moisture, clouds, and weather chapter 19 (part 1 of 3)
TRANSCRIPT
Moisture, Clouds, and Moisture, Clouds, and WeatherWeather
Chapter 19Chapter 19
(part 1 of 3)(part 1 of 3)
From Water to Water Vapor
The warmer the air, the more water vapor it can hold
Humidity• 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
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
Death Valley
Sierra Nevadas/ Panamint Range
Pacific Ocean
Orographic Lifting
Rising Air and PrecipitationFrontal 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”Wisp of hair”– High (6,000-15,000 m)High (6,000-15,000 m)– Made of ice crystalsMade of ice crystals– Thin due to dry airThin due to dry air
Stratus
• ““Layer”Layer”– Horizontal, sheet-likeHorizontal, sheet-like– Typical of… a cloudy dayTypical of… a cloudy day– Occur when condensation stops rising and Occur when condensation stops rising and
spreads outspreads out• NimbostratusNimbostratus if accompanied by rain or snowif accompanied by rain or snow
CumulusCumulus
• ““Heap” or “pile”Heap” or “pile”– Think of it like an acThink of it like an accumulcumulation of cloudsation of clouds– Fluffy: display flat bottoms and billowy topsFluffy: display flat bottoms and billowy tops– Base of cloud at level of dew point in airBase of cloud at level of dew point in air
Cumulonimbus
• Cumulus clouds which produce precipitationCumulus clouds which produce precipitation– Top sheared by winds, spreads at tropopauseTop 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
• Wind speed determined by pressure difference
• Earth is unequally heated, pressure changes widely
Large pressure difference
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
ThunderstormsThunderstorms
Thunderstorms• A weather phenomenon characterized by the
presence of lighting (and thunder)– also--Heavy rain, high winds, hail, and-possibly-
tornadoes
• 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– 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
• Truth– 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 BuildingLightning safety
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 SupercellDirection 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
C
A. Dust whirlB. OrganizingC. MatureD. ShrinkingE. 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
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