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51 For the ABC-S 75/25 base case, the frequency of tests gen- a) Using information on METAR visibility and time of day erating a percentage difference greater than 20% increased (daylight or darkness), the snowfall intensity can be read from: from a visibility chart that is part of the HOT guidance 1% at the shortest distance to material. That snowfall intensity and temperature can 14% at mid-range distance and then be used to select the appropriate cell in the HOT 28% at the longest distance. table; and For the ABC-S 50/50 base case, the frequency of tests gen- b) The METAR report also gives a direct indication of snow- erating a percentage difference greater than 20% increased fall intensity (light, moderate, or heavy). This indicated from: snowfall intensity, along with temperature, can be used by 0% at the shortest distance to the pilot to select the appropriate cell in the HOT table. 38% at mid-range distance and 42% at the longest distance. In addition, in an actual operation, the pilot has the option of visually estimating visibility distance (based on runway markers or local landmarks) and converting that value to Comparison of HOTDS Results snow intensity using the visibility table. This approach was to Current Operational Practices not available for this comparison. A brief comparison was made of HOT guideline times that were in effect during the testing versus HOT times as gener- Comparison of Snow Intensity Indicated ated by HOTDS systems using the precipitation measure- by METAR Reports and Test Data ments at the two test sites. This analysis is based on the "base case" and does not consider the CARs exemption criteria. To examine the differences in snow intensity from the dif- The values that could have been in use by pilots were con- ferent sources, Table 34 was developed for four selected tests. structed from the current HOT guidelines, existing weather The column headings show the source for the indicated snow information (METAR reports), and the visibility chart that is intensity. used to convert visibility to snowfall rate. Test 95 offers a good illustration of the variance in METAR is a routine aviation weather report that typically METAR-indicated intensity that pilots have to deal with in comes from airports or permanent weather observation sta- actual operations, with one indication being heavy and the tions. Reports are generated once an hour. If conditions change other one light, whereas the actual measured intensity was significantly, they can be updated in special reports. A typical moderate. METAR report contains data for the temperature, dew point, Test 97 also shows a significant variance in METAR- wind speed and direction, precipitation, cloud cover and indicated intensity, with one indication being heavy and heights, visibility, and barometric pressure. A METAR report the other one light, whereas the measured intensity was light may also contain other information including precipitation and very light. amounts. To establish the HOT values that could have been in effect, Comparison of HOT Values Based on METAR actual METAR reports in effect during selected tests were re- and Test Data trieved from archives. The METAR report gives the pilot two alternative ways to establish a value for snow intensity, which The snow intensity indications shown in Table 34 were is then used to extract holdover time from the HOT guidelines: then used to construct Table 35 with holdover times. Table 34. Comparison of snow intensity from different pilot aids. Snow Intensity HOTDS Visibility METAR Site 1 Site 2 Test Time Daylight/ OAT METAR (Statute Visibility Measured Snow Measured Snow # Interval Darkness (C) Snow Intensity Miles) Report and Intensity Intensity Report 2 2 Visibility Chart (g/dm /h) (g/dm /h) 95 00:10 - 00:20 Darkness 3/4 0 Heavy Light 14.5 18.1 107 15:10 - 15:20 Daylight 1/2 -4 Moderate Moderate 28.3 20.5 97 00:50 - 01:00 Darkness 1/2 0 Heavy Light 6 3.6 123 22:40 - 22:50 Darkness 3 -8 Light Light 5.8 3.6