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Page 1© Crown copyright Snow forecasting Techniques.

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1 Page 1© Crown copyright Snow forecasting Techniques

2 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.

3 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.

4 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.

5 Page 5© Crown copyright Boston Blizzard January 2005

6 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

7 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%

8 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

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

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

11 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.

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

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

14 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

15 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%

16 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

17 Page 17© Crown copyright Boyden’s Snow Forecasting Table

18 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

19 Page 19© Crown copyright Boyden’s Snow Forecasting Table

20 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

21 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

22 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

23 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

24 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

25 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!

26 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

27 Page 27© Crown copyright Any questions?

28 Snow case study UK, 25 th November 2005 Newquay Airport

29 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.

30 : 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

31 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

32 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

33 Now lets’ see what really happened!


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