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) DepositionIn 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 advectionupslope fog ice fog

(2) evaporation fog steam fogfrontal fog

Clouds

Buoyant masses of visible H2O or ice crystalsVisible sign of atmospheric stability or instabilityProduct 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 precipitation-bearing 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

ThunderstormsBest 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 mid-latitude 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 StagesThough 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

DestructionForms:

(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 ScaleHurricane Watch/Hurricane Warning

Some Interesting Ones

No Name; Galveston Bay,TX 1900

No Name; Okeechobee, FL 1928

Camille 1969

Agnes 1972

Hugo 1989

Gilbert 1994

Dennis, Floyd, Irene 1999

Andrew 1992

Katrina 2005

Catarina 2004