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Weather and Climate
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Atmosphere: layer of gases held close to earth by gravity. 99% of total mass of
atmosphere is within 20 miles of Earth’s surface.
I. Weather: short term atmospheric conditions in a certain area—Results from warming
of atmospheric gases.
A. Air mass: an extremely large body of air whose temperature and humidity are
very similar –can cover hundreds of thousands of square miles—there can be a
small variation in temperature/humidity. Air masses are typically classified
according to their source region and vary in source temperatures and source
humidity.
1.Source temperatures: Fill in the temperature you expect each air mass to
have:
A. Polar Latitudes (P): Cool
B. Tropical latitudes (T):Warm
As gases warm they expand and rise (because less dense): This creates
VERTICAL convection currents. These currents transfer heat energy
As cool air rushes in to fill the empty space: HORIZONTAL air flow is created.
2.Source Humidity’s: Fill in the humidity you expect each air mass to have:
A. Continental (C): Dry
B. Marine or maritime(M):Moist
Warm air masses hold more water than cool air
Masses can be classified as continental polar, maritime polar, continental
tropical, marine tropical, etc.
B. Front: leading edge of an air mass—boundaries between different air masses.
The boundary between the warm and cold air masses always slopes upwards
over the cold air. This is due to the fact that cold air is much denser than
warm air. The sloping of warm air over the cold air leads to a forced uplifting
of the warm air if one air mass is moving toward the other. In turn, this
uplifting causes condensation to occur and the possibility of precipitation
along the frontal boundary.
1. Warm Front: Advancing warm subtropical, moist air mass replaces a
retreating cold, dry polar air mass. The formation of clouds and precipitation
ahead of the frontal zone is caused by gradual frontal lifting. High altitude
cirrus and middle altitude altostratus clouds are found well in advance of the
front. These clouds produce precipitation in the form of snow or rain.
Warm air rises, cools, loses water as rain
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2. Cold Front: When cold air is moving forward and pushes warm air on top
of it. A cold front is the transition zone in the atmosphere where an
advancing cold, dry stable air mass displaces a warm, moist unstable
subtropical air mass. High altitude cirrus clouds are found well in advance
of the front. Precipitation is normally found just behind the front where
frontal lifting has caused the development of towering cumulus clouds.
Thuderhead clouds
After the front passes, cool, clear skies follow
3. Stationary: Neither air mass is moving
C. Air Pressure –Caused by molecules colliding. Higher near Earth’s surface
WHY? Molecules packed more closely together.
1.High Pressure System—high pressure and high density
High pressure systems usually form where the air converges aloft. As the air
converges in the upper-levels of the atmosphere, it forms an area of higher
pressure and is forced to sink. The sinking air spirals outward, clockwise in
the Northern Hemisphere, counterclockwise south of the Equator.
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High pressure systems are associated with clear, cool weather. Air tends to
sink near high-pressure centers, which inhibits precipitation and cloud
formation. This is why high-pressure systems tend to bring bright, sunny days
with calm weather.
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Moist warm air rises then loses water as it cools and falls back down a s a
high pressure . then the air is warmed when it’s near earth’s surface, gains
water and rises again low pressure
2. Low Pressure System—low pressure and low density
Low pressure areas form when an air mass warms, either from being over a
warm land or ocean surface, or from being warmed by condensation of water
vapor in large rain or snow systems. This warmed air then begins to rise. As
air rises it cools. As the air cools, the humidity in it begins to condense into
tiny drops of water, or if it's cold enough, into tiny ice crystals. If there's
enough water or ice, rain or snow begin to fall. This is why low pressure is
associated with bad weather.
Low pressure systems are usually associated with heavy precipitation and
overcast conditions.
Lows have low pressure and low density so center rises up and warem air
then expands out.
Created when warm (water heavy) air rises and leaves cooler, drier air
behind.
D. Weather Extremes
1.Tornadoes start deep within vast thunderclouds, where a column of
strongly rising warm air is set spinning by high winds streaming through the
clouds's top. As air is sucked into this swirling column, it spins very fast,
stretching thousands of feet up and down through the cloud, with
corkscrewing funnel descending from the cloud's base - the tornado.
Wind speeds up to 300 mi/hour.
Low pressure at center
US is most tornado prone—Australia is 2nd
In USA, April-July is prime time Esp. in the center of the country “tornado
alley”
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2.Hurricanes have deepening low-pressure centers that take in moist air and
thermal energy from the water’s surface. Convection lifts the air, and as the
air continues to rise, it eventually hits high pressure and is pushed outward.
Environmental Effects:
Storm surge—increase in water height near eye because winds draw warm
water upward.
Wind damage—strips vegetation of leaves
Erosion-loss of breeding grounds in wetlands
Flood waters may contain toxins from factories, streets
LONG TERM EFFECTS: flood waters flush excess nutrients from wetlands
(which are nurseries). This causes decrease in algae growth which leads to
increase in sea grasses (which is good for nursery areas)
Called typhoon in Pacific Ocean
Winds: minimal hurricane 75 m/hr up to 130 mi/hr
Katrina--$75 billion damage/ 1800 dead
Begin over warm water where the trade winds converge
A subtropical hi pressure leads to hot daytime temps with lo humidity. So
there is then lots of evaporation of ocean water into the air mass
1. Separate thunderstorms develop and converge
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2.
3.
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They pick up moisture and heat from the ocean
The heat increases the wind speed (because it adds energy)
Eye: descending air and lo pressure—calm
Energy dissipates as it goes over land or cooler waters.
II. Climate: long term atmospheric conditions—temperature and precipitation
A. Global Air Circulation Patterns—determined by
1. Uneven heating of earth’s surface: more heat at _Equator____, less at
___Poles_.
a. The light that hits near the poles (top pair of arrows) travels through
more atmosphere before reaching the surface than light near the
equator (bottom pair of arrows).
b. Because of the angle relative to the path of the rays, the sunlight near
the poles is spread over a larger area (orange box) than at the equator.
2. Seasonal Changes in Temperature and Precipitation.
Diff. regions of earth receive more sun at diff.
times of year—opposite for N and S
hemisphere
3.
Rotation of Earth on axis:
Earth’s surface moves faster at Poles
because less surface to cover. This
deflects air masses moving N and S to
the E and W (in N hemisphere –air
deflects to the right and to the left in
the S. Hemisphere
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Hadley cells: Vertical (convection currents) –warm air rises at equator, collides
with colder air coming from poles which cools it and causes it to descend. When
air reaches surface it is warmed and rises again, Process repeats at 60 lat. And
poles.
Coriolis effect: deflection to the right in the northern hemisphere and to
the left in the southern hemisphere due to Earth’s rotations.
Prevailing winds are determined by Coriolis effect:
Easterlies: occur between 60 and 90 lat. N Hemisphere: blow from NE; S
Hemisphere: blow from SE
Westerlies:
Blow from the west to east. Occur with falling air from 3060 lat .In N Hemisphere: move from SW; in S hemisphere: move from NW
Horse Latitudes: 30 lat. Weak winds—sailors threw horses overboard
Trade Winds 0-30 lat strong and steady. Move ships across ocean—Blow
from east –N Hemis: NE Trades S Trades: SE Trades
Doldrums Equator—no winds because air is constantly rising. 5latnorth
to 5 lat south is also called Intertropical Convergence Zone . Lots of
precipitation as convection storms form
4. Properties of air, water and land: Convection currents
Convection over large lakes because air over land warm faster during day, rises
and cool air over lake moves in—Lake breeze
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Rain shadow effect: as warm water rises and hits a mountain it cools (increase in
altitude) and rains on front side of mountain. Explains deserts on back sides of
mountains
III. Ocean Circulation
A. Surface Currents: Surface Circulation
1. Winds—blow on surface of ocean and push water—wind comes
from________differences in air temp/presure______________________
2. Complications to wind flow:
a. Coriolis effect
b. Land masses disrupt flow
B. Thermocline: The oceans have a well-mixed surface layer where the water
temperatures are relatively constant. Below the mixed layer is the thermocline, a
zone where temperature changes rapidly with depth. Below the thermocline the
temperature is relatively uniform with depth, showing only a small decrease to the
ocean bottom.
The temperature structure of the upper ocean varies during the year. The density
of water varies with temperature and salinity; with cold water denser than warm,
and highly saline water denser than less saline. During the summer the surface
water warms. As warmer water is less dense than cold, this warm water remains
at the surface and the water column is “stable”. There is little wind so the mixed
layer is shallow, as is the thermocline. During the fall and winter, cooler
temperatures and wind from storms cool the surface waters. This increases the
density of the surface water, which then sinks to a level of similar density. This
combination of cooling and wind mixing causes a deep mixed layer. In the
spring, the water warms again and the thermocline reforms.
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C. Deep Water Circulation: Thermohaline circulation
Temperature (thermo) and salinity (haline) differences cause flow
1. Step one: Deep Water Formation—water transport to poles cools and
sinks, or cools, freezes, becomes saltier and sinks (Ice formation releases
heat and increases salinity as very little salt freezes into the ice)
2. Step two: water flows at rates of kilometers/month deep at the bottom of
the ocean, taking 150 years or so to resurface
3. Step Three: prevailing winds off the coast (e.g., Peru, California) blow
surface waters away, causing lower, cooler waters to rise up this is called
UPWELLING. These deeper waters tend to be nutrient rich, and cause
phytoplankton and thus fish populations to bloom.
IV.ENSO (El Nino Southern Oscillation) of the Pacific Ocean—The entire ENSO
cycle lasts usually about 3-7 years and often includes a cold phase (La Nina) as well
as the better known warm phase (El Nino)
During Non-El-Nino years, the following occurs:
Normal conditions are defined as upwelling off the western coast of S. America
due to prevailing easterly winds. Remember the easterly winds blow from the east to the
west. Ocean conditions during Normal Years:
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trade winds transport water from eastern to western side of tropical Pacific Ocean
divergence along S. America coast causes upwelling (source of nutrient-rich
waters)
western tropical Pacific is several degrees warmer than eastern tropical Pacific
the thermocline slants from the eastern tropical Pacific to the western tropical
Pacific
sea-surface height is about 1/2m greater in western tropical Pacific than in eastern
tropical Pacific
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El Niño (the “warm event”) is the name given to the occasional development of
warm ocean surface waters along the coast of Ecuador and Peru (warm phase of
ENSO). When this warming occurs the usual upwelling of cold, nutrient-rich deep
ocean water is significantly reduced. El Niño normally occurs around Christmas and
lasts usually for a few weeks up to 14 months. .
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El Nino ocean conditions:
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trade winds decrease causing western Pacific waters to slosh eastward
warmer than normal surface waters in the eastern Pacific Ocean
cessation or weakening of upwelling along S. America coast
thermocline gradient is reduced
sea-surface height gradient is reduced
North US and Canada: warmer winter/less intense hurricanes
Eastern US and Peru/Ecuador: higher than average rain—can break drought
Phillipines, Indonesia and Australia: DRYER than normal (forest fires increase)
Impact of El Nino on Environment:]
Offshore fish population decreases therefore shore birds decline
Fishing industry-economic impact
Warmer water in Pacific leads to more rain/big storms in Califo.
BIOMES and CLIMATE
Doldrums (equator)—hi humidity, hi clouds, heavy rain
Tropical rainforests
Trades: Tropical and deciduous forests—precip. All year
Horse Lats: Descending dry air—DESERTS Lo relative humidity, few clouds warm or
hot summers, mild winters
Westerlies: Deciduous forests
60 lat: Boreal forests, taiga
Easterlies: Tundra/taiga—cold dry air falls : COLD DESERT
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La Nina: some years the trade winds can become extremely strong and an abnormal
accumulation of cold water can occur in the central and eastern Pacific (Figure 4). This
event is called a La Niña. (cold phase of ENSO) During La Niña years, the trade winds
are unusually strong due to an enhanced pressure gradient between the eastern and
western Pacific. As a result, upwelling is enhanced along the coast of South America,
contributing to colder than normal surface waters over the eastern tropical Pacific and
warmer than normal surface waters in the western tropical Pacific
La Nina can be thought of as the reversal of El Nino or the intensification of
normal conditions in the tropical Pacific Ocean.
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