Clouds ATS351 Lecture 5 Outline • Review CCN •Fog • Clouds Types • Use of Satellite in Cloud Identification Cloud Condensation Nuclei • Aerosols may become CN; and CN may become CCN – But not all CN are CCN – Larger concentrations over land • Cloud droplets can form on both insoluble and soluble particles • A particle that will serve as CCN is called hygroscopic or hydrophilic – condensation may occur at RH <100% • A particle that will not serve as a CCN is called hydrophobic. – condensation may occur at RH >100% • CCN Sources may include: – dust, volcanoes, factory smoke, forest fires, sea salt 1 Cloud Condensation Nuclei • Described by the size of the particle Haze • Dry haze – Just the small particles themselves, maybe a little bit of condensation • Wet haze – Begins at RH ~ 75% – Much more light scattering than dry (3x)‏ Fog • Going above 70% RH to 100% • Condensation on less active nuclei • Essentially, a thick wet haze on the ground or a low cloud • Called fog when visibility is less than 1 km • 4 Main Types – Radiation – Advection – Upslope – Evaporation/Mixing 2 Fog • Type of CN affects fog • Over the ocean – Fewer, larger drops • Over urban areas – More, smaller drops – Lower visibility – London Fog • Chemical reactions can cause fog to become acidic • If temperatures drop below freezing, freezing fog may result Radiation/Ground Fog • Surface radiatively cools • Light breeze helps more air contact cold surface • Common in the fall and winter (associated with ridges)‏ • Common in low-lying areas – River valleys • Form upward from the ground – Deepest around sunrise – May intensify after sunrise (dew evaporation)‏ • “Burns Off” with more insolation – Dissipates from bottom up – Dissipates easily around edges (thin, mixing)‏ Advection Fog • Warm air moves over a cold surface. • Breeze required • Pacific Coast Cold Current • Gulf Stream and British Isles • May combine with radiation fog 3 Upslope Fog • Moist air rises up the side of a mountain • Just like a cloud forming, but touching the surface • Would be seen on western slopes (not around Fort Collins as much except in the case of easterly winds and associated upslope)‏ • Can last for days under the right conditions Evaporation (Mixing) Fog • Just like breathing out when it’s cold • Cold air over warm water (steam fog)‏ – Seen over a pool or spa • Increased moisture raises the dew point – Maintained through vapor pressure difference and mixing • Steam devils on a lake • Precipitation Fog Foggy Weather Annual number of days with fog in the US 4 Clouds • Clouds result when saturation occurs • Properties: – Variety of sizes – Contain water drops and/or ice crystals – Various heights – Stratospheric clouds rare, but important for creation of the ozone hole Cloud Classification 5 Cirriform Clouds • Usually exist above 16,000 feet • Generally thin, sometimes partially translucent • Comprised of ice crystals • Absorb longwave radiation, but highly reflective of visible radiation (high albedo)‏ • Rarely precipitate – Virga • Examples: Cirrus (Ci): Called “mares tails” Wispy Cirrostratus (Cs): Usually present when halos around the sun are observed Halo 6 Cirrocumulus (Cc): Called “fish scales” or “mackerel sky” Stratiform clouds • Characterized by a horizontally uniform base (Layered)‏ • May or may not precipitate • May exist at any level Stratus (St): Uniform and gray, maybe lifted fog 7 Stratocumulus (Sc): Small “empty” spaces, large coverage Nimbostratus (Ns): Patchier and rainier than regular stratus Marine stratus 8 Cumuloform clouds • Large vertical extent • May or may not precipitate • Result from vertical motion • Cumulus (Cu) Examples: Cumulus Humilis: “Fair weather” Cumulus Congestus: Towering Cumulus 9 Cumulus Congestus Cumulonimbus: Anvil Cloud 10 Other cloud types Mammatus clouds • Precipitation evaporates out of anvil • Evaporation cools the air and it sinks • If drops are large, mammatus will be long lived Fractus Clouds • Also known as scud clouds • Low, detached clouds caught in the outflow of a thunderstorm • Can also be seen below stratus clouds. 11 Pyrocumulus • Caused by fire, volcano or industry • Caused by intense heating of moist air • Only forms in Calm wind situations Contrails • Condensed exhaust from jet aircraft • Important when considering climate effects of clouds Cloud Formations • Lenticular • Kelvin-Helmholtz waves • Cloud streets • Wall Clouds • Shelf Clouds 12 Lenticular Clouds • Stationary, lens-shaped clouds over mountains at high altitude • Stable, moist air flows over mountain, creating a large scale standing wave • Indicates region of turbulence Kelvin-Hemholtz Waves • Form when two parallel layers of air are moving at different speeds and in different directions • Upper layer is usually faster • Very short lived Cloud Streets • Form due to horizontal rolls in the atmosphere • Also due to uneven surface heating • Clouds form over updrafts in rolls • Occurs more frequently over the ocean 13 Shelf and Roll Clouds • Low, horizontal, wedge-like cloud • Shelf: Attached to Parent Storm • Roll: Removed from Parent Storm • Formation is due to gust front from thunderstorms Wall Cloud • Associated with severe thunderstorms • Indicates area of strongest updraft • The strongest tornados form here Really High Clouds • Nacreous Clouds (mother of pearl)‏ – Form in stratosphere – Seen best at polar latitudes in winter – Composition not well known • Noctilucent Clouds – Sometimes seen in the mesosphere – Stars shine through them – Made of tiny ice crystals 14 Nacreous Clouds Nacreous Clouds Noctilucent Clouds 15 Satellite Imagery • Visible imagery: essentially a black and white camera on a satellite. Measures brightness in the visible spectrum. • Infrared imagery: measures infrared radiance from the surface it is pointed at. From blackbody theory, the temperature of the surface can be found; since temperature changes with height, the cloud-top height can then be estimated. Visible Satellite • Pros - good at showing low clouds and fog - available in high spatial resolution • Cons - only works in daylight - clouds can be confused with reflective features like snow - optically thin clouds like cirrus don’t show as well IR Satellite • Pros - available at all hours - provides an estimate of cloud-top height • Cons - lower spatial resolution - low clouds don’t show because their temperatures are close to the surface temperature • Color enhancement table often applied to bring out important temperatures Raw Enhanced 16 Clouds and Satellite Imagery • The bright, puffy areas in the visible image on the right are cumulus and cumulonimbus clouds (the cumulonimbus are fuzzier around the edges). Notice how the cloud tops over the Front Range are cold in the IR imagery Cirrus in visible vs. IR • Because cirrus are cold and optically thin (meaning the sun can be seen through the cloud), they are more easily seen in the IR than the visible Low clouds/fog in visible vs. IR • Because low clouds are bright and warm, they are easily seen in the visible, but not the IR 17.