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WEAX-201 Chapter 6--Condensation, Dew, Fog, and Clouds Condensation……WITH STYLE!! 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 Formation of Dew and Frost ► Dew forms when the temperature cools to the dewpoint 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. 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 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 Note: vapor does not readily condense on these (wax on car) 1cm3 is about the size of your thumb Type of Particle # of Particles per cm3 Approximate Radius (mm) range (typical) Small CCN < 0.2 1000 Large CCN 0.2 - 1.0 100 Giant CCN > 1.0 1 Fog and cloud droplets > 10.0 300 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 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 Types of Fog: ► Radiation Fog ► Advection Fog ► Upslope Fog ► Steam Fog 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. 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 Valley Fog Advection Fog ► Common off the west coast of the U.S. Cold current along coastline Warm water further to the west 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 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? 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 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 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 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? 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 Introduction to Cloud Types - Know the cloud types - Be able to identify clouds Introduction to Cloud Types ► High Clouds: ►Cirrus (Ci) ►Cirrostratus (Cs) ►Cirrocumulus (Cc) Low Clouds: ►Stratus (St) ►Stratocumulus (Sc) ►Nimbostratus (Ns) Middle Clouds ►Altostratus (As) ►Altocumulus (Ac) Clouds with vertical development: ►Cumulus (Cu) ►Cumulonimbus (Cb) NOTE: cloud names come from Latin words: • cirrus - curl • stratus - layer • cumulus - heap • nimbus - violent rain ► 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 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 High Clouds - Cirrus (Ci) Cirrus Clouds (Ci)- high, thin wispy clouds at jet stream level in the upper troposphere Associated with fair weather 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) Cirrocumulus Clouds (Cc) ► High clouds ► Resemble fish scales or small rounded white puffs ► About the size of your thumbnail 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 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 Cc Ac Special type of Ac cloud that forms in high speed wind conditions Usually downwind of mountain ranges 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 Low Clouds ► Cloud-base heights for low clouds: Tropical Region//Middle Latitudes//Polar 0-2 km 0-2 km 0-2 km Stratus Clouds (St) -Uniform grayish cloud covering the entire sky -Fairly common here in the winter -Light, continuous drizzle St As Nimbostratus Clouds (Ns) ► Darker gray, "wet" looking low clouds ► Produce light/moderate precipitation over a large region 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 Vertically Developed Clouds Cumulus (Cu) ► Cumulus Clouds Look like cotton balls/cauliflower in the sky Whiter than Sc, and often more verticallydeveloped ► Sub-categories of cumulus: cumulus humilis - slightly developed Cu cumulus congestus (or “moderate Cu”) moderately developed Cumulus ► Cumulus humilis congestus Developing Cumulus Cumulonimbus Clouds (Cb) ► Thunderstorms Develop from growing Cu Can extend up to the troposphere Can contain both water and ice Produce precipitation (rain, snow, hail, etc) Produce lightning and severe weather Form a distinctive "anvil" cloud at the top of the storm Other unusual clouds - Scud ► Scud are ragged low clouds drifting beneath the actual cloud base ► Often form due to turbulent mixing of air: Warm air from the updraft Cool air from the downdraft Cause huge problems for general aviation Other Unusual clouds - Lenticular Clouds ► Form as air flows over mountains ► Look like pancakes, UFOs ► Appear to stay stationary Lenticular Cloud Time Lapse Other Unusual Clouds - Pileus ► Forms as a growing thunderstorm deflects moist air up and over the top of the building cumulus congestus or cumulonimbus Mammatus clouds - Found in the anvil portion of intense thunderstorms - Indicate strong downdrafts in upper portions of a thunderstorm - Indicate hail or tornadoes Other Unusual Clouds - Pollution Induced ► Hot air from a smoke stack can rise high enough to produce a cloud Unusual Clouds—”Hole Punch Clouds” ► Occurs in Ac clouds Ci above dropped light snow that fell into the lower Ac deck Acted much like cloud seeding (chap 8) Cloud droplets in the Ac cloud coalesced on the falling snow—this cleared a hole in the Ac deck From the WKRG-TV page Occurred in December in southern Alabama Effects of Aircraft on Clouds Contrails ► Jets passing through thick Ci clouds can raise RH to the point where precipitation begins, and clouds begin to clear Courtesy WKRG-TV ► If a jet is flying through air with low humidity, the moist air might produce a short-lived contrail. Thus, the forecast would be for the weather will remain fair. However, if the contrail is a thick, long lasting trail it indicates that high humidity is in the atmosphere and it could be a sign of a storm approaching. Cloudiness has increased by about 20% over some portions of the US - These areas are along the major air traffic routes - The cloud increases are due to contrails produced by aircraft - Implication for climate?? - Sky Conditions Description ASOS (NWS) Human Meaning Clear (CLR or SKC) 0 to 5% 0 No Clouds Few >5 to <25% >0 to 2/8 Few clouds visible Scattered (SCT) > 25 to <50% 3/8 to 4/8 Partly cloudy Broken (BKN) > 50 to <87% 5/8 to 7/8 Mostly cloudy Overcast (OVC) > 87 to 100% 8/8 Sky is covered by clouds NA Sky is hidden by surface-based phenomena, such as fog, blowing snow, smoke and so forth, rather than by cloud cover Sky obscured NA