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Page 1© Crown copyright Snow forecasting Techniques

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Page 2© Crown copyright Session Objectives Understanding of the impact of snow on aviation operations Understanding of the strengths and weaknesses of a variety of snow forecasting techniques Be able to apply the techniques to a real case study.

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Page 3© Crown copyright Impact of snow when aircraft in flight Poor visibility and low cloud base Snow ‘packing’ restricting: airflow into engines preventing retraction of landing gear Blocking or Pitot tubes ‘Wet’ snow (T>0°C) will result in airframe icing.

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Page 4© Crown copyright Impact of snow on aircraft at airfield Poor visibility and low cloud base Snowfall accumulation on airframe: Aerodynamics all up weight of aircraft windshield obscured Runway contamination: degrading braking action. obscuring runway and runway lights 1mm of rain = 1cm of snow.

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Page 5© Crown copyright Boston Blizzard January 2005

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Page 6© Crown copyright Probability of snow90%70%50%30%10% Thickness (gpm) hPa Thickness Advantages: Easy to use Disadvantages: Not necessarily representative of the lowest levels of atmosphere

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Page 7© Crown copyright hPa thickness chart SNOW PROBABILITY (AMSL): DM ≈ 40% DM ≈ 80% DM ≈ 95% DM <10% WHAT IS THE SNOW PROBABILITY AMSL AT POINTS: A? B? C? A B C 30-40% >95% <10%

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Page 8© Crown copyright Probability of snowMainlyReadily turnsMainlySnow snow to snowrainvery rare Height of wet-bulb<300 M<600 M 600 M 900 M 0 °C level AGL Height of wet-bulb freezing level Watch for cold surface air undercutting warm air! Advantages: Easy to use Takes account of evaporative cooling (though not precipitation intensity) Disadvantages: Too course in borderline situations

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Page 9© Crown copyright HEIGHT OF WET-BULB FREEZING LEVEL Wet-bulb freezing level ≈900m AGL Snow unlikely 0 C

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Page 10© Crown copyright HEIGHT OF WET-BULB FREEZING LEVEL Wet-bulb freezing level ≈600m AGL Rain readily turning to snow 0 C

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Page 11© Crown copyright Probability of snow90%70%50%30%10% Height of 0 °C isotherm AGL (hPa) Height of zero degree isotherm Advantages: Easy to use Disadvantages: Too coarse in borderline situations Takes no account of precipitation intensity or evaporative cooling if low level air is dry.

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Page 12© Crown copyright HEIGHT OF ZERO DEGREE ISOTHERM 0°C isotherm level ≈110hPa AGL <10% probability of snow 0 C

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Page 13© Crown copyright HEIGHT OF ZERO DEGREE ISOTHERM C 0°C isotherm level ≈45hPa AGL 30% probability of snow

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Page 14© Crown copyright Probability of snow90%70%50%30%10% Surface temp (°C) Surface temperature Advantages: Easy to use Disadvantages: Takes no account of warm air aloft Takes no account of precipitation intensity

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Page 15© Crown copyright Surface temperaturea WHAT IS THE SNOW PROBABILITY AMSL AT POINTS: A? B? C? A B C 90% 70% 50% 30% 10% % 40% 20%

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Page 16© Crown copyright Boyden’s Snow Forecasting Technique Where: C is the corrected value of the 1000–850 hPa thickness (gpm) A is the actual 1000–850 hPa thickness (gpm) H 1000 is the height of the 1000 hPa surface AMSL H GR is the height of the station AMSL Probability of snow 90%70%50%30%10% C

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Page 17© Crown copyright Boyden’s Snow Forecasting Table

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Page 18© Crown copyright Example Boyden’s Snow Forecasting Technique : 128.7DMMSLP: 992hPaH GR : 100M 1hPa ≈ 10m C = ( )/30= 1287 – 180/3= 1287 – 6= 1281 = 90% 90%70%50%30%10% C

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Page 19© Crown copyright Boyden’s Snow Forecasting Table

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Page 20© Crown copyright Boyden’s Snow Forecasting Technique Advantages: Samples crucial low levels of atmosphere Gives precise values Disadvantages: Inaccurate if there is a cold or warm undercut near surface Takes no account of precipitation intensity

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Page 21© Crown copyright RAIN TURNING TO SNOW AT SURFACE Melting starts as 0°C isotherm is reached DRY Low level air temperature initially above freezing Snow falls into the lower levels of this atmosphere Snow falling into a layer with an above freezing temperature melts and may evaporate if layer is unsaturated Large amounts of latent heat required 0 C

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Page 22© Crown copyright Cooling occurs as snow melts just below this level Temperature profile changes Profile starts to follow the 0°C isotherm down toward the surface Dew point increases slightly C RAIN TURNING TO SNOW AT SURFACE

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Page 23© Crown copyright Rough guide 1hr continuous melting snow feet of isothermal 4hrs continuous melting snow feet of isothermal Rain increasingly turning to snow at surface C RAIN TURNING TO SNOW AT SURFACE

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Page 24© Crown copyright If melting snow is of heavy intensity then isotherm can reach surface LESSON: In borderline snow situations, if precipitation is forecast to be heavy and prolonged, forecast snow C RAIN TURNING TO SNOW AT SURFACE

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Page 25© Crown copyright Summary Each snow forecasting technique has strengths and weaknesses Crucial forecasting points: 1)Temperature and humidity of the lowest 1500M of the atmosphere 2)Intensity and duration of precipitation 3)Height of airfield Small changes in 1 to 3 above can lead to big forecast errors Each technique is a probability forecasting assuming that precipitation is occurring If it is dry then probability of snow = 0 no matter how cold it is!

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Page 26© Crown copyright Percentage probability of snow TECHNIQUE90%70%50%30%10% Ht of 0°C isotherm hPa Based on 900 hPa108m225m315m405m550m Surface temperature-0.3°C1.2°C1.6°C2.3°C3.9°C Ht of 0°C wet-bulb temp<250m370m600m750m900m hPa thickness5180m5238m5258m5292m5334m

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Page 27© Crown copyright Any questions?

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Snow case study UK, 25 th November 2005 Newquay Airport

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Page 29© Crown copyright Scenario Please write down the following: Newquay Airport: EGDG 51°N 05°W Height 150M You will be given snow forecasting information for 0300Z, 0900Z and 1500Z Calculate the snow probability using the techniques taught this morning Use the tephigrams to forecast the intensity of of any precipitation eg TEMPO +SHRA.

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: 129.9DM : MSLP: 1006hPa T: +4.0°C Time is now Z Probability of snow90%70%50%30%10% hPa Thickness (gpm) Probability of snowMainly snow Rain turns to snow Mainly rain Snow rare Height of 0°C wet-bulb AGL <300M <600M ≥600M ≥900M Probability of snow90%70%50%30%10% Height of 0 °C isotherm AGL (hPa) Probability of snow90%70%50%30%10% Surface temp (°C) Prob of snow 90%70%50%30%10% Boyden C % 60hPa 300m <10% 1296 TEMPO -SHRA

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Time is now Z Probability of snow90%70%50%30%10% hPa Thickness (gpm) Probability of snowMainly snow Rain turns to snow Mainly rain Snow rare Height of 0°C wet-bulb AGL <300M <600M ≥600M ≥900M Probability of snow90%70%50%30%10% Height of 0 °C isotherm AGL (hPa) Probability of snow90%70%50%30%10% Surface temp (°C) Prob of snow 90%70%50%30%10% Boyden C : 130.1DM : MSLP: 1000hPa T: +4.0°C 80% 35hPa <300m <10% 1296 TEMPO SHRASN

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Time is now Z Probability of snow90%70%50%30%10% hPa Thickness (gpm) Probability of snowMainly snow Rain turns to snow Mainly rain Snow rare Height of 0°C wet-bulb AGL <300M <600M ≥600M ≥900M Probability of snow90%70%50%30%10% Height of 0 °C isotherm AGL (hPa) Probability of snow90%70%50%30%10% Surface temp (°C) Prob of snow 90%70%50%30%10% Boyden C : 128.3DM : MSLP: 995hPa T: +0.0°C >90% 12hPa <300m 90% 1276 TEMPO +SHSN

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Now lets’ see what really happened!

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