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CHAPTER 5 - PowerPoint PPT Presentation

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CHAPTER 5

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  1. CHAPTER 5 FORMS OF CONDENSATION AND PRECIPITATION

  2. Clouds - Buoyant masses of visible droplets of H2O vapor or ice crystals - May be as low as the ground (fog), or as high as the Troposphere (ice clouds) - Play role in maintaining heat balance by reflecting insolation and terrestrial radiation

  3. Clouds, cont • effect of cooling and condensation of H2O in the atmosphere - Forms of precipitation originate here - Sign of atmospheric stability / instability - Clouds are classified according to height and appearance - Average man’s key to weather

  4. Cloud Height and Appearance Height cirrus - high alto – middle stratus – low clouds of vertical formation Appearance cirrus – white; thin cumulus – globular; pillowy stratus – sheet; layered

  5. Cloud Height and Appearance, cont - Height and appearance are functions of temperature and H2O vapor available at each altitude for cloud formation - Clouds identified with precipitation we give the prefix “nimbo” or suffix “nimbus”

  6. Cloud Height and Appearance, cont • Specialized location conditions may give rise to sub-groups of clouds fractus – broken uncinus – hook shaped mammatus – rounded/udder-shaped lenticular – lens-like banner – banner-like nacreous – pearly; Stratospheric nactilucent – wispy; Mesospheric

  7. Cloud Formation • Cooling to, and below, the dew point • Cooling is the effect of vertical lifting / lifting requires a lifting force • Cooling of the parcel of air is controlled by the adiabatic lapse rate • Rising air becomes stable when it loses upward buoyancy and has the same density as surrounding air • Cloud formation assumes condensation nuclei (hydroscopic particles)… without which air below dew point becomes supersaturated

  8. Cloud Classes: High • visible because temperature produces ice crystals --- cirrus(Ci) – “mares tail” --- cirrocumulus(Cc) – “mackerel sky” --- cirrostratus(Cs) – partially or totally cover the sky

  9. Cloud Classes: Middle/Low Middle --- altostratus(As) --- altocumulus(Ac) Low *may be advective rather than convective* --- Stratus (St) --- Stratocumulus (Sc)

  10. Fog - Generally accepted to be a cloud with its base at, or near, ground level - Clouds are the result of lifting to saturation and condensation… fog results from (1) radiation cooling (2) cooling of air mass (3) adding additional moisture

  11. Fog, cont Cooling fogs - radiation fog – cool, clear, calm nights - advection fog – warm, moist air blown across a cold surface - upslope fog – most like a cloud; adiabatic cooling of valley breeze

  12. Fog, cont Evaporation fogs - steam fog – cool air passing over warm water - frontal fog – result of frontal wedging

  13. How Precipitation Forms All clouds are composed of H2O vapor Why do some clouds make precipitation and others just float there?

  14. How Precipitation Forms, cont • Cloud droplets are extremely small – about 0.02 mm dia … abundance of condensation nuclei results in many, small droplets --- gravity has very little effect on them --- they fall very slowly and evaporate very quickly in the atmosphere (average size droplet would take hours to fall from 1000 m; but it would evaporate too quickly)

  15. How Precipitation Forms, cont (2) Billions of cloud droplets are in competition for available moisture and growth by condensation is also very slow Rain drop large enough to fall to the ground is about 1 million times larger than a cloud droplet

  16. How Precipitation Forms, cont In order to grow large enough to fall without evaporation, cloud droplets must coalesce into large droplets… 2 ways (1) Bergeron Process (2) Collision-Coalescence

  17. Bergeron Process Dependent on the simultaneous existence of H2O vapor, liquid water droplets and ice crystals Dependent on two properties of water (1) Droplets do not freeze at 0o C, they freeze at –40o C (liquid water below 0o C is supercooled) (2) Saturation vapor pressure over ice crystals is much lower than over supercooled liquid droplets

  18. Aside Saturation vapor pressure is the increase in atmospheric pressure resulting from the motion of H2O molecules in the air through evaporation

  19. Aside, cont As more and more molecules escape, increasing atmospheric pressure (and slowing molecular motion through collision), forces some to return to the liquid state Where returning molecules equal evaporation saturation vapor pressure exists

  20. Bergeron Process, cont … air saturated with respect to water will be supersaturated for ice crystals … the ice crystals will then capture more H2O vapor then they will lose through sublimation (remember: more energy is necessary to sublimate than evaporate cloud droplets)

  21. Bergeron Process, cont … continued evaporation provides ice crystals with moisture “to rob” … larger ice crystals are produced that compete with supercooled droplets for available moisture and altering the relative humidity… increasing evaporation (constantly altering both temp. and moisture)

  22. Bergeron Process, cont Growing rapidly, ice crystals begin to fall: (1) some are “shattered” to become freezing nuclei and rob more liquid water (2) some crystals collect “shattered” material and become even larger *precipitation form these take depends on atmospheric conditions*

  23. Collision-Coalescence - Clouds below freezing altitudes (air temp > 0o C) large cloud droplets are necessary to produce precipitation ---these droplets fall quickly, colliding with smaller droplets and coalesce together --- they become larger and fall faster

  24. Collision-Coalescence, cont -Two opposing forces act of droplets: gravity and friction • Gravity pulls the drop to the Earth surface, and increases as drop size increases • Friction (drag) increases as the drop accelerates – more air molecule collisions

  25. Collision-Coalescence, cont • Eventually gravity and friction balance and the drop has reached terminal velocity --- as droplet size increases, gravitational force is affected more than drag … smaller drops have lower terminal velocity - Terminal velocity is important because speed-of-fall determines the probability that a drop will reach the surface before evaporation

  26. Collision-Coalescence, cont • Process is more efficient in clouds of vertical formation (abundant moisture) • Creation of “super drops” is limited to about 5 mm by drag from the air exceeding drop surface tension

  27. Collision-Coalescence, cont Presence of “super drops” does not ensure collision-coalescence (1) was from falling drop may push smaller drops away (2) electrical charge may be a factor

  28. Precipitation Form Rain Sleet Glaze Dry Snow Hail Wet Snow Rain