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10/20/16 Condensation: Dew, Fog, and Clouds Dew and Frost This chapter discusses: 0. How vertical motion effects parcel thermodynamics 1. How dew, frost, haze, fog, and clouds form from atmospheric moisture 2. Classification of fog and cloud types from observation Formation of Dew & Frost Figure 6.1 • Recall that solid surfaces (blades of grass, glass windshields, airframe surfaces) outside efficiently radiatively cool at night, especially if the winds are calm and there are no clouds (WHY?) • When this happens, they quickly become colder than the overlying air. • Dew and frost occur when surfaces cool to a temperature below the dewpoint(frostpoint) temperature (DPT) of the surface air. • When this happens, water condenses out on the cold surface, causing dew or frost to form Haze & Water Seeking Nuclei Figure 6.2 As air cools to its saturation, or dew point, vapor molecules slow down and can adhere as dew on the ground surface or as frost when air temperature drops below freezing. Daily temperature lows often occur by radiational cooling, forming dew at night or early morning. Dew is most likely when winds are calm, and the nocturnal inversion is steep and shallow Cloud Condensation Nuclei (CCN) Above the ground surface, cooling and slowing water vapor instead condenses upon cloud condensation nuclei (CCN, more on this later). Hygroscopic particles such as salt or dust seek condensing vapor, and can form a wet white haze when relative humidity is above 75%, or a dry blue haze when drier. Hygroscopic particles (e.g., salt, many dusts) seek condensing vapor, and can form a wet white haze when relative humidity is above 75%, or a dry blue haze when drier. A few percent of the aerosol in the atmosphere are referred to as Cloud Condensation Nuclei (CCN) DEFINE: Cloud Condensation Nuclei (CCN) are aerosols that are hygroscophic– they have an affinity for (i.e. attract) water vapor molecules. CCN serve as building sites for cloud (not rain) droplets. CCN are essential for cloud droplet growth Attracts water vapor molecules Repels water vapor molecules 1 10/20/16 How do clouds form? At least 2 things are necessary: • A saturated parcel • Abundant CCN Cooling by expansion Moving a parcel upward to a region of lower pressure causes the parcel to EXPAND and COOL If the parcel cools enough, it will become saturated, and condensation– in the form of cloud droplets– will occur. Conversely, moving a parcel downward (higher pressure) causes the parcel to CONTRACT and WARM. If initially saturated, as sinking parcels warm, become subsaturated, and the cloud will evaporate. Cloud formation • For clouds to form, the atmospheric parcel must saturate. the temperature needs to become the same as the DPT (VP needs to equal SVP) This can be done by (1) cooling the air parcel to DPT, or (2) adding water vapor to raise the DPT. • (1) is much more common than (2). Cooling can happen through – Longwave Radiation from the surface or cloud top, or, more commonly – Cooling the parcel by expansion (raising to region of lower pressure). FOG • FOG— Simply put, fog is a stratus-type cloud on the ground, not necessarily dense, but often is (visiblity below 10 km for aviation purposes, 6 km for NWS purposes and 1 km for WMO purposes). • Unlike most other clouds, fog is typically driven by some cooling mechanism other than lifting (expansional cooling). Ground Based Radiation Fog Radiation fog, CA Central Valley radiation fog— occurs in clear-sky conditions when ground (and atmos boundary layer) radiatively cools to below dewpoint. Typically occurs in low-lying areas with light winds. Dew formation is more likely if winds are dead calm. 2 10/20/16 Advection Fog Figure 6.5 Advection fog— occurs when warm, humid air mass advects over a cold surface (snow, lake, ice), chilling that airmass to the dewpoint temperature This fog often forms above the ocean due to mixing currents, or when warm ocean air rolls into the cooler waters at the Pacific Coastline. Other fogs… • upslope fog— simply a surface-based stratus cloud that forms as the result of orographic lifting (flow “up-slope”). The kind of fog you experience in mountain tops. • ice fog— occurs not from cooling, but from injection of moisture into the atmospheric boundary layer. It is composed of ice crystals rather than cloud droplets • Usually occurs only in situations of strong temperature inversions • only occurs at ~ -35° C or colder Fog forest, Costa Rica • typically found near anthropogenic (man-made) sources of water vapor (car exhaust, smokestacks, cooling towers at power plants, etc) 3 10/20/16 Fog & Human Safety Foggy days in the US have a predictable distribution due to ocean and mountain influences. Fog can help crop growth in California, but can also cause severe automobile, airplane, and boating accidents. Figure 6.8 4