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Snow forecasting Techniques
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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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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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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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Boston Blizzard January 2005
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Probability of snow 90% 70% 50% 30% 10%Thickness (gpm) 5180 5238 5258 5292 5334
1000 - 500 hPa Thickness
Advantages: Easy to use
Disadvantages: Not necessarily representative of
the lowest levels of atmosphere
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1000-500hPa thickness chart
SNOW PROBABILITY (AMSL):
528.0 DM ≈ 40%
522.0 DM ≈ 80%
516.0 DM ≈ 95%
534.0 DM <10%
WHAT IS THE SNOW PROBABILITY AMSL AT POINTS:
A?
B?
C?
A
B
C
30-40%
>95%
<10%
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Probability of snow Mainly Readily turns Mainly Snowsnow to snow rain very 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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HEIGHT OF WET-BULB FREEZING LEVEL
0 10
850
1000
2 3 5
79
Wet-bulb freezing level ≈900m AGL
Snow unlikely
0 C950
900
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HEIGHT OF WET-BULB FREEZING LEVEL
0 10
850
1000
2 3 5
79
Wet-bulb freezing level ≈600m AGL
Rain readily turning to snow
0 C950
900
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Probability of snow 90% 70% 50% 30% 10%Height of 0 °C isotherm AGL (hPa) 12 25 35 45 61
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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HEIGHT OF ZERO DEGREE ISOTHERM
0 10
850
1000
2 3 5
79
0°C isotherm level ≈110hPa AGL
<10% probability of snow
0 C950
900
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HEIGHT OF ZERO DEGREE ISOTHERM
0 10
850
1000
2 3 5
79
0 C950
900
0°C isotherm level ≈45hPa AGL
30% probability of snow
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Probability of snow 90% 70% 50% 30% 10%Surface temp (°C) 0.3 +1.2 +1.6 +2.3 +3.9
Surface temperature
Advantages: Easy to use
Disadvantages: Takes no account of warm air aloft Takes no account of precipitation
intensity
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1 0
1 0
5
5
0
0
5 0
5 05 0
5 55 55 5
059999M02
1010
SCT050FEW035
039999
00
1006
BKN080FEW020
7000SHRA
00
M05
999
CAVOK
00
M04
997
CAVOK
M019999M01
1004
FEW012
02
M02
1005
CAVOK
004000M01
998
-SHGSBKN025CB
SCT010
9999 G33
009999M06
999G23
FEW040
019999M01
1016G29
BKN100SCT045
BKN016CB
059999
02
986
BKN015CB
002000
00
1009
SHSNSCT010
00
M05
1002
CAVOK
00
M05
1003
CAVOK
01
M05
1001
CAVOK
046000M03
1005G26-SHGS
FEW025
009999M03
1005
FEW022
2800 G49-SHRA
00
M06
1000
CAVOK
029999M02
994
OVC050SCT025
029999M01
1015
FEW015
009999M01
1011
-SHSNBKN050SCT025FEW012
039000
00
1012G26SHRAGR
BKN023CBSCT017CB
02
M02
1006
CAVOK
9999
9999
9999 A
009999M04
1003
FEW045
019999M03
1004
FEW030
00
M05
996G33
CAVOK
029999
00
1016
SCT060FEW018CB
049999M01
996G35
BKN033FEW024
9999
3200-SN
009999M05
1002
FEW030
029999
00
1008
VCSHSCT025TCU
FEW010
9999 A
9999 A
01
M05
1000
CAVOK
9999 G35
Surface temperature
aa
WHAT IS THE SNOW PROBABILITY AMSL AT POINTS:
A?
B?
C?
A
B
C
90% 70% 50% 30% 10%0.3 +1.2 +1.6 +2.3 +3.9
70%
40%
20%
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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 AMSLProbability of snow
90% 70% 50% 30% 10%C 1281 1290 1293 1298 1303
30
)( 1000 GRHHAC
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Boyden’s Snow Forecasting Table
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Example
Boyden’s Snow Forecasting Technique
1000-850: 128.7DM MSLP: 992hPa HGR: 100M
1hPa ≈ 10m
C = 1287 + (-80-100)/30 = 1287 – 180/3 = 1287 – 6 = 1281
= 90% 90% 70% 50% 30% 10%C 1281 1290 1293 1298 1303
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Boyden’s Snow Forecasting Table
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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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RAIN TURNING TO SNOW AT SURFACE
0 10
850
1000
2 3 5
79
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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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 slightly0 10
850
1000
2 3 5
79
0 C
RAIN TURNING TO SNOW AT SURFACE
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Rough guide 1hr continuous melting
snow - 600 feet of isothermal
4hrs continuous melting snow - 1200 feet of isothermal
Rain increasingly turning to snow at surface
0 10
850
1000
2 3 5
79
0 C
RAIN TURNING TO SNOW AT SURFACE
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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.
0 10
850
1000
2 3 5
79
0 C
RAIN TURNING TO SNOW AT SURFACE
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Summary
Each snow forecasting technique has strengths and weaknesses
Crucial forecasting points:1) Temperature and humidity of the lowest 1500M of
the atmosphere2) Intensity and duration of precipitation3) 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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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
500-1000 hPa thickness 5180m 5238m 5258m 5292m 5334m
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Any questions?
Snow case studyUK, 25th November 2005
10
10
5
5
0
0
50
5050
555555
059999M02
1010
SCT050FEW035
039999
00
1006
BKN080FEW020
7000SHRA
00
M05
999
CAVOK
00
M04
997
CAVOK
M019999M01
1004
FEW012
02
M02
1005
CAVOK
004000M01
998
-SHGSBKN025CB
SCT010
9999 G33
009999M06
999G23
FEW040
019999M01
1016G29
BKN100SCT045
BKN016CB
059999
02
986
BKN015CB
002000
00
1009
SHSNSCT010
00
M05
1002
CAVOK
00
M05
1003
CAVOK
01
M05
1001
CAVOK
046000M03
1005G26-SHGS
FEW025
009999M03
1005
FEW022
2800 G49-SHRA
00
M06
1000
CAVOK
029999M02
994
OVC050SCT025
029999M01
1015
FEW015
009999M01
1011
-SHSNBKN050SCT025FEW012
039000
00
1012G26SHRAGR
BKN023CBSCT017CB
02
M02
1006
CAVOK
9999
9999
9999 A
009999M04
1003
FEW045
019999M03
1004
FEW030
00
M05
996G33
CAVOK
029999
00
1016
SCT060FEW018CB
049999M01
996G35
BKN033FEW024
9999
3200-SN
009999M05
1002
FEW030
029999
00
1008
VCSHSCT025TCU
FEW010
9999 A
9999 A
01
M05
1000
CAVOK
9999 G35
Newquay Airport
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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.
1000-850: 129.9DM
1000-500: 525.6
MSLP: 1006hPa
T: +4.0°C
Time is now 250300Z
Probability of snow 90% 70% 50% 30% 10%1000-500hPa Thickness (gpm) 5180 5238 5258 5292 5334
Probability of snow Mainly snow Rain turns to snow Mainly rain Snow rareHeight 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) 12 25 35 45 61
Probability of snow 90% 70% 50% 30% 10%Surface temp (°C) 0.3 +1.2 +1.6 +2.3 +3.9
Prob of snow 90% 70% 50% 30% 10%Boyden C 1281 1290 1293 1298 1303 30
)( 1000 GRHHAC
50%
60hPa
300m
<10%
1296
TEMPO -SHRA
Time is now 250900Z
30
)( 1000 GRHHAC
Probability of snow 90% 70% 50% 30% 10%1000-500hPa Thickness (gpm) 5180 5238 5258 5292 5334
Probability of snow Mainly snow Rain turns to snow Mainly rain Snow rareHeight 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) 12 25 35 45 61
Probability of snow 90% 70% 50% 30% 10%Surface temp (°C) 0.3 +1.2 +1.6 +2.3 +3.9
Prob of snow 90% 70% 50% 30% 10%Boyden C 1281 1290 1293 1298 1303
1000-850: 130.1DM
1000-500: 519.4
MSLP: 1000hPa
T: +4.0°C
80%
35hPa
<300m
<10%
1296
TEMPO SHRASN
Time is now 251500Z
30
)( 1000 GRHHAC
Probability of snow 90% 70% 50% 30% 10%1000-500hPa Thickness (gpm) 5180 5238 5258 5292 5334
Probability of snow Mainly snow Rain turns to snow Mainly rain Snow rareHeight 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) 12 25 35 45 61
Probability of snow 90% 70% 50% 30% 10%Surface temp (°C) 0.3 +1.2 +1.6 +2.3 +3.9
Prob of snow 90% 70% 50% 30% 10%Boyden C 1281 1290 1293 1298 1303
1000-850: 128.3DM
1000-500: 515.9
MSLP: 995hPa
T: +0.0°C
>90%
12hPa
<300m
90%
1276
TEMPO +SHSN
Now lets’ see what really happened!
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