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CLOUDS Dr. Sam Miller Weather & Climate – MTDI 1200OL Plymouth State University

CLOUDS Dr. Sam Miller Weather & Climate – MTDI 1200OL Plymouth State University. 1. Ahrens, Ch. 5. Formation Processes. What do you need to form a cloud?. Condensation of water vapor in the air into liquid cloud droplets Occurs when T = T d (RH = 100%)

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CLOUDS Dr. Sam Miller Weather & Climate – MTDI 1200OL Plymouth State University

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  1. CLOUDS Dr. Sam Miller Weather & Climate – MTDI 1200OL Plymouth State University 1

  2. Ahrens, Ch. 5

  3. Formation Processes

  4. What do you need to form a cloud? • Condensation of water vapor in the air into liquid cloud droplets • Occurs when T = Td (RH = 100%) • A way to reduce the temperature to the dew point • The best way to cool air in the atmosphere is LIFT IT • Cooling is adiabatic

  5. Rising air parcels expand and cool • Sinking parcels compress and warm

  6. Cloud bases Form at the LCL • Lifting Condensation Level • Altitude at which temperature within a rising parcel drops down to the dew point • Altitude at which lifted parcel reaches saturation • Visible cloud droplets appear LIFTING CONDENSATION LEVEL

  7. Type of cloud that forms • Type of lifting mechanism • Convection • Topography • Low-level convergence and fronts • Scale and duration of lifting mechanism • Widespread or localized • Short or long term • Amount of water vapor available • Stability

  8. Type of cloud that forms • Type of lifting mechanism • Convection • Topography • Low-level convergence and fronts • Scale and duration of lifting mechanism • Widespread or localized • Short or long term • Amount of water vapor available • Stability

  9. Convection • Surface heating causes hot “bubbles” of air (thermals) to break away from the surface and rise • Warm air rises, since it is less dense than cold air • Can produce “popcorn” thunderstorms during warm, humid afternoons

  10. Topography • Air is forced over mountain by strong, perpendicular wind • Rising air expands and cools on windward side of mountain • Often produces showery precipitation • Air descending leewardside of mountain is compressed and heated • Produces a rain shadow on the leeward side of the mountain

  11. Low-Level Convergence • Air near the surface moves toward a low pressure system • Away from a high pressure • “Fronts” are areas of low pressure • When air converges at surface, it is forced to rise • Often assisted by divergence aloft • Upper-level divergence sometimes occurs without low-level convergence

  12. Fronts • Warm air meets cold air and is forced to rise • Type of clouds depends on the type of front • Cold fronts can produce lines of thunderstorms called squall lines • Warm fronts usually produce widespread, flat clouds via overrunning

  13. Type of cloud that forms • Type of lifting mechanism • Convection • Topography • Low-level convergence and fronts • Scale and duration of lifting mechanism • Widespread or localized • Short or long term • Amount of water vapor available • Stability

  14. Type of cloud that forms Long-term, widespread lifting mechanisms will create the largest clouds • Type of lifting mechanism • Convection • Topography • Low-level convergence and fronts • Scale and duration of lifting mechanism • Widespread or localized • Short or long term • Amount of water vapor available • Stability

  15. Type of cloud that forms • Type of lifting mechanism • Convection • Topography • Low-level convergence and fronts • Scale and duration of lifting mechanism • Widespread or localized • Short or long term • Amount of water vapor available • Stability

  16. Type of cloud that forms Very moist airmasses (high dew point) can create larger clouds with lower bases • Type of lifting mechanism • Convection • Topography • Low-level convergence and fronts • Scale and duration of lifting mechanism • Widespread or localized • Short or long term • Amount of water vapor available • Stability

  17. Type of cloud that forms • Type of lifting mechanism • Convection • Topography • Low-level convergence and fronts • Scale and duration of lifting mechanism • Widespread or localized • Short or long term • Amount of water vapor available • Stability

  18. Type of cloud that forms Stable airmasses will produce little, if any, cloud cover – What does form will be “stratiform” Neutral airmasses are more likely to produce stratiform clouds and continuous precipitation Unstable airmasses will produce puffy, “cumuliform” clouds and showery precipitation. • Type of lifting mechanism • Convection • Topography • Low-level convergence and fronts • Scale and duration of lifting mechanism • Widespread or localized • Short or long term • Amount of water vapor available • Stability

  19. Cloud Classification

  20. Sources • International Cloud Atlas, published in 1956 by the World Meteorological Organization. • My webpage: vortex.plymouth.edu/~stmiller/. See “Tutorials,” then “Cloud Classification.” This is based on the Federal Meteorological Handbook and the International Cloud Atlas. • Textbook: Essentials of Meteorology (Ahrens, 2005).

  21. Sources • International Cloud Atlas, published in 1956 by the World Meteorological Organization. • My webpage: vortex.plymouth.edu/~stmiller/. See “Tutorials,” then “Cloud Classification.” This is based on the Federal Meteorological Handbook and the International Cloud Atlas. • Textbook: Essentials of Meteorology (Ahrens, 2005). Based on work of Luke Howard, 19th Century British pharmacist, chemist and amateur meteorologist Cloud nomenclature originally proposed in 1802

  22. Cloud classification is based on: • Height of the base (determines “etage”) • Physical appearance (determines “form” and “genera”)

  23. Cloud Etages (Mid-Latitudes) • Low etage • Bases below 2,000 m (6,500 ft) • Usually composed of water droplets • May build upward into the mid- and high etages • Middle etage • Bases between 2,000 and 7,000 m (6,500 and 23,000 ft) • Some overlap with high clouds • Composed of water droplets • (mixed with ice crystals when temp is low enough) • High etage • Bases generally above 5,000 m (16,000 ft) • Usually composed of ice crystals

  24. Cloud Forms and Genera FORMS Cumuliform: “Heaped up” – significant vertical development – Unstable Stratiform: Flat and veil-like; often covering widespread areas – Neutral/Stable Cirriform: Feather-like, generally composed of ice crystals LOW ETAGEMIDDLE ETAGEHIGH ETAGE (St) Stratus (AS) Altostratus (Ci) Cirrus (NS) Nimbostratus (AC) Altocumulus (CS) Cirrostratus (SC) Stratocumulus (CC) Cirrocumulus Low etage clouds with significant vertical development (Cu) Cumulus (Cb) Cumulonimbus Note: This is not exhaustive.

  25. Cloud Forms and Genera FORMS Cumuliform: “Heaped up” – significant vertical development – Unstable Stratiform: Flat and veil-like; often covering widespread areas – Neutral/Stable Cirriform: Feather-like, generally composed of ice crystals LOW ETAGEMIDDLE ETAGEHIGH ETAGE (St) Stratus (AS) Altostratus (Ci) Cirrus (NS) Nimbostratus (AC) Altocumulus (CS) Cirrostratus (SC) Stratocumulus (CC) Cirrocumulus Low etage clouds with significant vertical development (Cu) Cumulus (Cb) Cumulonimbus Note: This is not exhaustive.

  26. Cloud Forms and Genera FORMS Cumuliform: “Heaped up” – significant vertical development – Unstable Stratiform: Flat and veil-like; often covering widespread areas – Neutral/Stable Cirriform: Feather-like, generally composed of ice crystals LOW ETAGEMIDDLE ETAGEHIGH ETAGE (St) Stratus (AS) Altostratus (Ci) Cirrus (NS) Nimbostratus (AC) Altocumulus (CS) Cirrostratus (SC) Stratocumulus (CC) Cirrocumulus Low etage clouds with significant vertical development (Cu) Cumulus (Cb) Cumulonimbus Note: This is not exhaustive.

  27. Cloud Forms and Genera FORMS Cumuliform: “Heaped up” – significant vertical development – Unstable Stratiform: Flat and veil-like; often covering widespread areas – Neutral/Stable Cirriform: Feather-like, generally composed of ice crystals LOW ETAGEMIDDLE ETAGEHIGH ETAGE (St) Stratus (AS) Altostratus (Ci) Cirrus (NS) Nimbostratus (AC) Altocumulus (CS) Cirrostratus (SC) Stratocumulus (CC) Cirrocumulus Low etage clouds with significant vertical development (Cu) Cumulus (Cb) Cumulonimbus Note: This is not exhaustive.

  28. 1. Low Clouds • base below 2,000 m (6,500 ft) • composed of water droplets • (St) Stratus • (NS) Nimbostratus • Considered a mid-cloud in the International Cloud Atlas and Federal Meteorological Handbook • (SC) Stratocumulus

  29. Stratus

  30. Stratus Lifting mechanism(s): None, or weak upslope Scale and duration: Local – regional; hours Amount of water vapor: Varied Stability: Stable

  31. Stratus Fractus

  32. Stratus Fractus Lifting mechanism(s): None Scale and duration: Local; minutes Amount of water vapor: Varied Stability: Stable

  33. Stratus Abbreviated St May cover the entire sky Stratus – Latin for layer or veil Stratus Fractus – Ragged bits of stratus Resembles fog above the ground No precipitation (or only very light drizzle) Usually cannot see Sun through it

  34. Stratocumulus

  35. Stratocumulus

  36. Stratocumulus Lifting mechanism(s): All Scale and duration: Local – regional; hours Amount of water vapor: small - moderate Stability: Neutral - stable

  37. Stratocumulus Abbreviated SC Marks a half-way point between Cu and St Lumpy cloud layer Blue sky visible in between Individual “lumps” Size of fist when arm extended

  38. Nimbostratus

  39. Nimbostratus and Stratus Fractus

  40. Nimbostratus Lifting mechanism(s): Warm frontal Scale and duration: Regional; hours - days Amount of water vapor: High Stability: Neutral - stable

  41. Nimbostratus Abbreviated NS Steady precipitation Nimbo – rain Stratus – layer or veil Often accompanied by Stratus Fractus Associated with widespread weather systems, esp. in winter months

  42. 2. Low clouds with significant vertical development • (Cu) Cumulus • (CB) Cumulonimbus

  43. Cumulus

  44. Cumulus Lifting mechanism(s): Surface heating (convection); weak upslope; or cold frontal Scale and duration: Local (elements); minutes - hours Amount of water vapor: Small Stability: Slightly unstable

  45. Cu congestus(Towering Cumulus)

  46. Cu congestus(Towering Cumulus) Lifting mechanism(s): Surface heating (convection), upslope, or cold frontal Scale and duration: Local (elements); minutes - hours Amount of water vapor: Small - moderate Stability: Unstable

  47. Cumulus Abbreviated Cu White or gray puffy clouds Generally fair weather Composition: water droplets Cumulus Congestus Also known as Towering cumulus (TCu) May produce light showers

  48. Cumulonimbus

  49. Cumulonimbus

  50. Cumulonimbus

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