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Moisture, Clouds, and Weather Chapter 19 (part 1 of 3)

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Presentation on theme: "Moisture, Clouds, and Weather Chapter 19 (part 1 of 3)"— Presentation transcript:

1 Moisture, Clouds, and Weather Chapter 19 (part 1 of 3)

2 From Water to Water Vapor The warmer the air, the more water vapor it can hold

3 Humidity Absolute –Mass of water in a given volume (g/m 3 ) –Air at 25ºC = 23 g/m 3 –Air at 12ºC = 11.5 g/m 3 Relative actual quantity of H 2 O (%) = per unit of air x 100% maximum quantity at the same temperature

4 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

5 How Does Air Reach the Dew Point?

6 Radiation Cooling Heat lost by giving off energy (infrared / thermal) Can be from atmosphere, land, or water

7 Contact Cooling Warm, moist air cools against cold material –Dew –Frost If dew point is below freezing Formed directly from vapor

8 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

9 Rising Air and Precipitation Death Valley Sierra Nevadas/ Panamint Range Pacific Ocean Orographic Lifting

10 Rising Air and Precipitation Frontal Wedging Convection-Convergence (unequal heating)

11 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


13 Cirrus Clouds Wisp of hairWisp of hair –High (6,000-15,000 m) –Made of ice crystals –Thin due to dry air

14 Stratus LayerLayer –Horizontal, sheet-like –Typical of… a cloudy day –Occur when condensation stops rising and spreads out Nimbostratus if accompanied by rain or snowNimbostratus if accompanied by rain or snow

15 Cumulus Heap or pileHeap 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

16 Cumulonimbus Cumulus clouds which produce precipitationCumulus clouds which produce precipitation –Top sheared by winds, spreads at tropopause


18 Precipitation and Cloud Formation

19 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

20 Surface Tension Water wants to be a sphere!

21 Snow, Sleet… and Glaze? If cloud is composed of ice and air near ground is cold…


23 Glaze

24 Hail Hailonly occurs in thunderstormscreated by up/down draftsHail 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: in (47.6 cm) Weight: 756 g (1.5 lbs.)

25 Pressure and Wind Warm air rises (~1 km/day), creates low pressure Cold air sinks Wind is caused by pressure differences

26 Pressure Gradient Change in pressure difference with distance Wind speed determined by pressure difference Earth is unequally heated, pressure changes widely Large pressure difference


28 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)

29 Cyclones and Anticyclones Remember the Coriolis Effect

30 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

31 Warm front Air moves up slowly, light precipitation

32 Cold front Air moves up rapidly, lots of precipitation

33 Occluded Fronts Two cold air masses trap a warm air mass –Storm is short-lived; fast-moving cold air cuts off moisture supply

34 Stationary Front If neither air mass is strong enough to replace the other, the system stalls

35 Formation of a Cyclone Eventually (1-3 days), air rushing in to P zone equalizes pressure, storm dissipates

36 Thunderstorms

37 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 –Supercellmost dangerous

38 Thunderstorm development Need: moisture, unstable air, lifting Tend to follow three stages of development…

39 Lightning


41 Strikes –1 st, downward passage of negative charge –2 nd, bright return stroke (of light, not electrons)… like thislike 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 10 8 m/s, sound = 344 m/s Divide # of seconds between flash and thunder by 5 Safety: 30 s time difference = take cover

42 Dos and Donts! If inside –Dont 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

43 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 currentlightning crouch –Lightning often strikes the same place repeatedly, especially if the object is tall Ex. Empire State Building Lightning safety


45 Tornados

46 What is a Tornado? Violently rotating, funnel-shaped cloud –Often associated with extremely violent thunderstorms--supercells

47 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

48 When Tornados Occur A typical late afternoon tornado

49 Tornado Developmentthe supercell A supercella thunderstorm containing a mesocyclonea large, spinning updraft –Caused by motion of surface wind and wind aloft-- vorticity vorticity

50 The Supercell Direction of storm motion

51 Tornado Formation Mesocyclone changes position Development of rear flank downdraft gust front –Tornado develops at occlusion point between RFDD and FFDD


53 Tornado Life Cycle A C A.Dust whirl B.Organizing C.Mature D.Shrinking E.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

54 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!

55 Mammatus clouds Green sky

56 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

57 Tornado Safety If inside Small, interior, windowless room on lowest level of building Basement or storm shelter Bathtubs and pillowsknow 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

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