WEAX-201 Chapter 6--Condensation, Dew, Fog, and Clouds

1. WEAX-201Chapter 6--Condensation, Dew, Fog, and Clouds

2. Condensation……WITH STYLE!!

3. A little less eye-catching, but the same principle: • A cold drink “sweats” because warm, moist air comes in contact with the cold surface • The air cools to below it’s dew point temperature • Condensation occurs

4. Formation of Dew and Frost • Dew forms when the temperature cools to the dew-point temperature • If T = Td < 32°F, frost forms instead of dew • Dew/frost often forms close to the ground, and not on objects just above the ground • Why?    • Dew and frost most often form on clear, calm nights • Why?  • Dew can be an important source of moisture during periods of low rain fall.

5. Formation of Haze, Fog, and Clouds: Condensation Nuclei • The process of condensation of vapor to form a cloud drop is not as simple as dew or frost formation • Must have Cloud Condensation Nuclei (CCN) to form cloud drops • CCN are small particles in the atmosphere: • Dust, volcanoes, factory smoke, forest fires • Ocean salt, sulfate particles from phytoplankton in ocean • They are most abundant in lower troposphere over urban areas • They are quite small compared to a rain drop or cloud droplet

6. Sizes and Amounts of CCN • Total mass of CCN put into atmosphere each year is about 2x1012 kg • Two types of CCN: • Hydroscopic (water seeking) • Water vapor readily condenses on these • Ocean salt is a good example (sticky salt shaker when humid) • Hydrophobic (water repelling) • Water vapor does not readily condense on these (wax on car) • Note: 1cm3 is about the size of your thumb

7. Formation of Haze • Two types of haze: • Dry haze - large/giant particles in the air (smoke, smog, dust) • Wet haze - H2O condenses onto hydroscopic CCN – can • Can occur at RH as low as 75% • Wet haze has a dull gray, white color

8. Formation of Fog • Fog forms as the RH increases to 100% • Haze particles grow into fog (cloud) droplets near the ground • Fog is really a cloud near the ground • International definition: Visibility less than 1 km • National Weather Service definition: Visibility is less than or equal to 6 miles and T-Td < 5°F • Fog in heavily polluted areas can be a health problem since it becomes acidic

9. Types of Fog: • Radiation Fog • Advection Fog • Upslope Fog • Steam Fog

10. Formation of Radiation Fog • Conditions needed: • Moist air near surface of the ground • Clear and calm nights • Light winds to bring a larger volume of air in contact with the cooler ground • Radiational cooling allows the air temperature to drop to the dew point temperature.

11. Formation of Radiation Fog • Once the T reaches Td, radiation fog begins to develop • Common in the fall - especially when weather is dominated by high pressure • Often forms in valleys first since this is where the coldest air is. This is called valley fog

12. Valley Fog

15. Advection Fog • Common off the west coast of the U.S. • Cold current along coastline • Warm water further to the west

16. Advection Fog • Westerly winds advect warm moist air over colder water • Warm, moist air to the cold water via conduction • The parcel reaches saturation. Fog forms, and is advected onshore • Need a light breeze for this process to occur

17. Advection Fog • Advection fog can be an important source of moisture for plant life along the California-it rarely rains there during the summer months • Why are advection fogs rare in the tropics?

18. Upslope Fog • A parcel of warm, moist air climbs from the Gulf of Mexico as it is advected toward Denver • As the parcel ascends up the slope, it expands, and the temperature cools to the dew point

19. Upslope Fog • As the parcel ascends, it expands and cools to the dew point (lapse rate =10oC per 1000 meters) • Upslope fog/clouds then form • Neccessary ingredients: • Moist air • Winds that move the air up the slope • A slope

20. Steam Fog • Common here in late fall and winter • Seen over lakes or heated pools in winter • Need cold air over a warm body of water

21. Steam Fog • Heat and moisture are transferred from the warm water to the cooler, drier air • This occurs in a shallow layer near the lake’s surface • This is an unstable situation with warm, saturated air at the surface below cooler air • Whisps of warmer, moister air rise into the cooler air—steam fog is formed • On a cold morning, you can see your breath. Why?

22. Foggy Weather • Where is it foggy???? • Pacific Coast • Appalachian highland region • New England • Foggiest spot in the U.S.: Cape Disappointment, WA • it's foggy for 2556 hours per year, or about 107 days. • Fog is a significant weather problem for aviation ops

23. Introduction to Cloud Types- Know the cloud types- Be able to identify clouds

26. Introduction to Cloud Types • Clouds are comprised of liquid droplets of various sizes and/or ice crystals • They are characterized according to their height location in the atmosphere and their vertical development: • High clouds • Middle clouds • Low clouds • Vertically developed clouds • NOTE: cloud names come from Latin words: • cirrus - curl • stratus - layer • cumulus - heap • nimbus - violent rain

27. High Clouds - Cirriform • High clouds are comprised largely of ice • Cloud-base heights for high clouds: • Tropical Region//Middle Latitudes//Polar Regions • 6-18 km 5-13 km 3-8 km

28. High Clouds - Cirrus (Ci) Cirrus Clouds (Ci)- high, thin wispy clouds at jet stream level in the upper troposphere Associated with fair weather

29. High Clouds - Cirrostratus (Cs) • High, thin, sheet-like clouds • Produce halos around the sun/moon • Many of the optical phenomenon we learned a couple of weeks ago are caused by Cs • A sign that poor weather is often approaching (12-36 hours away)

31. Cirrocumulus Clouds (Cc) • High clouds • Resemble fish scales or small rounded white puffs • About the size of your thumbnail

33. Middle Clouds – Alto__ • Middle clouds are composed of water and/or ice • Cloud-base heights for middle clouds: • Tropical Region//Middle Latitudes//Polar Regions • 2-8 km 2-7 km 2-4 km

34. Altocumulus Clouds (Ac)- Shallow, puffy or wave-like in appearance Appear to be larger than your thumb, but smaller than your fist when holding your arm up to the sky

35. Cc Ac

36. Special type of Ac cloud that forms in high speed wind conditions Usually downwind of mountain ranges

37. Altostratus Clouds (As) • Grayish/blue-gray appearance • Thin layer covering entire sky uniformly • Found ahead of approaching storms • Can see the sun through altostratus, but NO halo will be observed

39. Low Clouds • Cloud-base heights for low clouds: • Tropical Region//Middle Latitudes//Polar • 0-2 km 0-2 km 0-2 km

40. Stratus Clouds (St) -Uniform grayish cloud covering the entire sky -Fairly common here in the winter -Light, continuous drizzle

44. St As

45. Nimbostratus Clouds (Ns) • Darker gray, "wet" looking low clouds • Produce light/moderate precipitation over a large region

48. Stratocumulus Clouds (Sc) • Low, lumpy, puffy clouds in patches or rounded masses • “Fair weather” clouds (usually) • Appear the size of your fist when holding your arm up to the sky