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Chapter 4, pages 94-100 : Condensation The formation of dew, fog, and clouds: removal of atmospheric water vapor psychrometric chart Condensation (or deposition) of excess water vapor produces dew, frost, clouds or fog condensation of excess water vapor T<Td clouds or dewfall Saturated T=Td Unsaturated T>Td Pop quiz: When is dew or frost on grass unlikely ? at night; on a windy, dry day; when it is calm; when the sky is clear; Condensation (or deposition) can be manmade as well 9-4-94, 07:10: STS-59 launch Contrails wing tip vortices Contrails are the result mainly of mixing of two air parcels, a really hot one and an ice cold one engine exhaust contrail forms measured Saturated contrail dissipates Unsaturated cold hot But most condensation processes on Earth are natural Two challenges to rain formation: 1. 2. Water vapor needs to condense on nuclei These droplets need to increase their volume a million-fold, to become a rain drop. Condensation Nuclei Nucleation is the condensation, freezing, or deposition of water vapor in free air (air not in contact with the surface). Almost all nucleation occurs on aerosol (heterogeneous nucleation). • condensation nuclei • ice nuclei Giant nuclei Large nuclei Aitken nuclei Size, m # density, cm-3 1 - 10 0.1 - 1.0 0.01 - 0.1 0.01 - 10 1 - 103 103 - 106 Because the small nuclei have such little mass, they are able to remain suspended in the atmosphere for many days. Condensation nuclei: sources breaking waves and air bubbles inject salt particles and dimethylsulfide aerosols into the air biomass burning and volcanoes anthropogenic industrial sources Condensation nuclei distribution: 1. there are plenty of CCN 2. therefore condensation occurs at a RH of 100% 3. there are more CCN over continents, so continental clouds tend to have more, but smaller droplets. Aitken particle concentrations Dust storm over the Mediterranean 25 Aug 2000, seen by TOMS on SEAWIFS (dust concentration estimated on right) global winds transport ~200 tonnes of aerosol /y Condensation nuclei: Hygroscopic particles Salt crystal Hygroscopic particle: water vapor can condense on the surface when the relative humidity is «100%. cloud droplet formation: the curvature effect the saturation vapor pressure is higher around a smaller droplet this is because of cohesion forces: the surface tension is trying to squish the droplet cloud droplet formation: the solute effect the saturation vapor pressure over a salty droplet is reduced this is because salt is hygroscopic Kohler curve the combined solute and curvature effects imply that the ambient RH needs to exceed 100% just slightly, for an embryonic drop to grow. Frost: deposition of water vapor cause: nocturnal radiational cooling (remember the energy budget on clear nights) Frost can also be due to the advection of supercooled clouds Ice nuclei Ice nuclei are less abundant, and more become active at lower temperatures Droplets can remain liquid (“supercooled”) even below freezing. Three types of ice nuclei freezing nuclei: droplets freeze when a nucleus becomes active contact nuclei: droplets freeze upon contact deposition nuclei: water vapor deposits directly as ice Fog: clouds near the ground Fog is defined as visibility to <1 km a day of widespread fog east of the Rockies Fog climatology I-80 summit webcam Fog or cloud formation Supersaturation may result from any of 3 processes: – A. air being cooled to saturation point – B. evaporation of water vapor – C. mixing of warm air with cold air. B A C discussion Think of situations in which any of these three processes causes fog – Land & sea … – Day & night… Fog by cooling: Radiation fog Radiation fog (“ground fog”) clear night sky, light winds, and an air mass that is relatively humid. ground fog is often confined to valleys: why ? the layer can grow vertically (radiational cooling from the top of the fog). Advection fog Radiation & advection fog Wintertime radiation fog in California’s Central Valley summertime advection fog along California’s coast. Advection fog is persistent in some areas Welwitschia mirabilis is entirely dependent on the Namib Desert’s advection fog. capturing water from the advection fog fog catcher beetle (onymacris unguicularis) fog harvesting net upslope fog steam fog Remember droplet formation mechanisms Fog/cloud is formed by: – – – A. air being cooled to saturation point B. evaporation of water vapor C. mixing of warm air with cold air. B steam fog is mainly the result of mixing of two types of air A C steam fog Forms when cold air drifts over a warmer body of water. The lowest layer of air becomes both more humid (evaporation) and warmer than the air above (unstable), through contact with the warmer water . The consequent mixing of warm, humid air with cold, dry air brings the air to saturation, and fog forms. Pop quiz: The most common type of fog, radiation fog, usually disappears during the day because: A: the cloud droplets absorb solar radiation; they then warm, resulting in a higher saturation vapor pressure around the droplets, higher than the vapor pressure in the ambient air; the differential leads to evaporation; B: a little sunshine trickles thru the fog; that warms the ground, which in turn warms the air; the air then can hold more water vapor, so the cloud droplets are happy to evaporate; C: the wind picks up during the day and advects the fog up; often the fog can be seen later in the afternoon, reshaped into a cumulus cloud D: the solar radiation is reflected by the fog, keeping the ground cold, and causing the fog droplets to fall out as dew. How do clouds form Rising air is the key process in the production of clouds: rising air expands and cools. Humidity increases until it reaches 100%. When this occurs, condensation occurs Cloud droplets form on condensation nuclei (CCN). Droplets may grow both by further condensation and by collision/coalescence rain What makes air rise ??? 4 mechanisms responsible for updrafts, clouds, & precipitation: convection spontaneous orographic buoyant Low-level convergence, OR divergence aloft, near jet stream frontal Calculating the height of cloud bases Lifting condensation level : level at which an air parcel becomes saturated when lifted (cooled adiabatically) You can use the following formulas to calculate the height (H) of a cumulus cloud base given surface temperature (T) and dew point temperature (Td). H (meters) = 125×(T– Td ) in ºC H (feet) = 222×(T – Td ) in ºF try out this applet estimate current cloud base Pop quiz: Cloud droplets form ______ when the air is saturated, and ice crystals form ______ when the air cools to ______ A: in pure air … in pure air … the freezing point; B: on cloud condensation nuclei … on ice nuclei … 32 °F; C: on cloud condensation nuclei … on ice nuclei … some temperature below 32 °F; D: on cloud condensation nuclei … on ice nuclei … -40 °F Cloudiness and climate change Do clouds cool or warm the Earth surface? cooling effect warming effect (from textbook page 111) Cloudiness and climate change The net effect of clouds depends on how high they are Low-level clouds block much solar radiation (high albedo) emit much LW radiation to space (they are warm) net effect: cooling High-level clouds more transparent to sunshine emit little LW radiation (they are cold) net effect: warming Cloudiness and climate change The effect of clouds on climate can be profound: a 4% increase in low-level cloudiness is sufficient to offset the warming due to a doubling of CO2. The predicted change in low-level cloud cover and albedo in the next few decades is very uncertain. The uncertainty relates to aerosol, both natural and manmade: more aerosol more cloud droplets higher albedo stronger cooling Note: cloud classification is discussed in Chapter 4 , p. 101- 110 (Ackerman & Knox). You will need this for Lab #7 (Wed 14 or Thu 15 Oct). next topic: adiabatic processes and stability