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

Humidity Absolute Relative 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 Sierra Nevadas/ Panamint Range Death Valley Pacific Ocean Orographic Lifting

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 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

Thunderstorms

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 Heats air to 50,000ºF (28,000ºC) 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 If outside 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 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 Building Lightning safety

Tornados Distributed by the Disaster Team Supercourse (www.pitt.edu/~super1) Developers Developed by Randall Schindler while at Steven F. Austin University Distributed by the Supercourse Tornado Team, Harold Brooks, NOAA, Daniel Mccarthy, NOAA, Schaefer, NOAA Eric Noji, CDC, Scott Lillianbridge, Uni. Texas Francois Sauer, Supercourse, Kansas City Faina Linkov, Mita Lovalekar, Ronald LaPorte, Supercourse Pittsburgh Doug Barrett, Janet Winterton, Fraser, Kevin Maney, USA Today

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 Anytime of the year- usually in the spring, summer, and fall Most tornados occur during late spring in the month of May Between the late afternoon and early evening is when most tornados are spawned The most dangerous time for formation during evening hours

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 Dust whirl Organizing Mature Shrinking Decaying or “rope” stage A Note: The time it takes to go from one stage to the next can vary considerably…not every tornado goes through all the stages 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 warning 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 If outside 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