Download Where Wind Comes From

Where Wind Comes From
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Friction occurs in two forms that are important
with regards to the weather and the
atmosphere: molecular viscosity (much less
important) and eddy viscosity (much more
important); both these act opposite to wind
direction and increases with increasing surface
roughness
Gravity accelerates air downward toward the
surface but does not modify horizontal winds
These individual forces are then combined in
hydrostatic equilibrium (the balance
between gravity and the vertical pressure
gradient), the geostrophic wind (an
unaccelerated, horizontal wind that follows
a straight path above the friction layer), the
gradient wind (blows parallel to isobars
and follows curved paths, also above the
friction layer), and surface winds (which
result from the effect of friction with the
surface combined with the Coriolis effect,
and act to direct wind across isobars and
toward low pressure).
Hydrostatic Equilibrium Illustrated
Air Pressure gradients in the horizontal dimension can
arise from the following: contrasts in air temperature
(primary cause) and water vapor (or both, usually),
and diverging and converging winds.
• Winds are monitored in several ways
o Examples of instruments used include the wind vane,
the wind sock, a cup anemometer, and a hot-wire
anemometer
o Wind speed and direction (= wind velocity…a brief
description of what a vector is goes here)
o The Beauford scale is used to estimate the speed of
the wind by observing its effects on the surface of
water and on common, land-based objects.
• Wind is always named for the direction from which it
comes
The Atmosphere’s Planetary Circulation
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• Chapter 18 is concerned with the genesis and
characteristics of a variety of weather systems.
• We examine these systems in order of decreasing spatial
scale beginning with the largest scale, the global or
planetary circulation. The principle components are:
– Intertropical Convergence Zone (ITCZ)
– Trade winds
– Subtropical Anticyclones
– Westerlies
– Subpolar lows
– Polar front
– Polar easterlies
Start with a layer
of gas, add sunlight, then mix…
 The global pattern of circulation undergoes seasonal
changes as follows:
– In winter, temperatures favor high pressure is favored over
continents and low air pressure over oceans.
– This pattern is reversed in the summer
– Subtropical latitudes have seasonal monsoons that arise
out of an interplay between several climate controls (solar
heating on land versus the ocean, topography, seasonal
shifts in global circulation and the ICTZ)
The average positions of the ITCZ in January and July
 Wave patterns in the westerlies and properties of
the jet stream determine how smaller scale
weather systems behave and what sort of weather
a particular mid-latitude location receives
– Zonal versus meridianal: zonal flow involves westerlies
blowing east to west with little amplitude; meridianal flow
involves east to west prevailing winds with much
amplitude north and south
– The shift between zonal and meridianal circulation fosters
air mass exchange, poleward heat transport, and cyclone
development and movement.
– Cutoff lows (cyclones) and anticyclones (highs) block the
usual east-west progression of storm systems and may
lead to extreme weather conditions (draught, temperature
extremes, rainfall extremes)
Zonal Westerly Flow
Meridianal Flow
Cutoff Features
• El Niño and La Niña are air-sea anomalies
that contrast with long-term average
(neutral) conditions in the tropical Pacific,
these anomalies can affect weather
conditions worldwide.
– El Niño—the lengthy period of unusually high
sea-surface temperature over a vast area of
the eastern tropical Pacific
– El Niño—accompanied by weather extremes
in the tropical latitudes and in other parts of
the world.
– La Niña—conditions opposite of El Niño,
again with global influences.
Effects of El Nino
On North America
El Nino as it appears in the data over the years…
Air Masses, Fronts, Cyclones, & Anticyclones
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• An air mass is a huge volume of air that is
relatively uniform horizontally in temperature and
humidity.
– Air masses modify as they travel away from source
regions (where initial characteristics are acquired).
– These source regions include maritime (ocean) and
continental (land), high latitudes (polar) and low
latitudes (tropical)
– The four basic types: cold and dry, cold and humid,
warm and dry, and warm and humid.
– The degree of modification depends on the stability of
the air mass and the temperature of the surface over
which it travels
• Where contrasting air masses meet, fronts and
frontal weather develop.
– Air that is forced to ascend along a front undergoes
expansional cooling that often spurs formation of
clouds and precipitation.
– Four types of fronts: warm, cold, stationary, and
occluded
– Warm frontal weather: broad cloud and precipitation
shield on the cold side of the warm front
– Cold frontal weather: narrow band of convective
clouds and brief rain / snow showers, or
thunderstorms
Cold Front Weather
Warm Front Weather
Stationary Front Weather
• The life cycle of an extratropical cyclone demonstrates
how air masses, fronts, and pressure systems are
interrelated.
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It starts as a wave along the polar front
It strengthens as the air pressure drops
Winds strengthen and frontal weather develops
The cyclone occludes as the faster-moving cold front catches
up with the slower moving warm front
– The upper level low-pressure trough becomes vertically
stacked over the surface cyclone
– The lifetime of this cycle varies considerably, with 1 to 4 days
needed to reach maximum intensity.
– The maximum strength of a cyclone also varies tremendously
• Anticyclones follow cyclones. Anticyclones are
fair-weather systems, may be warm core or
cold core, and are accompanied by
characteristic patterns of air mass advection.
• Land and sea (or lake) breezes, chinook winds,
desert winds, and mountain and valley breezes
are small-scale systems that are not
significantly influenced by the Coriolis effect.
• Planetary- and synoptic-scale winds set the
boundary conditions for local and regional
circulation systems.
Thunderstorms and Tornadoes
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• A thunderstorm is a mesocscale weather system
produced by strong convection currents that surge high
into the troposphere.
• The three stages in the life cycle of a thunderstorm cell
are cumulus, mature, and dissipating.
– Cumulus stage occurs when the storm builds upward and
laterally, but no rain is falling yet
– Mature stage begins when rain reaches the ground. At this time,
strong updrafts occur along downdrafts and the system peaks in
intensity.
– Dissipating stage happens when subsiding (sinking) air spread
throughout the entire cell and the clouds vaporize.
• We describe the special synoptic situation that favors
development of severe thunderstorms, including
atmospheric stability.
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They develop along fronts within a mass of maritime tropical air
They can develop on mountain slopes
Via convergence of surface winds, or
Through intense solar heating of Earth’s surface
• Thunderstorm hazards include lightning, downbursts,
flash floods, and hail. Some may become severe while
others may not for various reasons, and some factors that
are favorable for the occurrence of severe thunderstorms
include:
– The presence of a fast-moving, well-defined cold front associated
with a mature mid-latitude cyclone.
– The jet stream causes dry air to sink over a surface layer of
maritime tropical air.
– This layering effect favors explosive convection and development
of severe thunderstorms.
• Thunderstorms are most common
continental interiors of tropical latitudes
over
the
– Sea breeze convergence over Florida make thunderstorms
more common in that state than anywhere else in North
America
– At higher latitudes, colder airmasses tend to be stabilized,
which inhibits the convection necessary for thunderstorm
development.
– Thunderstorm hazards include lightning, downbursts, flash
floods, and hail (you can read more about these in your text)
• A tornado is a small mass of air that whirls rapidly
around a nearly vertical axis and is made visible by
water condensation and dust and debris drawn into
the system
• The special combination of atmospheric conditions
that favors tornado development occurs most
frequently in spring in the central United States.
– This is in a corridor that stretches from Texas northward to
Nebraska and eastward to Indiana and Illinois
– Conditions favorable for tornadic development travel
northward (with the sun) from the Gulf Coast in early Spring
to southern Canada by early summer)
• Based on rotational wind speeds reconstructed from
property damage, tornadoes are rated from 1 to 5 on
the Fujita-scale.
• Most North American tornadoes are spawned by
supercell thunderstorms that develop along or ahead
of well-defined cold fronts.
• Tornadoes also spin off gust fronts and are
associated with land-falling hurricanes.
Cognitive Skill of the Week: Testing models
A front is a conceptual model that is represented
graphically on a surface weather map. As is the case
with all scientific models, a front approximates the
transition zone between air masses that differ in
temperature and or humidity. As part of this week’s
investigation, you will analyze a meteorgram record
of the changing atmospheric conditions at a particular
location before, during, and after the passage of a
cold front. By analysis of the meteorgram, you have
an opportunity to compare the types of atmospheric
changes (e.g., temperature, wind) that theoretically
take place with the passage of a cold front versus
what actually happens during a real cold front
passage.
Affective Attribute of the Week: Objective
An objective learner acts and interprets information
independently of personal feelings, opinions, or
bias. In this week’s investigation, you are asked to
be objective in comparing what you understand
about fronts and frontal passages with atmospheric
changes recorded during an actual frontal passage.