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What are clouds and how do they form? What determines the height of a cloud base? ■ Does cloudy air have the same adiabatic lapse rate as cloud-free air? ■ What different types of clouds are there? ■ How are clouds described in METAR reports? ■ How are clouds observed by satellite? ■ ■ Unit 14 Clouds and Satellite Imagery Requirements for cloud formation What are clouds? ■ Requirements for formation: ■ – Abundant water vapor – Condensation nuclei ■ High RH, small T-DP spread ■ Condensation needs a surface (e.g. window) the atmosphere, water vapor condenses on condensation nuclei ■ In A cooling mechanism. What are the two ways that cooling can occur? A. ______________ B. ______________ – Condensation nuclei Aerosol (or particulates) - sea salt, dust, combustion by-products, etc. ■ Without these, cloud formation would be nearly impossible ■ Small, so can remain suspended for days ■ Some have a special affinity for water (hygroscopic), so condensation can start for RH less than 100%. eg - sea salt ■ Condensation and subsequent precipitation scavenge particulates from the atmosphere ■ Supercooled water Ice nuclei ■ Cloud droplets are surprisingly pure and may not freeze until cooled to -40 °C – ■ Therefore water droplets exist at temperatures below 0 °C--called “supercooled” water Supercooled droplets freeze when: – – They collide with a solid object (e.g., ground or airplane) or another cloud particle that is frozen Or when they contain an impurity called an ice nucleus (e.g., clay particle) ■ Ice nuclei are much less abundant in the atmosphere than condensation nuclei ■ Between 0°C and -15°C, most cloud particles are supercooled water droplets – Many are cold, but few are frozen ■ At colder and colder temperatures, the percentage of particles that are frozen increases ■ Colder than -40°C, all cloud water is ice 1 Fog Types of Fog Surface-based cloud (composed of water droplets or ice) ■ Fog is reported when clouds base < 50 ft. ■ Mist when vis < 7 miles, fog < 5/8 mile ■ Tends to form when T-DP is < 3°C & decreasing ■ Would you expect fog to be worse in rural or urban areas? ■ Is fog more common in stable/unstable air? ■ What causes fog to form? Dissipate? ■ What makes fog dissipate more slowly? ■ ■ Advection fog forms by contact cooling where warm air moving over colder surface – – – – ■ Occurs when warm moist air moves over colder bodies of water (sea fog), or over cold land Needs winds up to about 15 kt Occurs mostly near coasts, day or night Along CA coast, over Labrador current, S. coast Radiation fog – – – – Air near ground cools by radiation to saturation Also called ground fog Occurs in inland valleys (eg Fresno CA) Needs clear nights, light breeze < 5 kts Types of Fog (cont’d) Radiation fog ■ Upslope fog where warm, moist air ascends terrain – – ■ Like advection fog, upslope fog needs wind Dry and warmer on downwind side of mountain Steam fog where very cold air moves over warm water – – Shallow and wispy Also called sea smoke or evaporation fog Advection fog (San Francisco) Finding cloud base - LCL Below cloud base, rising dry surface air cools at DALR ■ DP decreases by about 5/9°C/1,000 ft ■ Rising parcels are saturated at the LCL Visual properties of clouds ■ ■ – – – – H=(T-DP)/2.5 (°C) H=(T-DP)/4.4 (°F) H in 1,000s of ft AGL What is H if T=72°F, DP=50°F? Two general types: – Cumuliform (cumulus): Puffy, cauliflower-like clouds (associated with instability/convection) ■ Can Dew point develop into TS if atmosphere conditionally unstable Cloud base – Dry H adiabat ■ Other visual properties: – – Surface DP Surface temp Stratiform (stratus): Layered clouds (associated with stability) – Fall streaks: lines extending downward from base of cloud, indicating precipitation Boundaries: water clouds usually have distinct edges; ice clouds usually appear wispy Dark base: cloud has deep vertical extent 2 Cloud types (pp. 6-18 & 6-19) ■ What are the following cloud types? Low clouds < 6,500 ft AGL – Stratus (ST), stratocumulus (SC), nimbostratus (NS), cumulus (CU), cumulonimbus (CB) ■ Middle clouds 6,500 to 20,000 ft AGL ■ High clouds > 20,000 ft AGL ■ Nimbus=rain, cumulus=heaped, stratus=layer, cirrus=high, alto=middle – – Altocumulus (AC), altostratus (AS) Cirrus (CI), cirrostratus (CS), cirrocumulus (CC) 3 Cloud & ceiling - METAR reporting Heights AGL 100’s of ft (<50 ft=mist or fog) Low (< 6,500 ft), middle (6,500-20,000 ft), high (> 20,000 ft) ■ Mist: 5/8 mi < vis < 7 mi. What is haze? ■ Fog: vis < 5/8 mi ■ Eighths of sky cover (FEW SCT BKN OVC) ■ VFR: ceiling* > 3,000 ft AGL, vis > 5 mi ■ Marginal VFR: ceiling 1-3 kft, vis 3-5 mi ■ IFR: ceiling < 1,000 ft and/or vis < 3 mi ■ Low IFR: ceiling < 500 ft and/or vis < 1 mi ■ ■ *Ceiling: lowest cloud layer having 5/8 or more coverage Infrared (IR) image Satellite imagery ■ IR imagery – – – ■ Satellite detects IR radiation emitted by earth and clouds Works at night as well as day Black means warm, white means cold Visible imagery – – Effectively a B&W snapshot of reflected visible light Only available during daylight hours ld co nt fro k ea W Area of sea fog (hard to see because similar temp to sea) High clouds Middle clouds Cirrus clouds Land is warm Visible (VIS) image nt fro ld o C Sea Area of fog 4