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Saunders fog point technique Saunders Technique Choose a representative midday temperature sounding (tephigram) − Same airmass as your area − Ideally upwind Amend the tephigram for time of maximum temperature − Draw on the station level pressure (QFE) − Mark on Tmax and Tdew at time of max temperature − Amend tephigram to fit maximum temperatures. © Crown copyright 2004 Page 2 Representative midday radiosonde © Crown copyright 2004 Page 3 Mark on Tmax and Tdew & modify ascent © Crown copyright 2004 Td Station Dew point T Station Dry bulb Page 4 Do a Normands point construction Normand’s point © Crown copyright 2004 Td Station Dew point T Station Dry bulb Page 5 Draw an isobar from Normands Point © Crown copyright 2004 Td Station Dew point T Station Dry bulb Page 6 Isobar intercepts dewpoint curve Follow SHMR from intercept down to surface Tf Fog point © Crown copyright 2004 Td Station Dew point T Station Dry bulb Page 7 Saunders Fog point Tf Fog point © Crown copyright 2004 Td Station Dew point T Station Dry bulb Page 8 Saunders Technique - 2 special cases Type I − Super-adiabat at surface Type II − Dry air aloft − Moist air near surface © Crown copyright 2004 Page 9 TYPE 1 – super-adiabat at surface Super-adiabat Td © Crown copyright 2004 Tmax Page 10 Ignore surface super-adiabat Type 1 Ignore Super-adiabat Tf Fog point © Crown copyright 2004 Td T Page 11 TYPE 2 – Much drier air aloft Much drier air aloft (large hydrolapse) Td © Crown copyright 2004 Tmax Page 12 Normands construction and isobar Fog point will be unrealistically low Type II Tf = ?? Td © Crown copyright 2004 Tmax Page 13 Extend ‘moister’ low level hydrolapse Extend dew point curve from lower level Type II Tf © Crown copyright 2004 Td Tmax Page 14 General fog points If a subsidence inversion is within 30hPa of the surface then Tf = Tdew © Crown copyright 2004 Page 15 28 20 16 12 9 7 5 2 5 0 .8 0 -1 -1 3 Inversion types 750 800 0 -2 850 900 48 Subsidence inversion 950 5 -2 1000 1050 28 20 16 12 9 7 5 2 3 0 -1 0 .8 40 30 20 10 0 0 -1 0 -2 5 -1 750 800 0 -2 850 48 900 950 5 -2 1000 Surface inversion 1050 40 30 20 10 0 0 -1 0 -2 © Crown copyright 2004 Page 16 General fog points If a subsidence inversion is within 30hPa of the surface then Tf = Td If it rains in the afternoon the fog point will be higher than the calculation If the radiosonde ascended through rain then the fog point will be lower If a sea breeze reaches your area after Tmax then use the coastal dew point as your fog point If your calculated fog point temperature is < 0C then the actual fog point will be lower due to hoar frost © Crown copyright 2004 Page 17 Fog or mist or nothing? Take your forecast minimum temperature and your fog point, If Tf - Tmin > 0 Fog is expected If Tf - Tmin < 0 Mist is expected with fog patches in river valleys If Tf - Tmin ≤ -2 Fog or mist are NOT expected © Crown copyright 2004 Page 18 When will the fog form? Temperature Night cooling curve (assuming no fog) Tf Amended cooling curve (formation of fog arrests surface cooling) Time of fog formation: 2100 Tmin 1200 1500 1800 2100 2400 0300 0600 0900 © Crown copyright 2004 Time Page 19 FOG FORMATION FORECASTING QUIZ 1) What are the 3 primary meteorological requirements for the formation of radiation fog? 2) Why is the dewpoint at midday often higher than the dewpoint at midnight? 3) When might the airmass dewpoint be equal to the fog point? © Crown copyright 2004 Page 20 FOG FORMATION FORECASTING QUIZ 1) What are the 3 primary meteorological requirements for the formation of radiation fog? ANS: Clear skies; low level moisture; calm or light surface winds 2) Why is the dewpoint at midday often higher than the dewpoint at midnight? ANS: Formation of dew during the evening 3) When might the airmass dewpoint be equal to the fog point? ANS: Humidity increasing with height near surface; subsidence inversion within 30hPa of surface. © Crown copyright 2004 Page 21 WARM ADVECTION FOG © Crown copyright 2004 Page 22 Warm advection fog formation WARM MOIST air moving over cool land or sea - Windward coasts Air cooled to dew point Light surface wind = fog - Overland Heating lifts fog into low cloud during the day, rapid in summer, slow in winter >10KT surface wind = low cloud (Tdry-Tdew)x350 = Stratus base above ground level © Crown copyright 2004 Page 23 UPSLOPE FOG Wind direction © Crown copyright 2004 Page 24 Upslope fog (stratus) formation Air forced to rise over hills Warm, moist, moderate to strong winds Stable air Air cools on ascent Very common on windward coasts and hills © Crown copyright 2004 Page 25 Upslope fog or stratus Airmass St/Sc Low-level wind © Crown copyright 2004 Page 26 Upslope fog or stratus Airmass St/Sc LCL Low-level wind © Crown copyright 2004 LCL = Lifting condensation level Page 27 Upslope fog or stratus Airmass St/Sc Upslope stratus LCL Low-level wind © Crown copyright 2004 Page 28 5 2 3 Forecasting upslope stratus 7 750 Too dry 9 LCL 1000 Select a representative ascent Then determine the lowest LCL This is a process of trial and error! Lowest LCL = Upslope Stratus Base 10 0 © Crown copyright 2004 Page 29 One last fog type, steam fog Cold advection fog ‘Arctic sea smoke’ Cold air flowing over relatively warm sea Low layer near surface becomes very unstable Convective swirls Evaporation then condensation © Crown copyright 2004 Page 30 Steam fog © Crown copyright 2004 Page 31 WHAT CLEARS FOG? Four main methods of clearance Increasing wind – lifts into stratus Increasing cloud cover – long wave radiation onto the top of the fog (most effective method) Advection of drier air – change of air mass or variations within an airmass Solar radiation – diurnal clearance. © Crown copyright 2004 Page 32 Solar radiation clearance technique Widely used in UK Representative ascent Usually a midnight or early morning ascent Ascent has to be modified for your station’s conditions at Tmin/dawn 3 types. © Crown copyright 2004 Page 33 Case A Sky Visible at Station – No inversion on ascent Representative midnight ascent QFE © Crown copyright 2004 Tdawn Plot QFE and Tdawn Page 34 Amend ascent for conditions at dawn ∆P is assumed to be a universal depth for all seasons and all locations ∆P = 10 hPa QFE © Crown copyright 2004 Tdawn Modified dawn ascent Page 35 Case A Sky Visible at Station – No inversion on ascent Fog Top (intersection of hydrolapse) SALR ∆P = 10 hPa QFE © Crown copyright 2004 Tdawn Tfog clear Page 36 Case B Sky obscured at station – inversion on ascent Representative midnight ascent © Crown copyright 2004 Page 37 Add p p in UK 6 hPa in May, Jun, Jul 12 hPa in Feb, Mar, Apr Aug, Sep, Oct 18 hPa in Nov, Dec, Jan © Crown copyright 2004 p Page 38 Plot Tdawn and determine fog top Interception with dewpoint curve SALR p Fog top QFE Tdawn © Crown copyright 2004 Tfog clear Tstratus clear Page 39 Case B Sky obscured at station – inversion on ascent Modified dawn ascent SALR Fog top QFE Tdawn © Crown copyright 2004 Tfog clear Tstratus clear Page 40 Case C Sky obscured at station – no inversion on ascent Representative 0000Z ascent © Crown copyright 2004 Page 41 Case C Sky obscured at station – no inversion on ascent p for UK 30 hPa in May, Jun, Jul 35 hPa in Feb, Mar, Apr, Aug, Sep, Oct 40 hPa in Nov, Dec, Jan p QFE © Crown copyright 2004 Page 42 Case C Sky obscured at station – no inversion on ascent Modified dawn ascent SALR Fog top p QFE Tdawn © Crown copyright 2004 Tfog clear Tstratus clear Page 43 Forecasting time of fog clearance 1) Calculate unadjusted Tmax temperature using the formula Tu = -192.65+0156h where h is the 1000850hPa thickness (in gpm) 2) Adjust Tmax for persistence of fog using Fig1 − For thick fog (≥10hPa deep) use curve 3 − For thin fog (sky visible) use curve 1 3) − − − − Plot temperature rise against time assuming: persistence of fog straight line connecting Tmin and Tmax Tmin = Fog point = sunrise + 1 hour Tmax = 1400 local time 4) Approximate fog clearance time is intersection of calculated fog clearance temperature with straight line © Crown copyright 2004 Page 44 Figure 1: Tmax adjustment graph © Crown copyright 2004 Page 45 FOG CLEARANCE FORECASTING QUIZ 1) What are the 4 primary means of clearing fog? 2) Describe the 3 types of tephigram fog clearance technique 3) What conditions are required for steam fog? © Crown copyright 2004 Page 46 FOG CLEARANCE FORECASTING QUIZ 1) What are the 4 primary means of clearing fog? ANS: Cloud spreading over top of fog; drier air; increasing wind; solar radiation 2) Describe the 3 types of tephigram fog clearance technique ANS: Sky visible, no inversion; sky obscured, inversion; sky obscured no inversion 3) What conditions are required for steam fog? ANS: Polar or arctic air over a comparatively warm sea. Low cloud © Crown copyright 2004 Page 47
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