1. Cloud Formation

2. Water Cycle Revisited Sources of water for clouds come from evaporation and transpiration Sinks of water for clouds come in the form of precipitation and evaporation What are the mechanisms that happen in between?

3. What is evaporation? Water that changes from a liquid to a gas from lakes, streams, and ocean.

4. What is transpiration? Water that evaporates from plants.

5. Thirty Seconds of Thermodynamics Temperature is a measure of molecular energy - if a collection of molecules has a lot of energy, it’ll have a higher temperature as well (and vice versa.) If H 2 O in the form of a liquid gains enough energy (which we call evaporation) it may undergo a phase change and become a gas. If H 2 O in the form of a gas loses enough energy (which we call condensation) it may turn back into a liquid.

6. Water Saturation Thought experiment - which beaker likely has a warmer temperature? Why? The temperature of a gas (say, water vapor) is closely related to its energy The amount of water vapor and liquid water will depend on the total energy of the system - obeying the laws of thermodynamics and staying in equilibrium

7. Describing Saturation Is the amount of water vapor that air can hold at a given temperature and pressure before the water vapor starts to condense. Humidity –% of water vapor in the air Hotter temperatures can hold more water! That is why it feels more humid in Florida or Hawaii than it does here in Lakewood. Or, in terms of a dewpoint temperature, which is the temperature that the vapor (at its current vapor pressure)would have to be cooled to to reach its saturation vapor pressure. Nothing here depends on the dry air!

8. Dew Surfaces cool strongly at night Strongest on clear, calm nights If a surface cools below the dew point, water condenses on the surface and dew drops are formed Surfaces cool strongly at night Strongest on clear, calm nights If a surface cools below the dew point, water condenses on the surface and dew drops are formed

9. Frost If the temperature is below freezing, the dew point is called the frost point If the surface temperature falls below the frost point water vapor is deposited directly as ice crystals The resulting crystals are known as frost, hoarfrost, or white frost If the temperature is below freezing, the dew point is called the frost point If the surface temperature falls below the frost point water vapor is deposited directly as ice crystals The resulting crystals are known as frost, hoarfrost, or white frost

10. Cloud droplet formation If the air temperature cools below the dew point, water vapor will tend to condense and form cloud/fog drops As with dew and frost, cloud drop formation prefers to condense on a surface of some sort - we call these particles cloud condensation nuclei (CCN) CCN surfaces facilitates condensation Without these particles clouds would not form in the atmosphere If the air temperature cools below the dew point, water vapor will tend to condense and form cloud/fog drops As with dew and frost, cloud drop formation prefers to condense on a surface of some sort - we call these particles cloud condensation nuclei (CCN) CCN surfaces facilitates condensation Without these particles clouds would not form in the atmosphere

11. Cloud Formation

12. Aerosol Sources Terrestrial Sources Dust/sand/dirt particles Smoke - volcanic, fires, and pollution (sulfates) Pollens and spores Oceanic Sources Sea Salts Terrestrial Sources Dust/sand/dirt particles Smoke - volcanic, fires, and pollution (sulfates) Pollens and spores Oceanic Sources Sea Salts

13. Typical sizes

14. Cloud Formation Mechanisms …can be anything that causes water vapor cool and condense Direct cooling… …lifting of an airmass…

15. Cloud Formation Mechanisms …can be anything that causes water vapor cool and condense …buoyant lifting……or mechanical forcing

16. Cloud classification High Clouds - generally above 16,000 ft at middle latitudes Main types - Cirrus, Cirrostratus, Cirrocumulus Middle Clouds – 7,000-16,000 feet Main types – Altostratus, Altocumulus Low Clouds - below 7,000 ft Main types – Stratus, stratocumulus, nimbostratus Clouds of Vertical Development Main types – Cumulus, Cumulonimbus Nimbo- and -nimbus prefix/suffix Denotes precipitation High Clouds - generally above 16,000 ft at middle latitudes Main types - Cirrus, Cirrostratus, Cirrocumulus Middle Clouds – 7,000-16,000 feet Main types – Altostratus, Altocumulus Low Clouds - below 7,000 ft Main types – Stratus, stratocumulus, nimbostratus Clouds of Vertical Development Main types – Cumulus, Cumulonimbus Nimbo- and -nimbus prefix/suffix Denotes precipitation Clouds are classified by height, appearance, precipitation, and category of vertical development

17. Low Clouds Stratus Uniform, gray Resembles fog that does not reach the ground Usually no precipitation, but light mist/drizzle possible Stratocumulus Low lumpy clouds Breaks (usually) between cloud elements Lower base and larger elements than altostratus Nimbostratus Dark gray Continuous light to moderate rain or snow Evaporating rain below can form stratus fractus Stratus Uniform, gray Resembles fog that does not reach the ground Usually no precipitation, but light mist/drizzle possible Stratocumulus Low lumpy clouds Breaks (usually) between cloud elements Lower base and larger elements than altostratus Nimbostratus Dark gray Continuous light to moderate rain or snow Evaporating rain below can form stratus fractus

18. Stratiform cloud layers

19. Stratocumulus cloud streets Stratus undulatus

20. Looking down on an eastern Atlantic stratus deck

21. Middle Clouds Altocumulus <1 km thick mostly water drops Gray, puffy Differences from cirrocumulus Larger puffs More dark/light contrast Altostratus Gray, blue-gray Often covers entire sky Sun or moon may show through dimly Usually no shadows Altocumulus <1 km thick mostly water drops Gray, puffy Differences from cirrocumulus Larger puffs More dark/light contrast Altostratus Gray, blue-gray Often covers entire sky Sun or moon may show through dimly Usually no shadows

22. Altostratus

23. Altocumulus

24. High Clouds High clouds White in day; red/orange/yellow at sunrise and sunset Made of ice crystals Cirrus Thin and wispy Move west to east Indicate fair weather Cirrocumulus Less common than cirrus Small, rounded white puffs individually or in long rows (fish scales; mackerel sky) High clouds White in day; red/orange/yellow at sunrise and sunset Made of ice crystals Cirrus Thin and wispy Move west to east Indicate fair weather Cirrocumulus Less common than cirrus Small, rounded white puffs individually or in long rows (fish scales; mackerel sky) Cirrostratus Thin and sheetlike Sun and moon clearly visible through them Halo common Often precede precipitation

25. Cirrus

26. Cirrocumulus

27. Cirrostratus Cirrostratus with Halo

28. Vertically developed clouds Cumulus Puffy “cotton” Flat base, rounded top More space between cloud elements than stratocumulus Cumulonimbus Thunderstorm cloud Very tall, often reaching tropopause Individual or grouped Large energy release from water vapor condensation Cumulus Puffy “cotton” Flat base, rounded top More space between cloud elements than stratocumulus Cumulonimbus Thunderstorm cloud Very tall, often reaching tropopause Individual or grouped Large energy release from water vapor condensation

29. Cumulonimbus with Pileaus caps

31. Mechanically Forced Clouds Lenticularis Wave clouds

32. Clouds - Why We Care Clouds transport energy from one area to another Evaporation takes latent heat out of warm surface waters Condensation releases same latent heat into atmosphere in a different location Heat has been ‘carried’ by the water inside a cloud Clouds transport energy from one area to another Evaporation takes latent heat out of warm surface waters Condensation releases same latent heat into atmosphere in a different location Heat has been ‘carried’ by the water inside a cloud

33. Latent Heat Release An average thunderstorm contains several thousand metric tons of water Condensing 1 kg of water releases ~ 2.26x10 6 J of latent heat energy An average thunderstorm containing around 1500 tons of water will release 3.45 billion Joules of energy

34. Clouds - Why We Care Clouds affect the radiation budget by reflecting visible light from the sun (thus cooling the planet) and by trapping infrared radiation from the surface (thus warming the planet) Reflection/trapping behavior depends on type of cloud… Clouds affect the radiation budget by reflecting visible light from the sun (thus cooling the planet) and by trapping infrared radiation from the surface (thus warming the planet) Reflection/trapping behavior depends on type of cloud…

35. Cloud Radiative Effects

36. Salient Tidbits In the infrared Clouds absorb radiation from below, re-emit at the temperature of the cloud Cirrus - very cold, emit little radiation Stratus - very warm, basically emit like the surface In the visible Clouds reflect radiation based on the amount of cloud droplets Cirrus - thin, not as reflective as thicker water clouds Stratus - thicker, many water droplets, highly reflective In the infrared Clouds absorb radiation from below, re-emit at the temperature of the cloud Cirrus - very cold, emit little radiation Stratus - very warm, basically emit like the surface In the visible Clouds reflect radiation based on the amount of cloud droplets Cirrus - thin, not as reflective as thicker water clouds Stratus - thicker, many water droplets, highly reflective

37. High Clouds Thin, cold ice clouds reflect less sunlight Extremely cold, emits infrared at colder temperatures, prevents warmer surface infrared from escaping to space NET EFFECT: Warming

38. Low Clouds Very thick water clouds reflect large amounts of sunlight Very near the surface, temperature of the cloud effectively the same as surface. Infrared radiation is therefore about the same - almost like the cloud wasn’t there! NET EFFECT: Cooling

39. Challenges Climate modelling with clouds - need to get both type and amount correct, which is difficult Overestimating high clouds - too much warming Overestimation low clouds - not enough Understanding cloud feedbacks What does CO 2 warming do to cloud populations? How does increased aerosol pollution affect cloud types and amounts? Observations Layered cloud structures not seen from satellites Climate modelling with clouds - need to get both type and amount correct, which is difficult Overestimating high clouds - too much warming Overestimation low clouds - not enough Understanding cloud feedbacks What does CO 2 warming do to cloud populations? How does increased aerosol pollution affect cloud types and amounts? Observations Layered cloud structures not seen from satellites