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Earth Science 101
Air Pressure and Wind
Chapter 19
Instructor : Pete Kozich
Atmospheric pressure
Force exerted by the weight of the air above
Weight of the air at sea level
• 14.7 pounds per square inch
• 1 kilogram per square centimeter
Decreases with increasing altitude
Units of measurement
• Millibar (mb) – standard sea level pressure
is 1013 mb
• Inches of mercury – standard sea level pressure
is 29.92 inches of mercury
Atmospheric pressure
Instruments for measuring
• Barometer
• Mercury barometer
• Invented in 1643
• Uses a glass tube filled with mercury and
inserted into a mercury filled reservoir
• Aneroid barometer
• "Without liquid"
• Uses an expanding/contracting air chamber
• Barograph (continuously records the air
pressure)
A mercury
barometer
Figure 18.2
A recording aneroid
barometer
Figure 18.4
Wind
Horizontal movement of air
• Results for pressure differences: out of areas
of high pressure, into areas of low pressure
• Pressure differences are caused by differential
heating of the earth’s surface
• Solar energy is the driving force. Thermal
gradients drive pressure gradients and,
from there, the wind.
Wind
Controls of wind
• Pressure gradient force (PGF)
• Pressure gradient – pressure change over distance
• We measure surface pressures at hundreds of locations
across the earth. This data is plotted on a map using
isobars.
• Isobars – lines of equal air pressure
• Closely spaced isobars indicate a steep pressure gradient
and high winds
• Widely spaced isobars indicate a weak pressure gradient
and light winds
• The magnitude of the PGF is determined by the
spacing of the isobars
• The direction it moves is always from higher to lower
areas of pressure
A weather map showing isobars
and wind speed/direction
Figure 18.5
Wind
Controls of wind
• Coriolis effect
• An apparent deflection in the wind direction due to Earth's
rotation
• Deflection is to the right in the Northern Hemisphere and to the
left in the Southern Hemisphere
• Varies with latitude and wind speed
• Maximum deflection at the poles; zero deflection at equator
• Deflection increases with increasing wind speed; deflection
decreases with decreasing wind speed
• Friction
• Only important near the surface
• Acts to slow the air's movement, also reduces Coriolis
• Bends air inward towards low pressure systems
The Coriolis effect
Figure 18.6
Wind
Upper air winds
• Generally blow parallel to isobars – called
geostrophic winds
• Jet stream
• Ribbon of air flowing at higher speed, due to
regionally enhanced pressure gradient force
resulting from strong temperature differences
• High altitude
• High velocity (120-240) kilometers per hour
• West to east direction
The geostrophic wind
Figure 18.7
Wind
Summary
• Upper airflow is nearly parallel to the isobars,
whereas the effect of friction causes the
surface winds to move more slowly and cross
the isobars at an angle (towards low
pressure).
Comparison between upper-level
winds and surface winds
Figure 18.9
Cyclones and Anticyclones
Cyclone
• A center of low pressure
• Pressure decreases from outer isobars to the center
• Winds associated with a cyclone
• In the Northern Hemisphere
• Inward (convergence)
• Counterclockwise
• In the Southern Hemisphere
• Inward (convergence)
• Clockwise
• Associated with rising air
• Often bring clouds and precipitation
Cyclones and anticyclones
Anticyclone
• A center of high pressure, particularly at low levels
• Pressure increases from outer isobars to the center
• Winds associated with an anticyclone
• In the Northern Hemisphere
• Outward (divergence)
• Clockwise
• In the Southern Hemisphere
• Outward (divergence)
• Counterclockwise
• Associated with subsiding air
• Usually bring "fair" weather
Cyclonic and anticyclonic winds
in the Northern Hemisphere
Surface
Figure 18.10
Airflow associated with surface
cyclones and anticyclones
Figure 18.12
General atmospheric circulation
 Underlying cause is unequal surface heating
• Unequal heating creates pressure imbalances
• Tropical regions receive more heating than can be radiated back
to space
• Polar regions receive less radiation than is radiated back to space
 Idealized global circulation (non-rotating Earth)
• Heated air would rise near the equator until it reached the tropopause,
which acts as a lid
• The air would then deflect poleward
• Upon reaching the poles, the air would sink back to the surface, spread
out in all directions, and return to the equator
• One cell in each hemisphere
• Surface air always moves equatorward
• Air aloft always moves poleward
General atmospheric circulation
On the rotating Earth there are three pairs of
atmospheric cells (four interfaces) that redistribute
the heat
Global circulation
• Intertropical Convergence Zone (ITCZ)
• Rising air
• Abundant precipitation
• Subtropical high pressure zone
•
•
•
•
Subsiding, stable, dry air
Near 30 degrees latitude
Location of great deserts
Air traveling equatorward from the subtropical high produces
the trade winds
• Air traveling poleward from the subtropical high produces the
westerly winds
General atmospheric circulation
Global circulation
• Subpolar low pressure zone
• Warm and cool winds interact
• Polar front – an area of storms
• Polar high pressure zone
• Cold, subsiding air
• Air spreads equatorward and produces polar easterly
winds
• Polar easterlies collide with the westerlies along the
polar front
• By far the weakest of the cells; highly variable
Global circulation
Figure 18.15
Global Atmospheric Circulation Model
General atmospheric circulation
Influence of continents
• Seasonal temperature differences disrupt the
• Global pressure patterns
• Global wind patterns
• Influence is most obvious in the Northern
Hemisphere (more land)
• Land heats up and cools off more quickly
• Mountains change air flow, and land-sea
temperature differences influence air flow
General atmospheric circulation
Influence of continents
• Monsoon
• Seasonal change in wind direction
• Occur over continents
• During warm months
• Air flows onto land (low pressure)
• Warm, moist air from the ocean
• Winter months
• Air flows off the land (high pressure)
• Dry, continental air
Average surface pressure and
associated winds for January
Figure 18.16 A
Average surface pressure and
associated winds for July
Figure 18.16 B
Circulation in the mid-latitudes
 The zone of the westerlies
 Complex
 Air flow is interrupted by migrating cyclones and
anticyclones
• Cells move west to east in the Northern Hemisphere
• Create anticyclonic and cyclonic flow patterns
• Paths of the cyclones and anticyclones are associated with the upperlevel airflow
 Upper-level airflow
• One of the most obvious features is the seasonal changes
• Stronger temperature gradient in the winter corresponds to a
stronger flow aloft
• Polar jet streams average position migrates south as winter
approaches and north as summer approaches in N.
Hemisphere
Local winds
Produced from temperature differences
Small scale phenomena
Types
• Sea breezes
• Coastal regions (daytime especially in the summer)
• Air over the land heats up creating a low pressure area over land,
allows cool air over the water to flow inland
• Begins around noon and strongest in late afternoon, often stormy
• Land breezes
• Coastal regions (nighttime)
• Land cools more rapidly than the water creating a low pressure
area over the warmer water, allows cool air over the land to flow
out to sea
• Land/sea breezes can also develop near lakes
Illustration of a sea breeze
and a land breeze
Figure 18.17
Vertical Structure of a Lake/Sea Breeze
Local winds
Types
For mountain and valley breezes, air in contact with
the ground heats up and cools off more rapidly over
course of a day at higher altitude (lower pressure)
• Mountain breezes
• Cold air at top of mountain late at night has higher pressure, blows
down mountain
• Valley breezes
• High in mountains, air heats up quickly, lowering air pressure;
winds blow up from the valley
• Chinook and Santa Ana winds
•
•
•
•
Near-freezing air passes over a mountain slope and descends
Air warms and dries as it descends
When it reaches the bottom, it is much warmer
Santa Ana winds are an example of a Chinook wind that occurs in
Southern California
Mountain and Valley Breeze
• Flow reverses over the course of the day
• This diurnal cycle of winds is best developed in clear
summer weather with light winds
Chinook Wind
•
•
•
•
A warm, dry wind
Typically descends on the Eastern Slope of the Rockies
Also found in Alps
Occur when strong westerly winds flow over a N-S oriented mountain
range
– Forces the air downslope; a trough forms on the east side of the mountains
– As the air descends it is compressed and warmed dry adiabatically
Fig. 9-33, p. 244
Santa Ana Winds
• A warm, dry wind that blows from the east or northeast into Southern CA
• The air descends from the desert plateau, funnels through the mountain
canyons, and spreads into the LA basin
• High pressure develops over the Great Basin and the cw circulation
forces the air downslope
• Compressional warming, warms the air as it descends
• The dry air becomes even drier
• Can cause brush fires
Satellite Images of
CA fires
from Earth
Observatory
Wind measurement
Two basic measurements
• Direction
• Speed (Magnitude)
Direction
• Winds are labeled from where they originate
(e.g., North wind – blows from the north
toward the south), reverse of ocean currents
• Instrument for measuring wind direction is the
wind vane
• Points into the wind
Wind measurement
Direction
• Direction indicated by either
• Compass points (N, NE, etc.)
• Scale of 0º to 360º
• Prevailing wind is the time averaged single
direction the wind comes out of
Speed – often measured with a cup
anemometer
Wind measurement
Changes in wind direction
• Meteorologists call them wind shifts
• Associated with locations of
• Cyclones (especially these)
• Anticyclones
• Often accompany changes in
• Temperature
• Moisture conditions
El Niño and La Niña
El Niño
• A countercurrent that flows toward the coasts
of Ecuador and Peru
• Warm
• Usually appears during the Christmas season
• Blocks upwelling of colder, nutrient-filled water,
and anchovies starve from lack of food
• Strongest El Niño events on record occurred
during 1982-83 and 1997-98
El Niño and La Niña
El Niño
• 1997-98 event caused
• Heavy rains in Ecuador and Peru
• Ferocious storms in California
• Related to large-scale atmospheric circulation
• Pressure changed between the eastern and western
Pacific called the Southern Oscillation
• Changes in trade winds creates a major change in
the equatorial current system, with warm water
flowing eastward
• Effects are highly variable depending in part on
the temperatures and size of the warm water
pools
Normal conditions
El Niño
El Niño and La Niña
La Niña
• Opposite of El Niño
• Triggered by colder than average surface
temperatures in the eastern Pacific
• Typical La Niña winter
• Blows colder than normal air over the Pacific
Northwest and northern Great Plains while warmer
in the southern tier of the United States
• Greater precipitation is expected in the Northwest
El Niño and La Niña
Global distribution of
precipitation
Relatively complex pattern
Related to global wind and pressure
patterns
• High pressure regions
•
•
•
•
Subsiding air
Divergent winds
Dry conditions
e.g., Sahara, Kalahari, Atacama, and MexicanSW US deserts, parts of the Outback
Global distribution of
precipitation
Related to global wind and pressure
patterns
• Low pressure regions
•
•
•
•
Ascending air
Converging winds
Ample precipitation
e.g., Amazon, Congo basins and Indonesia
Average annual precipitation
in millimeters
Figure 18.23
Global distribution of
precipitation
Related to distribution of land and water
Colder at poles, less water vapor in air and less
precipitation, but also less evaporation
• Mountain barriers also alter precipitation
patterns
• Windward slopes receive abundant rainfall from
orographic lifting
• Leeward slopes are usually deficient in moisture
Seasonal Pressure and Precipitation Patterns
End of Chapter 19