Chapter 18/19

Ch 18

Pressure •  Force of the weight of the air above •  At sea level: 14.7 lbs/in2 •  Decreases with altitude •  Inches of mercury (29.92 @ sea level) •  Barometer - instrument used to measure air

pressure –  Mercury barometer - glass tube filled w/ mercury –  Aneroid barometer - no liquid, uses an expanding

chamber •  High pressure - sinking air (cool, clear) •  Low pressure - rising air (warm, cloudy,

stormy)

Wind

•  Horizontal movement of air •  Moves from areas of High pressure to areas

of low pressure •  Isobar - lines of equal pressure •  Pressure gradient - pressure change over

distance •  Coriolis effect - apparent deflection in wind

direction due to Earth’s rotation •  Deflects to the right in the northern

hemisphere

Upper air winds

•  Geostrophic winds - generally blow parallel to isobars

•  Jet Stream - “river of air” – Travels at speeds from 75 mph - 150 mph – Generally separates 2 types of air masses:

•  Cold & warm •  This is where fronts form along

cyclones

•  A center of low pressure (L) •  Pressure decreases towards the center •  In N. hemisphere = counterclockwise •  Rising air •  Associated w/ Clouds & precipitation

anticyclone

•  A center of high pressure (H) •  Pressure increases towards the center •  In N. Hemisphere = clockwise •  Sinking air •  Usually brings “fair” weather

Circulation of air

•  Unequal heating of earth causes H’s and L’s

•  Rotation of earth causes 3 pairs of atmospheric cells

•  Land heats up and cools off faster than water creating seasonal H’s & L’s

•  Monsoon - seasonal change in wind direction (wet & dry seasons)

Circulation in the mid latitudes

•  (We live in this latitude) •  Zone of the Westerlies

–  Winds are named in the direction they are blowing from

•  Complex •  Air flow is interrupted by cyclones:

–  Cells move west to east (N. Hemp) –  Create anticyclonic/cyclonic flows –  Paths of the anticyclonic/cyclonic flows are

associated with the upper-level airflow (jet stream)

Local winds

•  Result from temperature differences •  Types

– Land/sea breezes – Mountain/Valley breezes

Measuring winds

•  2 basic measurements –  Direction –  Speed

•  Direction: Labeled according to the direction they originate (wind vane)

•  Speed: (cup anemometer) •  Changes in wind direction

–  Placement of H’s & L’s –  Temp –  Moisture conditions

El Nino/La Nina

•  Read on your own in textbook •  Pages: 518 - 522

Distribution of Precip.

•  Mainly in the U.S. (we will concern ourselves with)

•  We get our atmospheric conditions from air masses (when the air takes on the characteristics of where they form)

•  Weather is moved by high pressures •  Depending which way the wind is blowing that

is the type of weather we will get. •  The moisture that reaches Pennsylvania

mainly originates from either the Pacific or Atlantic Ocean or Great Lakes.

Chapter 19

Weather Patterns & Severe Storms

Air Masses

•  Air that takes the characteristics of the Earth’s surface.

•  Can be several miles thick •  Temperatures & moisture levels are

similar at any given altitude in that mass

Types of Air Masses

•  Polar (P) - cold •  Tropical (T) - warm •  Continental (c) - dry •  Maritime (m) - wet, moist •  Ex. If we are affected by an air mass coming

up the east coast originating from the Gulf of Mexico that air mass would be considered maritime tropical (mT)

In the Eastern U.S.

•  cP & mT are the most important air masses in North America especially east of the Rockies.

•  cP air masses: come from Canada & interior of Alaska –  Winter brings: cold, dry air –  Summer brings: cool relief –  Responsible for Lake Effect Snow

•  cP air moves across the Great Lakes, picks up moisture, snow falls on the other side (usually brings 2-hr delays)

•  mT air masses: originates in the Gulf of Mexico or Atlantic Ocean. –  Warm, moist, Unstable air (stormy) –  Brings Precipitation

•  cT air masses: don’t reach us –  (more so localized to the region they form in, the

desert) •  mP air masses: occasional influence in our

area, however are responsible for nor’easters* with cold temperatures & heavy rain or snow fall –  (1-2 days off if snow)

Fronts

•  Boundary that separates air masses of different densities –  Cold air heavier than warm air –  Warm air will be forced upward then it collides with

cold air –  This forms clouds –  Which eventually =‘s precipitation –  We can conclude whenever a front passes by us

there will be precipitation somewhere located along that front

Types of Fronts

•  Warm front: –  Warm air replaces cold air –  Slow moving (why? An area of low pressure

occupies this area, if no H pressure is close enough behind that L then the winds won’t be strong enough to drive it forward)

–  Clouds advance at a slow rate •  Cirrus cirrostratus altostratus

stratusnimbostratus –  Light-to-moderate precipitation lasting long –  Gradual temperature change when passing –  Precipitation will enter the area before the actual

front is over us.

Cold Fronts •  Cold air replaces warm air •  Travels fast •  Precipitation can be intense & violent, but

brief (T-storms) •  Temperature changes occur suddenly •  Clear conditions will occur after the storm

passes (because a H pressure is behind the front, and that is why it moves so fast)

•  Precip is located right on the front •  Acts like an “air bulldozer” driving warm moist

air high up in the atmosphere very quickly (cumulonimbus clouds)

Stationary Front

•  Air or winds move parallel to front •  At a standstill •  Ex. Warm air moves one way / cold air

moves the other way. Front stays in one spot.

•  Precipitation is similar to a warm front

Occluded Front

•  Fast moving cold front takes over the slow moving warm front.

•  Precipitation is similar to a warm front

What they look like

Cold fronts

stormy Nice, but cooler

Warm Fronts

cirrus (Starts getting cloudier) Rain starts Rain ends

Can take a day (give or take a few hrs. to completely pass)

Mid-lattitude cyclones •  Move slow (2-4 days to pass over a region) •  Consists of a cP & mT air mass •  Warm front and cold front connected to a L

pressure system •  Cold front eventually takes over the warm

front (forming an occluded front) •  Largest weather contrasts occur in the spring •  Can evolve into a nor’easter

–  Basically the mid lattitude cyclone passes by us, enters the Atlantic Ocean, picks up energy, we get hit with the back end of the system and get womped again with more precipitation than the initial pass by.

Severe Storms

•  Thunderstorms –  Cumulonimbus clouds, Heavy rainfall, Lightning,

Occasional hail –  2000 occurring any given time on earth –  100,000 / yr. in U.S. on avg.

•  Is the Parent of all bad storms –  Hurricanes, tornadoes, blizzards, noreasters, all

start-off as a T-storm •  Can happen in the winter (not unusual but not

as frequent as in the spring, summer, & fall.

Tornados

•  Violent funnel shaped wind storms •  770 / yr. on avg. in U.S. •  April - June •  Associated w/ huge T-Storms (Supercells) •  Difficult to predict due to being a small size

storm •  Can occur in other countries, but nearly all

happen in the U.S.

Hurricanes

•  Most violent and largest storm on earth –  Eastern U.S - Hurricane –  Western Pacific - Typhoon –  Southern Hemisphere (Indian Ocean) - Cyclone

•  North pacific has the greatest number per year

•  Forms in ocean near the equator •  Winds must maintain speeds of 74 mph or

greater

Parts of a Hurricane

•  Eye – Center of hurricane – Winds subside – Precipitation ceases – Very calm – Air sinks but heats up due to compression – Warmest part of a hurricane

Eyewall area of rising air greatest wind speed heaviest rainfall wall of cumulonimbus clouds

Fuel for hurricanes

•  Warm moist air – They die out when

•  Move onto land •  Move over cooler ocean waters •  Large scale flow aloft is unfavorable (upper level winds are messing things up)

Other hurricane info

•  Intensity is based on the Saffir-Simpson scale. –  Identifies the intensities of the hurricane

•  Damage: –  Storm surge- large dome of water pushed onto

shore when the eye makes landfall –  Wind damage –  Inland flooding from torrential rains –  Most damage from flooding will occur on the

(eastern) side of the storm.

Other storms

•  Tropical depression - beginning stage of a hurricane (winds do not exceed 38 mph)

•  Tropical storm - winds range in speed from (38 - 74 mph)

•  Blizzard - snow storm, winds must maintain speeds of 35 mph, and visibility must be reduce to less than 1/4 of a mile for a minimum of 3 hours.

And now you know •  advisory - When a significant storm or

hazardous weather is occurring, imminent, and is an inconvenience.

•  watch - A significant winter weather is expected, but not imminent, for the watch area; provides 12 to 36 hours notice of the possibility of severe weather.

•  warning - A significant storm or hazardous winter weather is occurring, imminent, or likely, and is a threat to life and property.

Check out this website

•  http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/home.rxml