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Storms - PowerPoint PPT Presentation

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Storms. Large scale circulation. 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

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Large scale circulation
Large scale circulation

  • 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



  • 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
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 masses1
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



  • 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?


  • 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/mid-latitudes) – also cyclones, usually in winter


  • 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
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
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


Extratropical cyclones1
Extratropical cyclones

  • Cyclone gets embedded in the westerly winds so moves eastward

  • Typically 1000-2500 km in diameter

  • Last 2-5 days

Extratropical cyclones2
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
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

Tropical cyclones1
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 cyclones2
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

Hurricanes over warm water)

  • 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)


Hurricane George over warm water)

Tropical cyclones3
Tropical cyclones over warm water)

  • Origin not well-understood

  • Power and strengthening are

Take home points
Take home points over warm water)

  • 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