Snow forecasting Techniques © 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. © 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. © 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. © Crown copyright

Boston Blizzard January 2005 © Crown copyright

1000 - 500 hPa Thickness Advantages: Easy to use Disadvantages: Probability of snow 90% 70% 50% 30% 10% Thickness (gpm) 5180 5238 5258 5292 5334 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. © Crown copyright

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% © Crown copyright

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! © Crown copyright

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 © Crown copyright

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 © Crown copyright

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 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. © Crown copyright

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 © Crown copyright

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 © Crown copyright

Surface temperature Advantages: Easy to use Disadvantages: Probability of snow 90% 70% 50% 30% 10% Surface temp (°C) 0.3 +1.2 +1.6 +2.3 +3.9 Advantages: Easy to use Disadvantages: Takes no account of warm air aloft Takes no account of precipitation intensity © Crown copyright

Surface temperature WHAT IS THE SNOW PROBABILITY AMSL AT POINTS: A? B? aa 70% C 40% 20% 90% 70% 50% 30% 10% 0.3 +1.2 +1.6 +2.3 +3.9 A B © 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) H1000 is the height of the 1000 hPa surface AMSL HGR is the height of the station AMSL Correct notes! 100-500 NO 100-850 thickness Use the nomogram not the equation Probability of snow 90% 70% 50% 30% 10% C 1281 1290 1293 1298 1303 © Crown copyright

Boyden’s Snow Forecasting Table Probs are a little tricky – 70 is not midway between 50 and 90 Use this as in exam © Crown copyright

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% 90% 70% 50% 30% 10% C 1281 1290 1293 1298 1303 © Crown copyright

Boyden’s Snow Forecasting Table Probs are a little tricky – 70 is not midway between 50 and 90 Use this as in exam © 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 Correct notes! 100-500 NO 100-850 thickness Use the nomogram not the equation Equation is ambigous as to whether heights are agl or amsl. © Crown copyright

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 © Crown copyright

RAIN TURNING TO SNOW AT SURFACE 10 850 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 © Crown copyright

RAIN TURNING TO SNOW AT SURFACE 2 3 5 0 C 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 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 © Crown copyright

RAIN TURNING TO SNOW AT SURFACE 2 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 © Crown copyright

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! © Crown copyright

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 © Crown copyright

Any questions? © Crown copyright

Snow case study UK, 25th November 2005 Newquay Airport

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. © Crown copyright

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 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 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

Now lets’ see what really happened!