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 modelAir moves vertically at the boundaries

Air moves horizontally within the cells from areas of high pressure to areas of low pressure

Tropical areas – Hadley cellsSurface 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 cellsWesterlies 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

• 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

• 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 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

Air masses

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/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

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 cyclones

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 airHigher pressure N of cold front so bending is towards lower pressureCold air pushes warm air/frontLow pressure intrusion into cold frontWarm air rises (with or without water) at both fronts & yields precipitation at the frontsCold front pushes warm frontEventually part of front is cut off and moves east

• Cyclone gets embedded in the westerly winds so moves eastward

• Typically 1000-2500 km in diameter

• Last 2-5 days

Extratropical cyclones

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

• Origin not well-understood

• Power and strengthening are

Tropical cyclones

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