Download Physical Geography Chapter 5

Chapter 5
Atmospheric Water and Weather
Supplemental notes are drawn from Lutgens and
Tarbuck, The Atmosphere
Significance of Water
(1) Vital to all organisms of the Earth
(2) Necessary for many Earth system processes
(3) Impacts the structure of the Earth’s surface –
chemically and physically
(4) Has definite functions in human activities
(5) Can exist in solid, liquid and gaseous states
under normal Earth atmospheric conditions
(6) It is slow to heat / slow to cool
Water and Atmospheric Moisture
-
Water on Earth: Location and Properties
Humidity
Atmospheric Stability
Clouds and Fog
Air Masses
Atmospheric Lifting Mechanisms
Midlatitude Cyclonic Systems
- Violent Weather
Ocean and Freshwater Distribution
Figure 5.3
Hydrologic Cycle
- Closed system movement of moisture in the
Hydrosphere
- Absorption and release of energy (latent heat…in
calories) “powers” the system
- As a proportion, the energy is small, the actual amount
is significant
- Gain-or-loss of energy results in three major processes
and two minor processes
Hydrologic Cycle
(1) Evaporation / transpiration
(2) Condensation
(3) Precipitation
(4) Sublimation
(5) Deposition
In general terms, precipitation=evaporation worldwide
--- in reality,
“too much, too little, too bad”
issues of frequency and dependability
Continents: precipitation>evaporation
Oceans: precipitation<evaporation
Water’s Heat Energy Characteristics
Figure 5.6
- Moisture in the hydrologic cycle is most
frequently locked in H2O vapor
- It is a small but, highly variable percentage of
the atmosphere by volume
- This H2O vapor is concentrated in the lower
18,000’ of the atmosphere
There are limits to the volume of H2O that the
atmosphere can hold in suspension
… temperature is the primary factor
As a general rule, the warmer the air, the greater
the volume of water vapor that air can hold
Saturation and Dew Point
saturation or point of saturation
- Achieved by:
(1) dropping temperature
(2) increasing moisture
Temperature of saturation is called dew point
After saturation, additional cooling or addition of
moisture results in condensation
Humidity
- General term for the measure of the volume of
H2O present in the air at a given temperature
--- reported as:
absolute; relative; specific
- We are interested in relative humidity
Ratio of H2O in the atmosphere at a given
temperature, to the volume of H2O the air
can hold at that temperature
(H2Oobserved / H2Opossible) x 100
Relative Humidity
Figure 5.7
Humidity Patterns
Figure 5.10
Atmospheric Stability
- Adiabatic Processes
--- Dry adiabatic rate (DAR) 10oC/1000m
--- Moist adiabatic rate (MAR) 6oC/1000m
--- Stable and unstable atmospheric
conditions
Condensation
Process by-which gaseous H2O is changed to a
liquid (600 cal release)
clouds; fog; dew; frost
[though frost is technically different]
Requires: (1) air cooled beyond saturation
(2) a surface on-which to
condensation
(condensation nuclei)
Fog
Simplest: a cloud with base at ground level
(1) cooling
radiation
advection
upslope fog
ice fog
(2) evaporation fog
steam fog
frontal fog
Clouds
Buoyant masses of visible H2O or ice crystals
Visible sign of atmospheric stability or instability
Product of any process encouraging air
movement
vertical
convection
convergence
subsidence
horizontal
advection
frontal lifting
Cloud Forms
Classed by altitude and appearance
- Altitude “families”
High – cirro
Middle – alto
Low – strato
Clouds of vertical formation
Cloud Forms, cont
- Appearance
stratus - sheet, layer (stability)
cumulus – globular, pillowy (instability)
cirrus – high, white, thin (stable, ice)
We also make use of the prefix / suffix
nimbo or nimbus to designate precipitationbearing clouds
Cloud Types and Identification
Figure 5.18
Cumulonimbus Development
Figure 5.19
Airmasses
- Large masses of air characterized by:
- (1) common properties of humidity
and temperature at a given
altitude
- (2) characteristics of their source
region
Source Region
Region whose terrestrial and atmospheric
conditions create airmasses
(1) extensive and uniform in area
(2) area of atmospheric stagnation
Airmasses are classified by:
(1) Latitude of source region
(gives temperature)
A; P; T; E; AA
(2) Surface area below the airmass (gives
humidity)
continental – “c” low moisture
marine – “m”
high moisture
* “k” and “w” are added for stability indices
Airmass Classification, cont
cA – continental Arctic
cP – continental Polar (*)
cT – continental Tropical (*)
mT – marine Tropical (*)
mP – marine Polar (*)
mE – marine Equatorial
cAA – continental Antarctic
* consistently influence North America
Air Masses
Figure 5.24
Front
Surface or zone of contact / conflict /
discontinuity between airmasses
Coined by Norwegian meteorologists in WWI –
Polar Front Theory
… Norwegian Cyclone Model
links cloud patterns, precipitation, wind,
barometer, flow aloft, etc
Frontal lifting occurs when one airmass is forced
to rise/ride above the other
Passing through a front frequently brings
weather change:
temperature; dew point spread; wind speed /
direction; atmospheric pressure
Atmospheric Lifting Mechanisms
Convectional Lifting
Orographic Lifting
Frontal Lifting
---Cold fronts
---Warm fronts
--- Occluded fronts
--- Stationary fronts
Atmospheric Lifting Mechanisms
Figure 5.27
Cold Front
Figure 5.31a
Warm Front
Figure 5.32
Midlatitude Cyclone
Figure 5.33
Average and Actual Storm Tracks
Figure 5.34
Thunderstorms
Best known disturbance weather pattern…
not strictly cyclonic flow
Worldwide approx 16 million annually
Product of warm, moist air lifted to
condensation… most are tropical almost
unknown at the Poles
- may be: convectional; orographic;
frontal
Thunderstorms
Figure 5.36
Thunderstorms, cont
Characterized by thunder/lightning;
torrential rainfall/hail; strong
up-and-downdraft winds; release of
latent heat
Stages:
(1) cumulus
(2) mature
(3) dissipation
Tornadoes
From the Spanish tornar – “to turn”
Intense center(s) of low pressure
… pressure gradient winds may exceed 300
mph
… a “whirl-pool” like column of air vortex
downward from a cumulonimbus cloud
A funnel of condensed H2O, funnel colored by
what the tornado contacts
Tornado Development and Occurrence
- Often produced in association with midlatitude cyclones
- < 1% of thunderstorms produce
tornadoes
- Typically North American (3/4) and
spawned in cP-mT air collisions
--- 700+ annually; North America
dominates
Twister!
Figure 5.38
Tornadoes
Figure 5.39
Life Stages
Though a tornado may have a “life” of only minutes, each
will go through some combination of the following
stages:
(1) Funnel cloud
(2) Tornado
(3) Mature Tornado
(4) Shrinking Tornado
(5) Decaying Tornado
Tornado Destruction
Millions of stories about what tornadoes can do
Destruction from:
(1) high winds - strong updrafts
(2) high speed projectiles
(3) subsidiary vortices /
“down blasts”
Tornado Destruction, cont
No one has accurately measured the
windspeed of a tornado
We rate tornadoes by extent of damage; the
Fujita Scale (F-Scale)
Tornado Watch and Tornado Warning
Hurricanes
Tropical cyclone with windspeed in excess
of 200 mph
Lowest pressure recorded in the Western
Hemisphere
Name from Huracan – Carib Indian god of
evil
Hurricane Development and Occurrence
Giant heat engines taking energy from oceanic
latent heat
Form over tropical waters 5o to 20o
… but not all the tropical waters
inside of 5o [no Coriolis]
So. Atlantic [cold water currents]
The notable exception to a lack of So. Atlantic hurricanes
is Hurricane Catarina (2004)
Hurricane Development and Occurrence,
cont
Three stages of development
(each can be an end in itself)
(1) tropical depression
(2) tropical storm
(3) hurricane
--- wind swirl/rain bands
--- eyewall [winds to 200 mph]
--- eye [winds approx 25 mph]
Profile of a Hurricane
Figure 5.42
Destruction
Damage from tropical hurricanes range from
complete devastation, caused by the passage of
the eyewall of a very intense hurricane along
the coast, to a minor nuisance, produced by a
weak hurricane whose effects resemble those of
a strong thunderstorm
Annually nearly every portion of the US is
effected directly or indirectly by hurricane
activity
Destruction
Forms:
(1) wind*
(2) storm surge
(3) inland flooding*
[eye wall may produce 10+” rainfall]
(4) ancillary vortices (tornadoes)
* function of ground speed
Saffir-Simpson Hurricane Intensity Scale
Hurricane Watch/Hurricane Warning
Some Interesting Ones
No Name; Galveston Bay,TX
No Name; Okeechobee, FL
Camille
Agnes
Hugo
Gilbert
Dennis, Floyd, Irene
Andrew
Katrina
Catarina
1900
1928
1969
1972
1989
1994
1999
1992
2005
2004