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Pg. 37
75% of deaths from Natural Disasters are from weather
More than EQ, MW, and volcanoes combined WHY???
OVERVIEW (we will be adding a few others but basically)
principles of weather
thunderstorms
wind
tornadoes
extreme heat and drought
Pg. 37
PRINCIPLES OF WEATHER
Water + Heat (sun) = weather
Warm air =more heat energy and more moisture
Cool Air = less heat energy and less moisture
WATER and WATER VAPOR has huge heat capacity –
ND pg. 273 table 10.1
Snickers vs. rice cake
absorbs and holds LOTS of heat & transports it
around the earth and releases it
HEAT - more of sun’s energy absorbed at equator
less at poles due to angle earth is tilted
Pole vs. equator sun energy
AIR MOVEMENTwarm air to poles cool air to equator
but things complicate this simplicity often = STORMS
Pg. 37
WHAT CAUSES WEATHER IN THE UNITED STATES??
1.) Global Wind Patterns
Air heated at equator heats, expands and rises.
As expands (lowers pressure/density) causes cooling
(adiabatic process) total heat stays the same
but volume increase causes temperature to drop
Cooling air sinks and compresses becoming dense so warms
heat at surface warms also.
GLOBAL WIND PATTERNS
Pg. 37 ADD
BUT WAIT We first need to talk about…
THE CORIOLIS EFFECT
***1.) Northern hemisphere -Air currents and
ocean currents are deflected to the right
2.) Southern hemisphere -Air currents and
ocean currents are deflected to the left
Pg. 37-38
WHAT CAUSES WEATHER IN THE UNITED STATES??
1.) Global Wind Patterns(continued)
Angle of Earth’s tilt causes different amounts of radiation
to hit surface at different points.
Differential surface heating
Produces Cells of global air circulation
Hadley - produce deserts
Ferrel - occur over continental masses = turbulence
and jet stream variation create severe
weather conditions
Polar – cool air sinks and moves toward equator
But remember Coriolis Effect
Pg. 38
2.Jet Stream - area of pressure gradient between
cold polar air and warm subtropical air = lots of
atmospheric disturbance
Occurs in troposphere(up high)
hot air=high pressure
cold air=low pressure
THIS IS OPPOSITE BAROMETRIC PRESSURE
Coriolis “bends” the wind to the right/east
MOVES LOTS OF AIR=weather
fast moving air (up to 370 mph)
from 5 to 8 miles high
moves from west to east over North America
position changes due to seasons and turbulence
Pg. 38
3.) Fronts - Air masses have different temperatures so
don’t mix, boundary = fronts
Air masses in U.S. related to latitude and
relationship of ocean/land
3 types of fronts
Cold - cold air under warm, warm moved
upward = thin band of intense thunderstorms
Warm - warm over wedge of cold = gentle
gradient = wide spread clouds
Occluded - 3 masses warm, cool and cold come
together
Pg.38
4.) High and Low pressure zones
Barometric Pressure - measured by a column of
mercury
Pg. 38
Low Pressure - rising warm air creates low pressure
so air rushes in to replace rising air produces
counterclockwise surface winds air moving upward
cools, water vapor condenses = precipitation
(stormy)
ADD Two things contribute or can cause rising air
1.) warm air rises
2. divergence aloft creates a vacuum and “sucks”
up air from below
BOTH cause upward movement of air=low pressure
Pg. 38
Low Pressure (continued)
Chicken or the egg?
Divergence or warm air?
Usually working together Aloft air diverges
Has to go some place
Barometer
Effect?
Warm Air rises
Air Rushes
in from side
Low Pressure
Air rising
Ground
Pg. 38
Low Pressure (continued)
Top View
Coriolis Effects bends wind to
Right (---)
Rushing in wind begins
to blow from one “arrow”
to the next creating
circular path-counterclockwise
Pg. 39
Cyclone - winds rotate counterclockwise about a
core of air moving upward
Northern Hemisphere-counterclockwise
Low pressure-air rises
ND pg. 281 fig. 10.17
Generally occurs in/downstream of trough of jetdivergence WHY???
Pg. 39
High Pressure - air comes together in upper atmosphere
and descends and/or cold air sinks
Produces clockwise surface winds
ADD Two things can cause air to sink
1.) convergence aloft “crashing” air aloft is forced
down
2.) cold air sinks
Pg. 39
High Pressure (continued)
Chicken or the egg?
Divergence or cold air?
Usually work together Air converges aloft and
is forced down
Barometer
Effect?
Cold Air Sinks
Air hits ground and
Rushes out to sides
High Pressure
Air sinking
Ground
Pg. 39
High Pressure (continued)
Top View
Coriolis Effects bends wind to
Right(---)
Rushing OUT air
begins to bend to
right and set up
clockwise rotation
Pg. 39
Descending air WARMS as it descends so
creates dry windy conditions = no precip.
(clear weather)
Anticyclone - winds moves clockwise around
downward moving air
ND pg. 281 fig. 10.17
Generally occurs at ridge of jet- Convergence
WHY??
Pg. 38
GENERAL RULES
Warm air rises
Cold air sink
land = dry
ocean = moist
Cold air sinks
OR convergence
High pressure
TEMP?
SUMMER -land is warmer
WHY??
RAIN and Storms?
WHY??
Warm air rises OR
divergence aloft
Low pressure
HEAVY
RAIN
Ocean
Land
Pg. 38
GENERAL RULES
Warm air rises
Cold air sink
land = dry
ocean = moist
Warm air rises OR
divergence aloft
Low pressure
TEMP?
WINTER –ocean is warmer
WHY??
RAIN and Storms?
WHY?
Cold air sinks
OR convergence
High pressure
Light
RAIN
Ocean
Land
Pg.39
5.MIDLATITUDE CYCLONES - (air mass rotating
counterclockwise around a low pressure core)
Produce most severe weather in U.S.-WHY???
Three Scales of cyclones
1) LARGEST Jet stream weather system –
cold front behind warm front = thunderstorms
over large area DO FIRST ND pg. 281 fig. 10.17
2) Smaller localized can produce thunderstorms
over smaller area
3) Tornado - smaller radius rotation of
wind (higher speed winds)
Pg. 40
6. Focus on THUNDERSTORMS
HOW DO THEY WORK?
1.warm moist air absorbs heat so less dense and
rises (hot summer afternoons)
2.as rises cools and water vapor condenses into
clouds (as water condenses releases heat so
rise even higher)
3.ice and water becomes heavy updraft can’t
support so falls (precipitation) creating downdrafts
that pull in cool air
Pg. 40
6. Focus on THUNDERSTORMS
HOW DO THEY WORK
4. updrafts and downdrafts occur in thunderstorm
violent thunderstorm move ice up and down and
create hail
5.once downdraft pulls in a lot of cool air
overwhelms updraft of warm air (no new
moisture moved upward) and thunderstorm
begins to die out
6. SUMMARY another summary
Pg. 40
MOST OCCUR WHERE?
1. Florida and inland as tropical moist air moves
off of Gulf WHY???
2. Mountains of western U.S. Moist Gulf air
contact dry desert air - moisture moves
up (mountains help force upward) and you
Moist Air get a thunderstorm WHY????
Forced aloft
Moist Air
by low pressure
Forced aloft
Over land
Ocean
cool
Florida
warm
Mountains
Pg. 40
WHAT TYPE OF WEATHER DO THEY BRING?
1. heavy rain - floods
(TEXAS 22 INCHES IN 2 3/4 HOURS!!) and mass wasting
Why Texas? - location next to Gulf, air flow onto land
over low-lying coastal plain is moved up over highland
created by a fault system (warm air moved upward=precip)
Moist Gulf Air sent aloft
Texas highlands
fault
Low-lying
coastal plains
Pg. 41
2.flash floods in desert especially (S. Utah canyons)
WHY?
low vegetation
hard surfaces
thin soils
topography-narrow passages
3. hail - nasty on crops - Texas again and Oklahoma
(need colder air in interior for ice to form) largest
17 in diameter!!
4. lightning - starts forest fires and direct strikes
cause deaths
Pg. 41
Lightning - a spectacular show in the atmosphere
Electric current seeks to balance areas with
excess charges cloud to cloud OR cloud to earth
Bottom of thundercloud builds negative charge
(due to colliding and shattering of supercooled
water drops)-not very well understood
The negative build up induces a positive build up on
the ground directly below.
ND Pg. 323 fig. 11.27
Diagram
Awesome site describing process
Pg. 41
Anatomy of Lightning:
step leaders -Negative charge heads toward
ground, when close positive charges moves
upward and they connect and see flash.
Animation
Thunder results from high temp of lightning
flash (up to 55,000 degrees F) heats air and
expands explosively
Pg. 41
5.winds –
SURFACE WINDS ALWAYS BLOW FROM
HIGH TO LOW PRESSURE
straight-line (derecho)
rotating (tornado)- a little different than H to L
straight-line wind up to 100 mph
tornado wind speed up to 310 mph
Small but deadly
WIND
High
Pressure
WIND Low WIND
Pressure
Pg. 42
WHAT MAKES A TORNADO?? (3 things)
1.) humid warm air from Gulf >75 degree F moves north
2.) meeting fast >50 mph cold dry air
3.) jet stream winds >150 mph
Courtesy NOAA
4.) single cell vs. super cell system
ND pg. 326 fig. 11.30
When these air masses meet warm air is lifted
upward, strong updraft is sheared and spun by
fast-moving cold air and twisted at upper
levels by jet = tornado
ND pg. 326 fig. 11.29
Pg. 42
HOW DO THEY (TORNADOES) DESTROY?
pressure differences - very low pressure in center,
pressure inside a building is much
higher - when pressure outside drops
Low
instantly - buildings explode.
Pressure
Utah-can occur but rare. WHY??
SLC =1 fatality
Kentucky/Indiana case
Onset is rapid: very little warning.
First siren warning at 1:49am.
High
Second at 1:59am tornado
Pressure
struck at 2:01am.
F3(force3) category.
158mph-206mph winds
ADD ON
Tornadoes are classified based on wind speed and
resultant damage
Fujita Scale
Here is a BIGGIE
Pg. 42 WHERE DO THEY OCCUR?
Good Old Okalahoma-WHY?
Can happen in Utah – uncommon-WHY
VIDEO????