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Clouds, Stability, Adiabatic Processes, Precipitation

Clouds, Stability, Adiabatic Processes, Precipitation. What is a cloud?. Dense mass suspended water droplets, supercooled water droplets and/or ice particles on condensation / ice nuclei of solid matter (sea salt, soil particles, organic matter, pollutants, aerosols). Clouds form.

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Clouds, Stability, Adiabatic Processes, Precipitation

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  1. Clouds, Stability, Adiabatic Processes, Precipitation

  2. What is a cloud? Dense mass • suspended water droplets, supercooled water droplets and/or ice particles • on condensation / ice nuclei of solid matter (sea salt, soil particles, organic matter, pollutants, aerosols)

  3. Clouds form • When a parcel of air rises and cools to the dew point temperature. • Then the water vapor in the air parcel condenses into water droplets or deposits into ice crystals

  4. Why would a parcel of air rise?

  5. If surrounding air is more dense / colder than the parcel, parcel will rise. warm Colder / more dense surrounding air THAT IS CALLED UNSTABLE

  6. A parcel will rise until it gets to an altitude at which surrounding air is at same density / temperature

  7. If surrounding air is less dense / warmer than the parcel, parcel will sink; or will not rise. Warmer / less dense air STABLE; NO CLOUDS cold

  8. Therefore, stability is determined by temperature difference between parcel and surrounding air.

  9. Temperature of surroundings decreases with altitude at AVERAGE environmental lapse rate: (3.5°F / 1000 ft; 6.4°C / 1000 m)

  10. What about parcel? As parcel rises, it expands due to lower atmospheric pressure of surrounding air; as parcel expands, its temperature drops.

  11. As a parcel of air descends, it contracts due to greater atmospheric pressure of surrounding air; as parcel contracts, temperature rises. .

  12. QUIZ 5 1. What is this? • Fog B. a cloud C. Frost D. dew • 2. What is this? • Fog B. a cloud • C. Frost D. dew

  13. 3. What is this? • Fog B. a cloud • C. Frost D. dew • 4. What is this? • Fog B. a cloud • C. Frost D. dew

  14. 5. For the previous 4 questions, which of the • following was required? • A legal ID B. saturation • C. Low relative humidity D. extreme heat • 6. What do you call a dense mass of water • droplets, supercooled water droplets and/or ice • crystals in the troposphere? • A cloud B. the frozen three • C. Dunno D. aerosols

  15. Yesterday’s heavy fog On radar

  16. On surface analysis

  17. On visible satellite image

  18. On thermal IR

  19. Water vapor

  20. Long range forecast A vigorous upper-level trough moving from the Rockies to the central U.S. on Wed is expected to quickly deepen and spin up a significant low pressure system. The surface low is expected to move from the Central High Plains Wed morning to the Midwest on Thu and the Great Lakes by Fri, before quickly moving northeast into Quebec by Fri night/Sat. Confidence in the forecast for this system has improved significantly over the past 24-48 hours.

  21. These changes in temperature: • NOT due to heat exchange with surroundings, • due to expanding and contracting ONLY!. called ADIABATIC • (change in temperature with no gain or loss of heat)

  22. Top & bottom elevations of clouds: Bottom: lifting condensation level: temperature of rising air reaches dew point Top: Cloud rises until cloud temperature reaches temperature of surroundings (becomes stable). Cloud thickness depends on thickness of unstable layer.

  23. Lifting condensation level

  24. General cloud forms: • Stratiform: layer • layer of air forced to rise over denser layers • Cumuliform: globular • parcels of air forced to rise above denser surrounding air

  25. Altitude classes: • High Cloud (cirro-): ice crystal clouds • > 7000 m (23,000 ft.) • CIRRUS, CIRROSTRATUS, CIRROCUMULUS • Middle Cloud (alto-) • 2000 – 7000 m (6500 - 23,000 ft.) • ALTOSTRATUS, ALTOCUMULUS • Low Cloud (stratus) • <2000 m (6500 ft) • STRATUS, NIMBOSTRATUS, STRATOCUMULUS • Vertically developed clouds (CUMULONIMBUS)

  26. Mechanisms causing air parcels to rise: • Spontaneous: • Convergence • Convection • Forced : • Orographic Uplift • Frontal Lifting

  27. GOES

  28. Cold front

  29. Warm front

  30. PRECIPITATION(rain , snow, sleet, freezing rain, hail) • Large parcel of air rises and cools • Adiabatic cooling • Cools to dew point • Cloud droplets grow large enough to fall

  31. Why do cloud droplets need to grow?

  32. Processes by which cloud droplets/crystals grow : A. Bergeron Findeisen Process Mid- and High latitudes: “Cold Clouds” (tops colder than 0°C) • Saturation vapor pressure is greater over water than ice • It takes more water vapor to saturate air over water than over ice • ice crystals grow at the expense of water droplets

  33. Bergeron Findeisen process

  34. Collision/Coalescence process Low latitudes: “Warm Clouds” (tops warmer than -15°C) • Collision and coalescence: some big drops form; they fall faster than small drops and collect small drops

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