Storms
Large scale circulation
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Movement of air masses
Circulation cells
Warm air rises, cools then sinks
Water vapor decreases air density and
warm air can hold more water vapor
Features of the model
Air moves vertically at the boundaries
Air moves horizontally within the cells from areas
of high pressure to areas of low pressure
Tropical areas – Hadley cells
Surface winds are strong and dependable
Trade winds or easterlies centered at ~15oN (northeast
trade winds) and ~ 15oS (southeast trade winds)
Surface wind moves from horse latitudes to doldrums
so come out of northeast in N hemisphere
Mid-latitude areas – Ferrel cells
Westerlies centered at ~ 45oN and ~45oS
Surface wind moves from horse latitudes to polar cells
so comes out of southwest in the N hemisphere
Polar front
• About 50o N and S
• Persistent boundary between converging warm
and cold air masses
• Get highly variable weather at these latitudes
• Made up of a succession of waves that appear
on weather maps as warm or cold fronts
– Succession of warm, moist, subtropical air
and cold dry polar air
– Weather typical of N America and Europe
• Narrow bands of strong winds called jet streams
at altitudes of about 10 km
Weather
• A result of smaller atmospheric motions and
eddies
• Usually caused by differences in atmospheric
pressure, temperature and humidity (remember
all of these affect density)
• Weather forecasts try to predict smaller scale air
movements
Pressure systems
• Low pressure where air masses or winds
diverge
• High pressure where air masses or winds
converge
• Air moves from high pressure areas to low
pressure areas
• Meteorologists map highs and lows to predict air
movement
Air masses
• Comparable to a water mass
• Large body of air with uniform temperature
and humidity (so density) throughout
• Air over land or water will take on
characteristics of surface below
– Cold, dry land yields cold, dry air (high
pressure)
– Warm ocean surface yields warm, wet air (low
pressure)
Air masses
• Air masses form over land and water acquiring
characteristics of their sources
– Dry, cold air forms over Canada and Siberia…
– Wet, moist air forms over equatorial waters…
• When air masses move, they change
characteristics
– Temperature changes
– Humidity or water content changes (lose water)
Air masses
• Air masses can move within or between
cells
• Density differences prevent air masses
from mixing (like water) – dense air slides
beneath
• Turbulence at boundaries between air
masses
• Fronts are boundaries between air masses
of different densities
– Fronts marked by changes in temperature and
humidity
Fronts
• A cold front is the leading edge of a cold-air
mass advancing on a warm air mass
– Displaces warm air
– Cold air pushes under warm air (more toe shaped)
– Get precipitation (rain or snow) just behind the front
• A warm front is the advancing edge of a warm
air mass
– Displaces cold air
– Rises over cold air in a wedge shape
– Drops water in front of its leading edge
Ocean influence on weather
• At mid-latitudes, warm and cold water masses
steer weather patterns on land
• Size and energy of water masses permits this
• Large cold water masses in the N Pacific shift
prevailing westerlies blowing across E North
America
• Cold, dry air from Canada displaces warm, moist
air from the Gulf of Mexico and the tropical N
Atlantic
• So get cooler winters in the SE USA
• Shifts in positions of water masses can cause
changes in patterns
• Warm equatorial surface waters in the Atlantic
cause prolonged drought in Africa?
Storms
• Regional atmospheric disturbances
characterized by strong winds and, often,
precipitation
• Cyclones are intense storms around low
pressure centers
• Tropical disturbances (in Hadley cells/tropics) –
cyclones (hurricanes)
• Extratropical disturbances (in Ferrel cells/midlatitudes) – also cyclones, usually in winter
Cyclones
• Low pressure air
• Rotates as winds converge and ascend
(may bring water with them so get
precipitation)
• Form between or within air masses
Extratropical cyclones
• Form at the boundary of polar and Ferrel cells
(polar front) – mid-latitudes
• Occur mainly in winter when temperature and
density differences across the front are most
pronounced
• Cold air poleward of front is moving from the
pole and east (more dense)
• Warm air equatorward of the front is moving
from the equator and west (less dense)
• Cold air tries to slide below the warm air at the
low pressure interface of a stationary front
Extratropical cyclone
• May get alternating high and low pressure
systems that bend the front
• May get a twist in front due to opposite wind
directions
• Twisting air mass becomes cyclonic and
circulates CCW in the N hemisphere (opposite
Coriolis)
• CCW flow is Coriolis driven because of the
dominant flow of air masses at the edges
• Part of the front is cut off
• Wind speed increases as storm condenses
• Air rushing toward center rises making a low
pressure zone (air rises and loses moisture)
Cold air tries to dive below or push under warm air
Higher pressure N of cold front so bending
is towards lower pressure
Cold air pushes warm air/front
Low pressure intrusion into cold front
Warm air rises (with or without water) at both
fronts & yields precipitation at the fronts
Cold front pushes warm front
Eventually part of front is cut off and moves
east
Extratropical cyclones
• Cyclone gets embedded in the westerly
winds so moves eastward
• Typically 1000-2500 km in diameter
• Last 2-5 days
Extratropical cyclones
• Precipitation begins as circular flow develops
• Precipitation caused by the lifting and cooling of
the mid-latitude air (warmer air from the Ferrel
cell) involved in the twist
• Cold air advances behind it and does the lifting
creating a cold front
• Warm front occurs as the warm air is lifted on
top of the retreating cold edge
• Often these are called frontal storms and are the
principle cause of weather in mid-latitudes
Nor’easters
• Most powerful wind approaches from the
east (polar cells)
• Occur along the east coast of the US in
winter
Tropical cyclones
• Masses of warm, humid, rotating air
• Occur in all tropical oceans except the equatorial
South Atlantic
• Large tropical cyclones (winds at least 119
km/hr) are:
– hurricanes in the North Atlantic & eastern Pacific
(about 100/year)
– Typhoons in the western Pacific
– Tropical cyclones in the Indian Ocean
– Willi-willis in the waters near Australia
• Smaller tropical cyclones are tropical storms or
depressions
• Appear as circular spirals
• May be 1000 km in diameter and 15 km
high
• Calm center is the eye & can be 13-16 km
• Occur June – November in N hemisphere
Internal structure of a mature hurricane.
Tropical cyclones
• Usually generated within one air mass
• Usually generated between 10o and 25o latitude
(Coriolis effect closer to equator is too weak to
initiate rotary motion)
• Typically last ~9 days
• Origins not well understood
– Convergence of warm, wet winds that rise
• Usually develop from a tropical depression
• Power if from the condensing water vapor and
rising air currents at the eye
Tropical cyclones
• Tropical depressions form in easterly waves
– areas of lower pressure within the easterly tradewinds
– thought to originate over a large, warm land mass.
• Air containing the disturbance is heated over
tropical water
• Circular winds begin to blow in the vicinity of the
wave
• Some warm, humid air is forced upward
• Condensation begins
Where hurricanes form (areas of high humidity and warm air over warm water)
Hurricanes
• Develop in 2-3 days from tropical cyclones
under ideal conditions
• Centers move westward and poleward (within
easterlies) in N hemisphere at 5 to 40 km/hr
• Poleward motion due to general atmospheric
circulation
• Hurricanes lose strength over land (friction and
loss of water vapor supply) or relatively cold
surface water (decreases rising wind speed in
eye)
Tropical cyclone tracks – breeding grounds shown in orange
Hurricane George
Tropical cyclones
• Origin not well-understood
• Power and strengthening are
Take home points
• Weather versus climate – scale of
circulation
• Frontal storms – extratropical cyclones
• Tropical cyclones (e.g., hurricanes)
• Pressure systems
• Affects of humidity, temperature and
pressure on air flow