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Snow forecasting Techniques. 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. Impact of snow when aircraft in flight.

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session objectives
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.
impact of snow when aircraft in flight
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.
impact of snow on aircraft at airfield
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.
1000 500 hpa thickness

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
1000 500hpa thickness chart
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?

B

A

C

30-40%

>95%

<10%

height of wet bulb freezing level
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!

height of wet bulb freezing level2
Wet-bulb freezing level ≈600m AGL

Rain readily turning to snow

HEIGHT OF WET-BULB FREEZING LEVEL

850

3

5

2

0 C

7

900

950

9

1000

10

0

height of zero degree isotherm

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.
height of zero degree isotherm1
0°C isotherm level ≈110hPa AGL

<10% probability of snow

HEIGHT OF ZERO DEGREE ISOTHERM

850

3

5

2

0 C

7

900

950

9

1000

10

0

height of zero degree isotherm2
0°C isotherm level ≈45hPa AGL

30% probability of snow

HEIGHT OF ZERO DEGREE ISOTHERM

850

3

5

2

0 C

7

900

950

9

1000

10

0

surface temperature

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
slide15

Surface temperature

90% 70% 50% 30% 10%

0.3 +1.2 +1.6 +2.3 +3.9

WHAT IS THE SNOW PROBABILITY AMSL AT POINTS:

A?

B?

C?

aa

70%

C

40%

20%

A

B

boyden s snow forecasting technique

90% 70% 50% 30% 10%

C 1281 1290 1293 1298 1303

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

Probability of snow

boyden s snow forecasting technique1

90% 70% 50% 30% 10%

C 1281 1290 1293 1298 1303

Boyden’s Snow Forecasting Technique

Example

1000-850: 128.7DM MSLP: 992hPa HGR: 100M

1hPa ≈ 10m

C = 1287 + (-80-100)/30 = 1287 – 180/3 = 1287 – 6 = 1281

= 90%

boyden s snow forecasting technique2
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
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

3

5

2

Melting starts as 0°C isotherm is reached

850

7

9

1000

RAIN TURNING TO SNOW AT SURFACE

0 C

10

0

rain turning to snow at surface1
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

3

5

2

850

7

9

1000

10

0

RAIN TURNING TO SNOW AT SURFACE

0 C

rain turning to snow at surface2
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

RAIN TURNING TO SNOW AT SURFACE

3

5

2

0 C

850

7

9

1000

10

0

rain turning to snow at surface3
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.

RAIN TURNING TO SNOW AT SURFACE

3

5

2

0 C

850

7

9

1000

10

0

summary
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!
scenario
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.
time is now 250300z
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%

1000-500hPa Thickness (gpm) 5180 5238 5258 5292 5334

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) 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

50%

300m

60hPa

<10%

1296

time is now 250900z
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%

1000-500hPa Thickness (gpm) 5180 5238 5258 5292 5334

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) 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

80%

<300m

35hPa

<10%

1296

time is now 251500z
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%

1000-500hPa Thickness (gpm) 5180 5238 5258 5292 5334

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) 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

>90%

<300m

12hPa

90%

1276

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