1. Snow forecasting Techniques
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2. 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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3. 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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4. 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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5. Boston Blizzard January 2005
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6. 1000 - 500 hPa Thickness Advantages: Easy to use Disadvantages:
Probability of snow 90% 70% 50% 30% 10%
Thickness (gpm)
Advantages:
Easy to use
Disadvantages:
Not necessarily representative of the lowest levels of atmosphere
Undercuts can give artificially high or low thicknesses. Snow is utterly dependent on the very bottom of the atmosphere so this technique is dependent on no cold or warm undercuts.
528 level comes from.
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7. 1000-500hPa thickness chart SNOW PROBABILITY (AMSL): 528.0 DM ≈ 40%
WHAT IS THE SNOW PROBABILITY AMSL AT POINTS: A? B? C? B A C
30-40%
>95%
<10%
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8. Height of wet-bulb freezing level
Probability of snow Mainly Readily turns Mainly Snow
snow to snow rain very rare
Height of wet-bulb <300 M <600 M 600 M 900 M
0 °C level AGL
Advantages:
Easy to use
Takes account of evaporative cooling (though not precipitation intensity)
Disadvantages:
Too course in borderline situations
Watch for cold surface air undercutting warm air!
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9. HEIGHT OF WET-BULB FREEZING LEVEL
850
Wet-bulb freezing level ≈900m AGL
Snow unlikely 2 3 5
0 C 7
900
950 9
1000 10
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10. HEIGHT OF WET-BULB FREEZING LEVEL
850
Wet-bulb freezing level ≈600m AGL
Rain readily turning to snow 2 3 5
0 C 7
900
950 9
1000 10
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11. Height of zero degree isotherm
Probability of snow 90% 70% 50% 30% 10%
Height of 0 °C isotherm AGL (hPa)
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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12. HEIGHT OF ZERO DEGREE ISOTHERM
850
0°C isotherm level ≈110hPa AGL
<10% probability of snow 2 3 5
0 C 7
900
950 9
1000 10
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13. HEIGHT OF ZERO DEGREE ISOTHERM
850
0°C isotherm level ≈45hPa AGL
30% probability of snow 2 3 5
0 C 7
900
950 9
1000 10
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14. Surface temperature Advantages: Easy to use Disadvantages:
Probability of snow 90% 70% 50% 30% 10%
Surface temp (°C)
Advantages:
Easy to use
Disadvantages:
Takes no account of warm air aloft
Takes no account of precipitation intensity
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15. Surface temperature WHAT IS THE SNOW PROBABILITY AMSL AT POINTS: A? B?aa
70% C
40%
20%
90% 70% 50% 30% 10% A B
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16. 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)
H1000 is the height of the 1000 hPa surface AMSL
HGR is the height of the station AMSL
Correct notes!
NO thickness
Use the nomogram not the equation
Probability of snow
90% 70% 50% 30% 10% C
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17. Boyden’s Snow Forecasting Table
Probs are a little tricky – 70 is not midway between 50 and 90
Use this as in exam
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18. Boyden’s Snow Forecasting Technique
Example
: 128.7DM MSLP: 992hPa HGR: 100M
1hPa ≈ 10m
C = ( )/30 = 1287 – 180/3 = 1287 – 6 = 1281
= 90%
90% 70% 50% 30% 10% C
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19. Boyden’s Snow Forecasting Table
Probs are a little tricky – 70 is not midway between 50 and 90
Use this as in exam
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20. 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
Correct notes!
NO thickness
Use the nomogram not the equation
Equation is ambigous as to whether heights are agl or amsl.
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21. RAIN TURNING TO SNOW AT SURFACE
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
850
1000 2 3 5 7 9
Melting starts as 0°C isotherm is reached
0 C 10
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22. RAIN TURNING TO SNOW AT SURFACE10
850
1000 2 3 5 7 9
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
0 C
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23. RAIN TURNING TO SNOW AT SURFACE2 3 5
0 C
Rough guide
1hr continuous melting snow feet of isothermal
4hrs continuous melting snow feet of isothermal
Rain increasingly turning to snow at surface
850 7
If sufficient cooling for snow to reach the ground without melting, an isothermal layer with temperature near zero is usually established near the ground with stratus pannus forming at, or very near, the surface. 9
1000 10
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24. RAIN TURNING TO SNOW AT SURFACE2 3 5
0 C
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.
850 7
If sufficient cooling for snow to reach the ground without melting, an isothermal layer with temperature near zero is usually established near the ground with stratus pannus forming at, or very near, the surface. 9
1000 10
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25. Summary Each snow forecasting technique has strengths and weaknesses
Crucial forecasting points:
Temperature and humidity of the lowest 1500M of the atmosphere
Intensity and duration of precipitation
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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26. Percentage probability of snow
TECHNIQUE
90%
70%
50%
30%
10%
Ht of 0°C isotherm hPa 12 25 35 45 61
Based on 900 hPa
108m
225m
315m
405m
550m
Surface temperature
-0.3°C
1.2°C
1.6°C
2.3°C
3.9°C
Ht of 0°C wet-bulb temp
<250m
370m
600m
750m
900m
hPa thickness
5180m
5238m
5258m
5292m
5334m
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27. Any questions?
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28. Snow case study
UK, 25th November 2005
Newquay Airport

29. 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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30. Time is now 250300Z 1000-850: 129.9DM 1000-500: 525.6 MSLP: 1006hPa
T: +4.0°C
TEMPO -SHRA
Probability of snow 90% 70% 50% 30% 10%
hPa Thickness (gpm)
Probability of snow Mainly snow Rain turns to snow Mainly rain Snow rare
Height of 0°C wet-bulb AGL <300M <600M ≥600M ≥900M
Probability of snow 90% 70% 50% 30% 10%
Height of 0 °C isotherm AGL (hPa)
Probability of snow 90% 70% 50% 30% 10%
Surface temp (°C)
Prob of snow 90% 70% 50% 30% 10%
Boyden C
50%
300m
60hPa
<10%
1296

31. Time is now 250900Z 1000-850: 130.1DM 1000-500: 519.4 MSLP: 1000hPa
T: +4.0°C
TEMPO SHRASN
Probability of snow 90% 70% 50% 30% 10%
hPa Thickness (gpm)
Probability of snow Mainly snow Rain turns to snow Mainly rain Snow rare
Height of 0°C wet-bulb AGL <300M <600M ≥600M ≥900M
Probability of snow 90% 70% 50% 30% 10%
Height of 0 °C isotherm AGL (hPa)
Probability of snow 90% 70% 50% 30% 10%
Surface temp (°C)
Prob of snow 90% 70% 50% 30% 10%
Boyden C
80%
<300m
35hPa
<10%
1296

32. Time is now 251500Z 1000-850: 128.3DM 1000-500: 515.9 MSLP: 995hPa
T: +0.0°C
TEMPO +SHSN
Probability of snow 90% 70% 50% 30% 10%
hPa Thickness (gpm)
Probability of snow Mainly snow Rain turns to snow Mainly rain Snow rare
Height of 0°C wet-bulb AGL <300M <600M ≥600M ≥900M
Probability of snow 90% 70% 50% 30% 10%
Height of 0 °C isotherm AGL (hPa)
Probability of snow 90% 70% 50% 30% 10%
Surface temp (°C)
Prob of snow 90% 70% 50% 30% 10%
Boyden C
>90%
<300m
12hPa
90%
1276

33. Now lets’ see what really happened!