roy rasmussen national center for atmospheric research presentation to ofcm meeting feb. 14, 2008
DESCRIPTION
On the use of Liquid Water Equivalent (LWE) for estimating snowfall intensity and freezing drizzle intensity versus the use of visibility. Roy Rasmussen National Center for Atmospheric Research Presentation to OFCM meeting Feb. 14, 2008. Background. - PowerPoint PPT PresentationTRANSCRIPT
On the use of Liquid Water Equivalent On the use of Liquid Water Equivalent (LWE) for estimating snowfall intensity (LWE) for estimating snowfall intensity
and freezing drizzle intensity versus and freezing drizzle intensity versus the use of visibilitythe use of visibility
Roy Rasmussen National Center for Atmospheric Research
Presentation to OFCM meetingFeb. 14, 2008
Background
• Rough ice accumulation on a wing, the thickness of a thumbnail, leads to a loss of lift and increase in drag of 25%, requiring that aircraft be deiced with Type I (deicing), and/or Type II or IV anti-icing fluids (both either ethylene or propylene glycol based) to remove ice and protect against re-icing before takeoff.
• Holdover time is the period that deicing/anti-icing fluids protect against re-icing.
BackgroundBackground Outdoor and indoor testing of deicing fluids for Outdoor and indoor testing of deicing fluids for
endurance time has shown that the liquid endurance time has shown that the liquid equivalent snowfall rate is the main cause for equivalent snowfall rate is the main cause for fluid failure:fluid failure:– Dilutes the fluid, leading to runoffDilutes the fluid, leading to runoff– Causes the freeze temperature of the fluid to Causes the freeze temperature of the fluid to
increaseincrease
Holdover times are also sensitive to fluid Holdover times are also sensitive to fluid temperaturetemperature
Deicing fluid endurance time as a function of liquid equivalent rate
Regression analysis that provides a power law relationship is used for each fluid and each dilution for each weather condition
0
10
20
30
40
50
60
70
80
90
100
110
120
0 5 10 15 20 25 30 35 40 45Plate Pan
Rate of Precipitation (g/dm²/h)
Fluid Fa
ilure T
ime on
Plate (m
in)
0 to -3°C-3 to -14°CCurrent SAE HOT (0 to -3°C)Current SAE HOT (-3 to -14°C)Current SAE HOT (Below -14°C)-3-14-25
# of Tests = 37
FAA TYPE I HOLDOVER TIME GUIDELINE
Table 1. FAA Guideline for Holdover Times SAE Type I Fluid Mixtures as a Function of Weather Conditions and OAT.
CAUTION: THIS TABLE IS FOR DEPARTURE PLANNING ONLY AND SHOULD BE USED IN CONJUNCTION WITH PRE-TAKEOFF CHECK PROCEDURES.
Outside Air
Temperature
Approximate Holdover Times Under Various Weather Conditions (hours: minutes)
Snow/Snow Grains Degrees Celsius
Degrees Fahrenheit
Active Frost
Freezing Fog Very
Light Light Moderate
Freezing Drizzle*
Light Freezing
Rain
Rain on Cold Soaked Wing**
Other‡
-3 and above
27 and above 0:45 0:11 - 0:17 0:18-0:22 0:11 - 0:18 0:06 - 0:11 0:09 - 0:13 0:02 - 0:05 0:02-0:05
below -3 to -6
below 27 to 21 0:45 0:08 - 0:13 0:14-0:17 0:08 - 0:14 0:05 - 0:08 0:05 - 0:09 0:02 - 0:05
below -6 to -10
below 21 to 14 0:45 0:06 - 0:10 0:11-0:13 0:06 - 0:11 0:04 - 0:06 0:04 - 0:07 0:02 - 0:05
CAUTION: No holdover time guidelines exist
below -10
below 14 0:45 0:05 - 0:09 0:07-0:08 0:04 - 0:07 0:02 - 0:04
THE RESPONSIBILITY FOR THE APPLICATION OF THESE DATA REMAINS WITH THE USER. * Use light freezing rain holdover times if positive identification of freezing drizzle is not possible ** This column is for use at temperatures above 0 degrees Celsius (32 degrees Fahrenheit) only ‡ Heavy snow, snow pellets, ice pellets, moderate and heavy freezing rain, hail TO USE THESE TIMES, THE FLUID MUST BE HEATED TO A MINIMUM TEMPERATURE OF
60C (140F) AT THE NOZZLE AND AT LEAST 1 LITER/M2 ( 2 GALS/100FT2) MUST BE APPLIED TO DEICED SURFACES SAE Type I fluid/water mixture is selected so that the freezing point of the mixture is at least 10 C
(18 F) below OAT. CAUTIONS:
THE TIME OF PROTECTION WILL BE SHORTENED IN HEAVY WEATHER CONDITIONS. HEAVY PRECIPITATION RATES OR HIGH MOISTURE CONTENT, HIGH WIND VELOCITY, OR JET BLAST MAY REDUCE HOLDOVER TIME BELOW THE LOWEST TIME STATED IN THE RANGE. HOLDOVER TIME MAY BE REDUCED WHEN AIRCRAFT SKIN TEMPERATURE IS LOWER THAN OAT.
SAE TYPE I FLUID USED DURING GROUND DEICING/ANTI-ICING IS NOT INTENDED FOR AND DOES NOT PROVIDE PROTECTION DURING FLIGHT. August 2006
2.5 – 4 g/d2/hrVery Light
4 – 10 g/d2/hrLight
10-25 g/d2/hrModerate
> 25 g/d2/hrHeavy
0
20
40
60
80
100
120
0 10 20 30 40 50
Octagon 50% data
Endurance Time (min) T = 0 CEndurance Time (min) T = -5 C Endurance Time (min) T = -10 C
Endu
ranc
e Ti
me
(min
)
Precipitation Rate (g/d2/hr)
Ver
y Li
ght
Lig ht
Mod
erat
e
Hea
vy
FAA TYPE IV HOLDOVER TIME GUIDELINE TABLE 4 - FAA Guideline for Holdover Times SAE Type IV Fluid Mixtures as a Function of
Weather Conditions and OAT.
CAUTION: THIS TABLE IS FOR DEPARTURE PLANNING ONLY AND SHOULD BE USED IN CONJUNCTION WITH PRE-TAKEOFF CHECK PROCEDURES.
Outside Air Temperature Approximate Holdover Times Under Various Weather Conditions (hours: minutes)
Degrees Celsius
Degrees Fahrenheit
Type IV Fluid Concentration
Neat-Fluid/Water (Volume %/Volume %)
Active Frost
Freezing Fog Snow/Snow Grains
Freezing Drizzle*
Light Freezing Rain
Rain on Cold Soaked Wing**
Other‡
100/0 12:00 1:15-2:30 0:35-1:15 0:40-1:10 0:25-0:40 0:10-0:50
75/25 5:00 1:05-1:45 0:20-0:55 0:35-0:50 0:15-0:30 0:05-0:35
-3 and above
27 and above
50/50 3:00 0:15-0:35 0:05-0:15 0:10-0:20 0:05-0:10
100/0 12:00 0:20-1:20 0:20-0:40 ***0:20-0:45 ***0:10-0:25 below
-3 to -14
below
27 to 7 75/25 5:00 0:25-0:50 0:15-0:35 ***0:15-0:30 ***0:10-0:20
CAUTION: No holdover time guidelines exist
below
-14 to -25
below
7 to -13
100/0 12:00 0:15-0:40 0:15-0:30
below -25 below -13 100/0 SAE Type IV fluid may be used below -25 °C (-13 °F) provided the freezing point of the fluid is at least 7 °C(13 °F) below the OAT and the aerodynamic acceptance criteria are met. Consider use of SAE Type I when SAE Type IV fluid cannot be used.
THE RESPONSIBILITY FOR THE APPLICATION OF THESE DATA REMAINS WITH THE USER. * Use light freezing rain holdover times if positive identification of freezing drizzle is not possible ** This column is for use at temperatures above 0 °C (32°F) only *** No holdover time guidelines exist for this condition below -10 C (14 F) ‡ Snow pellets, ice pellets, heavy snow, moderate and heavy freezing rain, and hail
CAUTIONS:
THE TIME OF PROTECTION WILL BE SHORTENED IN HEAVY WEATHER CONDITIONS. HEAVY PRECIPITATION RATES OR HIGH MOISTURE CONTENT, HIGH WIND VELOCITY, OR JET BLAST MAY REDUCE HOLDOVER TIME BELOW THE LOWEST TIME STATED IN THE RANGE. HOLDOVER TIME MAY BE REDUCED WHEN AIRCRAFT SKIN TEMPERATURE IS LOWER THAN OAT.
SAE TYPE IV FLUID USED DURING GROUND DEICING/ANTI-ICING IS NOT INTENDED FOR AND DOES NOT PROVIDE PROTECTION DURING FLIGHT.
Moderate Light
0
20
40
60
80
100
120
0 10 20 30 40 50
Octagon 50% data
Endurance Time (min) T = 0 CEndurance Time (min) T = -5 C Endurance Time (min) T = -10 C
Endu
ranc
e Ti
me
(min
)
Precipitation Rate (g/d2/hr)
Upper range of snow cell
at -5 C
Lower range ofsnow cell
at -5 C
Type IV
Snowfall Intensity by Visibility
NWS Category
Light
Moderate
Heavy
Visibility Liquid Equivalent Snowfall Rate
> 5/8 mi < 1.0 mm/hr
5/8-5/16 mi 1.0 to 2.5 mm/hr
1/4 mi or less > 2.5 mm/hr
- SN
SN
+ SN
Background
• An analysis of five aircraft ground deicing accidents showed that the snowfall rate as determined by a weighing snowgauge was nearly identical for each accident (near 2.5 mm/hr), but that the snowfall intensity reported to the pilot from the METAR using visibility ranged from light to heavy.
“Common Snowfall Conditions Associated with Aircraft Takeoff Accidents”, 2000: R.
Rasmussen, J. Cole, R.K. Moore, and M. Kuperman. J. Aircraft, Vol. 37, 110 – 116.
Liqu
id-e
quiv
alen
t Pre
cipi
tatio
n R
ate
(in/h
r)
LightS-
ModS
HeavyS+
Snow Intensity Based on Visibility Alone
.004 in/hr
.04 in/hr
.08 in/hr
.10 in/hr
.4 in/hr
x*
+
Accidents Plotted Against Visibility AndLiquid-Equivalent Precipitation Rate
Background• Since the current method to estimate snow intensity is based on
visibility, I decided to do a follow up study to examine this relationship in more detail.
• Study based on observations from well shielded weighing snowgauges and calibrated automatic visibility sensors. Visibility sensor calibrated during snow conditions by a manual observer.
• Theoretical study also performed and compared to the observations.
“The Estimation of Snowfall Rate Using Visibility”, 1999: R. Rasmussen, J. Vivekanandan, J. Cole, B. Myers, and C. Masters. Journal of Applied Meteorology, Vol. 38, 1542 – 1563.
0.1
1
10
0.01 0.1 1 10
Vis
ibili
ty [k
m]
Snowfall Rate [mm/hr]
'94-'95
SN (Light)
SN (Moderate)
+SN (Heavy)
Boston
LaGuardia
Washington, D.C.
Denver Newark
Observations of Visibility and Liquid Equivalent Snowfall Rate from the NCAR Marshall Field Site
0.1
1
10
0.01 0.1 1 10
Vis
ibili
ty [k
m]
Snowfall Rate [mm/hr]
'94-'95
SN (Light)
SN (Moderate)
+SN (Heavy)
Boston
LaGuardia
Washington, D.C.
Denver Newark
Observations of Visibility and Liquid Equivalent Snowfall Rate from the NCAR Marshall Field Site
Light Mod Heavy
0.1
1
10
0.01 0.1 1 10
Vis
ibili
ty [k
m]
Snowfall Rate [mm/hr]
'94-'95
SN (Light)
SN (Moderate)
+SN (Heavy)
Boston
LaGuardia
Washington, D.C.
Denver Newark
Observations of Visibility and Liquid Equivalent Snowfall Rate from the NCAR Marshall Field Site
Light Mod Heavy
0.1
1
10
0.01 0.1 1 10
Vis
ibili
ty [k
m]
Snowfall Rate [mm/hr]
'94-'95
SN (Light)
SN (Moderate)
+SN (Heavy)
Boston
LaGuardia
Washington, D.C.
Denver Newark
Observations of Visibility and Liquid Equivalent Snowfall Rate from the NCAR Marshall Field Site
Light Mod Heavy
0.1
1
10
0.01 0.1 1 10
Vis
ibili
ty (k
m)
Snowfall Rate (mm/hr)
Wet Snowflakes
Dry snowflakes(aggretated snowcrystals)
Theoretical Relationship between Visibility andLiquid Equivalent Snowfall Rate for Dry and Wet snow
Theoretical Relationship between Visibility and Liquid Equivalent Snowfall Rate for various crystal types and degree of riming.
0.1
1
10
0.01 0.1 1 10
Vis
ibili
ty (k
m)
Snowfall Rate (mm/hr)
Wet Snow
DrySnow
1/17 - 1/18/96
0.1
1
10
0.01 0.1 1 10
Vis
ibili
ty (k
m)
ETI Snowfall Rate (mm/hr)
N1a R3b
C1h
P1a
Wet
R1a Dry
P1e
94-95
R2a Key
Wet - Wet SnowflakesDry - Dry SnowflakesP1e - DendritesP1a - Hexagonal platesC1h - Thick platesN1a - Elementary needleR2a - Densely rimed plateR3b - Heavily rimed dendriteR1a - Rimed elementary needle
Boston
LaGuardia
Washington, D.C.
DenverNewark
Theory, Observations, and Accident Data Combined
LOG LOG PLOTVISIBILITY VS. RATE (1995-96 to 2001-02)
1.0 (100%)
2.5 (100%)
0.5 (85%)
1.0 (94%)
253
0.7 (~95%)
1.1 (~95%)
1.9 (~95%)
0.1
1
10
1 10 100Plate Pan Rate (g/dm2/h)
AES
Visib
ility (
Statu
te Mi
les)
TCFAACaptures >95%
Night, <-1ºC
Data Points = 3816Days Represented = 89Hours of Data = 381.6
Courtesy of APS Aviation
Seven years of data from Montreal
LOG LOG PLOTVISIBILITY VS. RATE (1995-96 to 2001-02)
1.0 (100%)
2.5 (100%)
0.5 (85%)
1.0 (94%)
253
0.7 (~95%)
1.1 (~95%)
1.9 (~95%)
0.1
1
10
1 10 100Plate Pan Rate (g/dm2/h)
AES
Visib
ility (
Statu
te Mi
les)
TCFAACaptures >95%
Night, <-1ºC
Data Points = 3816Days Represented = 89Hours of Data = 381.6
Courtesy of APS Aviation
Seven years of data from Montreal
Light
Moderate
Heavy
0
0.5
1
1.5
2
2.5
3
0 0.5 1 1.5 2 2.5 3
Day versus Night Visibility using Vd=-3Vn/ln(Vn*0.00336)
A
Nig
httim
e V
isib
ility
, Vn
(mile
s)
Daytime Visibility, Vd (miles)
Day vs Night Visibility• Visibility at night is twice as good as the visibility during the day for the
same snowfall rate
• Means that going from day to night decreases the snowfall rate from heavy to moderate, or from moderate to light.
• The LaGuardia accident (3/22/92) occurred at night
– Occurred under wet snow conditions
– Snow intensity of light
– Occurred at night
Summary• Both observations and theory show that a given visibility is not
uniquely related to only one snowfall rate due to:
– a variety of crystal types– varying degrees of riming – varying degrees of aggregation– wet snow– difference in visibility between day and night for a given
extinction coefficient
• Above suggests that the variation in visibility during the previous deicing accidents is real and potentially misleading.
FREQUENCY OF DE/ANTI-ICING OPERATIONS
(ALL AIRPORTS) – SURVEY 2000-03
Type I / IV 2000-2003
Frost33%
Snow 56%
ZP7%
Other 4%
Visibility Criteria for Snow Intensity
Time of Day
Temp.Visibility (Statute Mile)
(C) (F) 2 1/2 2 1 1/2 1 3/4 1/2 1/4
Day
colder/equal-1
colder/equal 30VeryLight
Very Light Light Light Moderate Moderate Heavy
Snowfall Intensity
warmer than-1
warmer than 30VeryLight Light Light Moderate Moderate Heavy Heavy
Night
colder/equal-1
colder/equal 30VeryLight Light Moderate Moderate Heavy Heavy Heavy
warmer than-1
warmer than 30VeryLight Light Moderate Heavy Heavy Heavy Heavy
NOTE: Based upon technical report, “The Estimation of Snowfall Rate Using Visibility,” Rasmussen, et al., Journal of Applied Meteorology, October 1999 and additional in situ data.
HEAVY = Caution - no holdover time guidelines exist
Modified Visibility Criteria for Snow Intensity Based on Temperature and Day or Night
Time of Day
Temp.Visibility (Statute Mile)
(C) (F) 2 1/2 2 1 1/2 1 3/4 1/2 1/4
Day
colder/equal-1
colder/equal 30VeryLight
Very Light Light Light Moderate Moderate Heavy
Snowfall Intensity
warmer than-1
warmer than 30VeryLight Light Light Moderate Moderate Heavy Heavy
Night
colder/equal-1
colder/equal 30VeryLight Light Moderate Moderate Heavy Heavy Heavy
warmer than-1
warmer than 30VeryLight Light Moderate Heavy Heavy Heavy Heavy
NOTE: Based upon technical report, “The Estimation of Snowfall Rate Using Visibility,” Rasmussen, et al., Journal of Applied Meteorology, October 1999 and additional in situ data.
HEAVY = Caution - no holdover time guidelines exist
PROBABILITY OF SNOW IN EACH HOT TABLE TEMPERATURE RANGE – TYPE I FLUIDS
Temperature (°C)
Very Light Snow
Light Snow
Moderate Snow
Heavy Snow Total
-3 and above 20.1% 8.0% 9.9% 1.2% 39.2%
below -3 to -6 12.4% 5.2% 6.1% 0.8% 24.4%
below -6 to -10 11.6% 4.1% 4.9% 0.6% 21.2%
below -10 9.0% 2.6% 3.1% 0.5% 15.2%
Total 53.2% 19.9% 24.0% 3.0% 100.0%
• The liquid equivalent rate of snow, freezing rain, snow pellets, ice pellets, and freezing drizzle is the primary factor causing aircraft deicing fluids to fail.
• The current NWS estimates of snowfall and drizzle intensity available to pilots via METARS through ASOS and ATIS is based on visibility, not liquid equivalent.
• The use of a modified visibility table is only a stop-gap measure. Need real-time liquid equivalent!
The lack of liquid equivalent and precipitation type real-time information updated every minute is having a significant impact on aircraft deicing operations and is a significant safety factor
Hotplate (Yankee)
Weighing Snowgauge (GEONOR)
Liquid WaterEquivalent Measurements
ASOS weighing precipitation gauge (AWPAG)Manufactured by OTT
CHANGE PARAGRAPH 2.6.6 TO INCLUDE CHANGES APPROVED BY ACCB AND APMC?CHANGE PARAGRAPH 2.6.6 TO INCLUDE CHANGES APPROVED BY ACCB AND APMC? 2.6.6 Algorithms Used by Automated Stations. 2.6.6 Algorithms Used by Automated Stations. Automated stations shall use algorithms that Automated stations shall use algorithms that
conform with the latest conform with the latest Federal Standard Algorithms for Automated Weather Observing Systems Federal Standard Algorithms for Automated Weather Observing Systems published by the OFCM, published by the OFCM, or or subsequently approved by the ASOS Configuration Control Board subsequently approved by the ASOS Configuration Control Board and ASOS Program Management Council.and ASOS Program Management Council. These algorithms do not apply to previously authorized These algorithms do not apply to previously authorized systems, which may continue to operate until replaced or modified.systems, which may continue to operate until replaced or modified.
8.4 Present Weather Observing Standards8.4 Present Weather Observing Standards 8.4.1 Qualifiers. 8.4.1 Qualifiers. Present weather qualifiers fall into two categories: intensity or proximity Present weather qualifiers fall into two categories: intensity or proximity
and descriptors. Qualifiers may be used in various combinations to describe weather and descriptors. Qualifiers may be used in various combinations to describe weather phenomena.phenomena.
a. a. Intensity/Proximity. Intensity/Proximity. The intensity qualifiers are: light, moderate, and heavy. The The intensity qualifiers are: light, moderate, and heavy. The proximity qualifier is vicinity.proximity qualifier is vicinity.
(1) (1) Intensity of Precipitation. Intensity of Precipitation. When more than one form of precipitation is occurring at a When more than one form of precipitation is occurring at a time or precipitation is occurring with an obscuration, the intensities determined shall be no time or precipitation is occurring with an obscuration, the intensities determined shall be no greater than that which would be determined if any forms were occurring alone. The greater than that which would be determined if any forms were occurring alone. The intensity of precipitation shall be identified as light, moderate, or heavyintensity of precipitation shall be identified as light, moderate, or heavy in accordance in accordance with one of the following:with one of the following:
SHOULD WE CHANGE INTENSITY OF SNOW AND DRIZZLE AS FOLLOWS?SHOULD WE CHANGE INTENSITY OF SNOW AND DRIZZLE AS FOLLOWS? (b) (b) Intensity of Snow and Drizzle. Intensity of Snow and Drizzle. The manually-derived intensity of snow and drizzle shall be The manually-derived intensity of snow and drizzle shall be
based on the reported surface visibility in accordance with Table 8-6, when occurring alone. based on the reported surface visibility in accordance with Table 8-6, when occurring alone. Where possible, the intensity of snow or drizzle may be automatically determined by sensors Where possible, the intensity of snow or drizzle may be automatically determined by sensors operating in accordance with the Federal Standard Algorithms for Automated Observing operating in accordance with the Federal Standard Algorithms for Automated Observing Systems Used for Aviation Purposes using a liquid water equivalent rate. Systems Used for Aviation Purposes using a liquid water equivalent rate.
Table 8-4. Table 8-4. Intensity of Snow Based on Liquid Water Equivalent RateIntensity of Snow Based on Liquid Water Equivalent Rate IntensityIntensity CriteriaCriteria Light Less than 1 mm*/hr Moderate 1 – 2.5 mm*/hr Heavy Greater than 2.5 mm*/hr Table 8-6. Estimating Intensity of Snow or Drizzle Based on Visibility Intensity Criteria Light Visibility > 1/2 mile. Moderate Visibility > 1/4 mile but < 1/2 mile. Heavy Visibility < 1/4 mile.
Proposed change #2:Proposed change #2: 2.5.2 Aviation Selected Special Weather Report (SPECI) 2.5.2 Aviation Selected Special Weather Report (SPECI) a. Criteria for SPECI a. Criteria for SPECI CHANGE CHANGE (6) PRECIPITATION. (6) PRECIPITATION. (a) hail begins or ends. (a) hail begins or ends. (b) freezing precipitation begins, ends, or changes intensity. (b) freezing precipitation begins, ends, or changes intensity. (c) ice pellets begin, end, or change intensity. (c) ice pellets begin, end, or change intensity. TOTO (6) PRECIPITATION. (6) PRECIPITATION. (a) hail begins or ends. (a) hail begins or ends. (b) freezing precipitation begins, ends, or changes intensity. (b) freezing precipitation begins, ends, or changes intensity. (c) ice pellets begin, end, or change intensity. (c) ice pellets begin, end, or change intensity. (d) snow begins, ends or changes intensity.(d) snow begins, ends or changes intensity.
Freezing drizzle
•
IntensityFMH-1 Criteria for Drizzle Intensity1
FZDZ Accretion Rate Criteria2
Light Visibility > ½ mile Rate ≤ 0.25 mm/hr (0.01 in/hr)
Moderate Visibility > ¼ mile but ≤ ½ mile
Rate > 0.25 mm/hr but ≤ 0.5 mm/hr
Heavy Visibility ≤ ¼ mile Rate > 0.5 mm/hr (0.02 in/hr)
Criteria for Determining Freezing Drizzle Intensity
1 Federal Meteorological Handbook 2 Glossary of the American Meteorological Society
Remarks on Current Methods to Estimate Freezing Drizzle Intensity
• Currently used method to estimate drizzle intensity (light, moderate, or heavy) reported on METAR’s is based on
visibility. This is true both with the U.S. National Weather Service, and also internationally.
•
Remarks on Current Methods to Estimate Freezing Drizzle Intensity
• Report by Allan Ramsay prepared for the U.S. National Weather Service ASOS program (Sept. 21, 1999) has shown that the use of visibility to
estimate drizzle intensity is highly inaccurate in terms of the actual ice accretion rate on an exposed surface.
•
Freezing Drizzle
Heatingcycles
Frequencydrop due to ice accretion on the sensor
Ramsay Algorithm for FZRA/FZDZIce Detector LEDWI
Present Wx Type
Temp Visibility Sky Cover
Present Weather Reported
AccretionFrequency < 39,967 Hz
and
15 min Accretion Rate>13 Hz in 15 min
RA,UP <2.8 C(<37°F)
Any Any FZRA
SN Any Any Any SN
AccretionFrequency < 39,967 Hz
and
15 min Accretion Rate>6 Hz in 15 min
No Precip
< 0 C(<32°F)
Any OVC FZDZ
Not OVC
None
AccretionFrequency < 39,967 Hz
No Precip
< 0 C(<32°F)
> 7 miles CLR or
SCT
FROST
Icing Rates• Generally based on rate per 15 minutes• .01 inch/hour = 66 Hz/hour = 16.5 Hz/15 min
FZFG .001 - .004 in/hr 1 – 5 Hz/15 min
Light FZDZ .004 - .01 in/hr 6 – 16 Hz/15 min
MDT FZDZ .01 - .02 in/hr 17 – 33 Hz/15 min
Heavy FZDZ > .02 in/hr > 33 Hz/15 min
0
2000
4000
6000
8000
10000
12000
14000
16000
Light Moderate Heavy
Visibility baseintensityAccretaion basedintensity
One minute freezing drizzle observations from ASOS siteswith freezing rain sensors at various locations in the U.S. from 1998-99 (Analysis by Al Ramsey, formerly of Raytheon).
Freezing Drizzle Intensity
Min
utes
0102030405060708090
100
Light Moderate Heavy
Visibility baseintensityAccretaion basedintensity
Freezing Drizzle Intensity
Perc
ent o
f tim
e
One minute freezing drizzle observations from ASOS siteswith freezing rain sensors at various locations in the U.S. from 1998-99 (Analysis by Al Ramsey, formerly of Raytheon).
Freezing Drizzle Intensity
Quotes from the Al Ramsey report to the U.S. National Weather Service:
“It is clear that visibility-based criteria provide little or no information on the intensity of ice accretion from freezing drizzle.”
“…data collected in this project clearly indicate that METAR/SPECI reports of FZDZ intensity, based on visibility, do not accurately represent the rate of ice accretion, and they may mislead users into selecting incorrect holdover times.”
Freezing drizzle ingest into engines can cause significant damage to fan blades (light freezing drizzle reported by METAR, heavy freezing drizzle actually occuring based on liquid equivalent)
SHOULD WE CHANGE INTENSITY OF SNOW AND DRIZZLE AS FOLLOWS?SHOULD WE CHANGE INTENSITY OF SNOW AND DRIZZLE AS FOLLOWS? (b) (b) Intensity of Snow and Drizzle. Intensity of Snow and Drizzle. The manually-derived intensity of snow and drizzle shall be The manually-derived intensity of snow and drizzle shall be
based on the reported surface visibility in accordance with Table 8-6, when occurring alone. based on the reported surface visibility in accordance with Table 8-6, when occurring alone. Where possible, the intensity of snow or drizzle may be automatically determined by sensors Where possible, the intensity of snow or drizzle may be automatically determined by sensors operating in accordance with the Federal Standard Algorithms for Automated Observing operating in accordance with the Federal Standard Algorithms for Automated Observing Systems Used for Aviation Purposes using a liquid water equivalent rate. Systems Used for Aviation Purposes using a liquid water equivalent rate.
Table 8-5. Table 8-5. Intensity of Freezing Drizzle Based on Liquid Water Equivalent RateIntensity of Freezing Drizzle Based on Liquid Water Equivalent Rate IntensityIntensity CriteriaCriteria LightLight Less than 0.254 mm*/hrLess than 0.254 mm*/hr ModerateModerate 0.254 – 0.508 mm*/hr0.254 – 0.508 mm*/hr HeavyHeavy Greater than 0.508 mm*/hrGreater than 0.508 mm*/hr Table 8-6. Table 8-6. EstimatingEstimating Intensity of Snow or Drizzle Based on Visibility Intensity of Snow or Drizzle Based on Visibility Intensity Intensity CriteriaCriteria Light Light Visibility > 1/2 mile.Visibility > 1/2 mile. Moderate Moderate Visibility > 1/4 mile but < 1/2 mile.Visibility > 1/4 mile but < 1/2 mile. Heavy Heavy Visibility < 1/4 mile.Visibility < 1/4 mile.