and can float a car
at two feet deep. Most flash flood deaths occur in vehicles, so it’s never a
good idea to drive through flooded roadways.
Tornadoes are rapidly rotating winds
that spin counterclockwise in the Northern Hemisphere. Wind shear starts columns of air rotating
horizontally, and the updraft of a supercell pushes this rotation into the
vertical. This creates the mesocyclone,
which can drop from the base of the storm in a lowering known as a wall cloud. Eventually the winds reach the surface,
forming a tornado. Tornadoes usually
only last a few minutes and can range in size from a football field to over a
mile wide. Doppler radar can estimate
wind speed and direction, making it a useful tool in detecting the rotating
mesocyclone. Tornadoes most often occur
in the plains states, with TX having the most total tornadoes, and OK having
the most per 10,000 miles. There is
also a maximum in tornadoes in FL spawned from hurricanes and tropical storms. Tornadoes have occurred at least once in all
50 states. Most tornadoes occur in late
spring and early summer when the upper level winds are strong and there is high
instability. They need the heating of
the day to get the storms going, so most tornadoes occur in the late afternoon
or early evening. Suction vorticies are
small intense rotations that occur within the main tornado, and can be
responsible for intense damage.
Tornadoes are rated in strength though the Fujita scale, which estimates
wind speed through damage. Fortunately
only about 2% of tornadoes are the violent F4 or F5, but that small amount is
responsible for the most fatalities.
The F-scale fails when the tornado occurs in an open area where there is
nothing to damage. A tornado that moves
onto water from land can be called a waterspout. Waterspouts can also occur when cumulus congestus clouds are in
the area and warm water heats the air from below. Fortunately, most waterspouts are small and weak.
Know:
·
What
conditions are necessary for thunderstorms to develop
·
The
life cycle of an air mass thunderstorm
·
The
different types of severe thunderstorms
·
How
lightning forms
·
Why
downbursts and flash floods are so dangerous
·
The
basics of tornado formation
·
When
and where most tornadoes occur
·
The
F-scale and its problems