Lecture 7Outline of Last Lecture I. Water Vapor in the AtmosphereII. HumidityIII. EvaporationIV. CondensationV. SaturationVI. ConclusionVII. Increasing TemperatureVIII. Decreasing TemperatureIX. Atmospheric HumidityX. Vapor PressureXI. Saturation Vapor PressureXII. Relative HumidityXIII. Changing Relative HumidityXIV. Dew Point TemperatureOutline of Current Lecture XV. Cloud FormationXVI. Mountains and PrecipitationXVII. Cloud DropletsXVIII. Collision and Coalescence XIX. Cloud DepthXX. Supercooled waterATMS 100 1st EditionXXI. Vapor PressureXXII. The Bergeron ProcessXXIII. Cold-Cloud ProcessXXIV. Rain and DrizzleXXV. VirgaXXVI. SnowXXVII. SleetXXVIII. Freezing RainCurrent LectureXXIX. Cloud Formationa. Most clouds form in rising airb. why is rising air important?i. as air rises, it expands and coolsii. amount of water vapor in air doesn’t change as air risesc. as temperature decreases, relative humidity increasesi. air eventually rises high enough (cools enough) to become satu-rated (RH=100%)ii. when air saturates, a cloud formsd. what causes air to rise?i. surface heating/convection ii. uplift by topography (mountains)These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.iii. surface convergence (after exam)iv. fronts (after exam)XXX. Mountains and Precipitationa. orographic ascent-forced lifting of air by topography (terrain)i. air cools as it risesii. often see enhanced clouds and precipitation on windward(side fac-ing toward wind) side of mountainsb. air warms as it descendsi. RH decreasesii. often see rain shadow on lee (sheltered) side of mountainsXXXI. What Are Clouds Made Of?a. clouds ar made of billions of microscopic water droplets or ice crystalsb. water droplets require small particles on which to condensei. small particles called condensation nucleiii. sizes .1-1.0 um in diameteriii. um=1 millionth of a meteriv. about 100-1000 nuclei per cm^3 of airc. sources of condensation nuclei:i. dust, ash, smoke, pollution, salt, plankton, skin, etc.XXXII. Cloud Dropletsa. most cloud droplets are small (20 um)i. this is 100 times smaller than an average raindropb. How do cloud droplets grow into rain?i. most clouds don’t produce precipitationc. Two ways:i. Collision and coalescenceii. Bergeron Process (Cold Clouds)XXXIII. Collision and Coalescencea. Collision: two droplets collideb. Coalescence: two droplets stick togetheri. collision doesn’t necessarily result in coalescencec. larger droplets fall faster than and collide with smaller dropletsd. also called “warm cloud” process because there is no ice involvedXXXIV. Cloud Deptha. a droplet takes longer to fall through a thick cloud than a thin onei. stronger currents of rising air (updrafts)ii. more collisionsiii. droplets of different sizesb. shallow cloud, weak updraft, all small droplets, no collision and coales-cenceXXXV. Supercooled Watera. water can remain liquid below freezingi. called supercooled waterb. freezes into ice when it contacts something coldXXXVI. Vapor Pressurea. More vapor molecules above water droplet than above ice crystal at same temperatureb. Why? Easier for molecules to becomes gas from liquid than from iceXXXVII. The Bergeron Processa. vapor molecules diffuse (travel) from near water droplets to near ice crys-talsi. they go from where there is more water vapor to where there is lessb. this reduces water vapor around droplet and increases water vapor around ice crystalc. air around water droplet becomes unsaturatedi. droplet beings to evaporated. air around ice crystal becomes supersaturatedi. ice crystal grows via deposition (gas to ice)e. vapor continues to diffuse, process continuesf. result: ice crystals grow at expense of water dropletsXXXVIII. Cold-Cloud Processa. ice crystals grow, become heavy, begin to fallb. snowflakes melt if they fall into warmer airi. if it’s cold, they don’t melt at allc. most precipitation, even during the summer, begins as snowflakes high in the atmospherei. precipitation types can change between the cloud base and the groundii. even a hurricane is a hellacious blizzard at 20,000 feetd. Bergeron Process also called Cold Cloud Process because ice is involvedXXXIX. Rain and Drizzlea. raini. liquid precipitation with diameter great than or equal to .5 mm1. drops do not look like teardropsb. drizzlei. liquid precipitation with diameter of drops less than .5 mmXL. Virgaa. virga is precipitation that evaporates before reaching the groundb. typically there is dry air below cloud baseXLI. Snowa. precipitation in the form of ice crystalsb. snow contains a lot of airi. can easily be compactedc. typically if you melt 10-12 inches of snow, you will only get one ince of liq-uid wateri. varies with temperature1. warmer=more water2. colder=less waterd. temperatures and vertical motions within clouds also affect snowflake growthXLII. Sleeta. falling snowflakes melt when they encounter warm airi. become raindropsb. if raindrops then fall into a deep layer of cold air, they refreeze into ice pel-letsc. sleet requires thin above freezing layer of air aloft and deep below freez-ing layer near surfaced. tiny ice pellet (frozen rain drop)e. produces pinging soundf. accumulates at 2:1 XLIII. Freezing Raina. falling supercooled raindropsb. snow falls into warm layer of air, meltsc. layer of cold air near surface is not thick enough to refreeze dropleti. droplets become supercooledd. droplets instantly freeze when they hit something below freezingi. creates icy glaze over everythingii. very hazadorousiii. Freezing rain requires deep above freezing layer of air aloft and thin below freezing layer near
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