Moisture, Clouds, and
Weather
Chapter 19
(part 1 of 3)
From Water to Water
Vapor
The warmer the air, the more water vapor
it can hold
• Absolute
– Mass of water in a given volume
(g/m3)
– Air at 25ºC = 23 g/m3
– Air at 12ºC = 11.5 g/m3
• Relative
actual quantity of H2O
(%) =
per unit of air
x 100%
maximum quantity
at the same temperature
Humidity
Relative Humidity (RH)
• Saturation – 100% RH
• Dew Point
– If you are at 100% RH, and you cool
it below, water vapor becomes liquid
– Water condenses on soil, grass,
airborne particles
• Supersaturation and supercooling
– Needs a nucleation site
How Does Air Reach the Dew
Point?
Radiation Cooling
• Heat lost by giving off energy (infrared / thermal)
• Can be from atmosphere, land, or water
Contact Cooling
• Warm, moist air cools against cold material
– Dew
– Frost
• If dew point is below freezing
• Formed directly from vapor
Cooling of Rising Air
• Adiabatic temperature changes
– Changes in T due to expansion or compression
– Air cools between 10°C/1,000 m when dry and
5°C/1,000 m when wet
• Adiabatic lapse rate
• Why are they different?
– Latent heat as gas
becomes water
Rising Air and Precipitation
Pacific Ocean
Sierra Nevadas/
Panamint Range
Orographic Lifting
Death Valley
Rising Air and Precipitation
Frontal Wedging
Convection-Convergence
(unequal heating)
What Controls Cloud Formation?
• Normal Lapse Rate (6°C/1,000 m)
– Air cools with elevation in troposphere
– Varies with altitude, latitude, time of day,
seasons
• Dry air mass
• “Wet” air mass
– Latent heat keeps it “warm,” rises very high
Cirrus Clouds
• “Wisp of hair”
– High (6,000-15,000 m)
– Made of ice crystals
– Thin due to dry air
Stratus
• “Layer”
– Horizontal, sheet-like
– Typical of… a cloudy day
– Occur when condensation stops rising and
spreads out
• Nimbostratus if accompanied by rain or snow
Cumulus
• “Heap” or “pile”
– Think of it like an accumulation of clouds
– Fluffy: display flat bottoms and billowy tops
– Base of cloud at level of dew point in air
Cumulonimbus
• Cumulus clouds which produce precipitation
– Top sheared by winds, spreads at tropopause
Precipitation and Cloud
Formation
The Formation of Rain
• Droplets in a cloud are small (0.01 mm)
• Coalescence of droplets
– In clouds above freezing
– 1 million to one “drop”
• Ice is less dense than water
– As air cools toward the dew point, water vapor
may spontaneously form ice… not water
– Ice then falls, remelts and makes rain
Surface Tension
• Water wants to be a
sphere!
Snow, Sleet… and Glaze?
• If cloud is composed of ice and air near
ground is cold…
Glaze
Hail
• Hail—only occurs in thunderstorms—created
by up/down drafts
• Most frequent in late spring-early summer
– Extreme T differences from surface to jet stream
– Air forced upward over mountains
Largest hailstone
ever recorded:
Diameter:
7.0 in (17.8 cm)
Circumference:
18.75 in (47.6 cm)
Weight: 756 g (1.5
lbs.)
Pressure and Wind
• Warm air rises (~1 km/day), creates low
pressure
• Cold air sinks
• Wind is caused
by pressure
differences
Pressure Gradient
• Change in pressure difference with
distance
Large pressure difference
• Wind speed determined by
pressure difference
• Earth is unequally heated,
pressure changes widely
What Causes All the Variation
• Solar heating from the Sun
– Ex: Creates deserts at 30° N and S
• Seasonal tilt of Earth
• Continental character (e.g., mountains)
• Ocean currents (temperature)
Cyclones and Anticyclones
• Remember the Coriolis Effect
Air Masses and Fronts
• Air mass: large body of air with ~uniform T
and humidity at any given latitude
– Ex: Atlantic O. above Gulf Stream circulation
• Air masses collide along a front
Warm front
• Air moves up slowly, light precipitation
Cold front
• Air moves up rapidly, lots of precipitation
Occluded Fronts
• Two cold air masses trap a warm air mass
– Storm is short-lived; fast-moving cold air cuts
off moisture supply
Stationary Front
• If neither air
mass is strong
enough to
replace the
other, the
system stalls
Formation of a Cyclone
• Eventually (1-3 days), air rushing in to P
zone equalizes pressure, storm dissipates
Thunderstorms
Thunderstorms
• A weather phenomenon characterized by the
presence of lighting (and thunder)
– also--Heavy rain, high winds, hail, and-possiblytornadoes
• Common in areas of moist air
– Equatorial regions
– SE US
• Can occur as
– Single-cell or multi-cell
– Squall line
– Supercell—most dangerous
Thunderstorm development
• Need: moisture, unstable air, lifting
• Tend to follow three stages of
development…
Lightning
Strikes
– 1st, downward passage of negative charge
– 2nd, bright return stroke (of light, not
electrons)… like this
• Heats air to 50,000ºF (28,000ºC)
– Massive expansion of air at supersonic speed
• Air moves at diff speed than sound it makes
– Speed of light ~ 3 x 108 m/s, sound = 344 m/s
• Divide # of seconds between flash and thunder by
5
• Safety: 30 s time difference = take cover
Do’s and Don’ts!
• If inside
– Don’t touch anything plugged in, or corded phones
– Do not take a bath/shower or wash dishes
• If outside
– Seek shelter in a “safe” house (one with plumbing and
wiring)
• Avoid “unsafe” structures like patios, tarps, picnic pavilions,
etc..
– Stay away from tall trees
– Crouch on the balls of your feet
– Shut off your car and touch nothing metal
Lightning Myths
• Myth
• Truth
– Jewelry and metal belt
buckles attract lightning
– If outside, lay flat on the
ground to decrease your
height
– Lightning never strikes
the same place twice
Lightning safety
– Being isolated, tall and
exposed make you more
prone to being struck
– Lying flat makes you
more prone to ground
current—lightning crouch
– Lightning often strikes
the same place
repeatedly, especially if
the object is tall
• Ex. Empire State
Building
Tornados
What is a Tornado?
• Violently rotating, funnel-shaped cloud
– Often associated with extremely violent
thunderstorms--supercells
Tornado Facts
• Tornados can occur almost anywhere in the world
– Occur most frequently in US (tornado alley)
•
•
•
•
Duration: a few minutes (can last several hours)
Diameter (Avg.): 150’(can be >1/2 mile wide)
Length of path (Avg.): 4 miles (can be >200 miles)
Funnel can travel from 0 mph up to ~70 mph,
usually travels at 30 mph
• Winds 65->210 mph
When Tornados Occur
A typical late
afternoon tornado
Tornado Development—the
supercell
• A supercell—a
thunderstorm
containing a
mesocyclone—a
large, spinning
updraft
– Caused by motion
of surface wind
and wind aloft-vorticity
The Supercell
Direction of storm motion
Tornado Formation
• Mesocyclone changes position
• Development of rear flank downdraft—
gust front
– Tornado develops at occlusion point between
RFDD and FFDD
Tornado Life Cycle
A.
B.
C.
D.
E.
Dust whirl
Organizing
Mature
Shrinking
Decaying or “rope” stage
Note: The time it takes
to go from one stage to
the next can vary
considerably…not
every tornado goes
through all the stages
A
C
Tornado Indicators
• A greenish colored sky (cause by light
scattering--hail)
• Mammatus clouds
• A sudden drop in barometric pressure
• Large hail of at least .75 in. diameter
• Strong winds > 60 mph
• Frequent and intense lightning
• A rotating wall cloud or a cloud that appears
to hang from the sky
• A loud rumbling noise- seek shelter!
Mammatus
clouds
Green sky
Early Warning Systems
• Tornado watch
– Tornado development is possible; take shelter and
remain alert
• Tornado warning
– Radar indication or direct observation of a Tornado
on the ground
Tornado Safety
• If inside
• Small, interior, windowless room on lowest
level of building
• Basement or storm shelter
• Bathtubs and pillows—know what is above you
• If outside
– Seek shelter indoors
– If no shelter available, find low spot and
cover head
– NEVER take shelter in a car