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