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12.3.1. General. The measurement of precipitation is expressed in terms of vertical depth of water (or water equivalent in the case of solid forms) which reaches the surface during a specified period. In METAR observations, requirements for the measurement of precipitation are established to include both liquid and frozen amounts which have fallen and the total depth of solid forms on the ground at the time of observation. NOTE: The term solid is sometimes used as a synonym for frozen forms of precipitation.

12.3.2. Unit of Measurement. The basic unit of measurement is the inch. Higher headquarters or MAJCOMs may require stations to report in millimeters (mm) for liquid precipitation (or water equiv- alent) and centimeters (cm) for frozen precipitation and snow depth. Table 12.1. provides guidance for converting values from inches to millimeters.

12.3.2.1. Liquid precipitation (or water equivalent): To the nearest 0.01 inch. Less than 0.005 inch is termed a trace.

12.3.2.2. Frozen/Freezing precipitation: To the nearest 0.1 inch. Less than 0.05 inch is termed a trace.

12.3.2.3. Snow depth (any solid form): To the nearest whole inch. Less than 0.5 inch is termed a trace.

12.3.3. Observation Periods. Precipitation and snow depth measurements are normally obtained at 3- and 6-hourly synoptic times and at midnight LST. Make measurements more frequently when nec- essary; i.e., to meet local or other support requirements.

12.3.4. Representative Area for Measurement of Solid Forms. In obtaining samples or measure- ments of snowfall and depth of snow on the ground, select an area that is smooth, level, preferably grass covered, and as free from drifting as possible. Avoid using paved areas and low spots where water tends to collect. Select an area that permits measurements to be taken in undisturbed snow.

12.3.4.1. As an aid in obtaining the measurement of new snowfall, place snowboards on top of the snow after each measurement. The next measurement can then be taken from the top of the snow to the snowboard.

12.3.4.2. A snowboard is an aid for measuring new snow fall. The snowboard can be a thin, light-colored wooden board or a thin light-colored, light-weight composite material board (com- posite material must be a poor conductor of heat). The snowboard must be at least 2 feet-by-2 feet (about 60 cm) square and should be at least ½ inch thick.

12.3.4.3. In using the area, start measurements along the edge nearest the station to avoid unnec- essary tracking of the snow. Unless the snow is very deep and drifting is pronounced, take a mea- surement 2 feet (about 60 cm) from previous measurements.

12.3.4.4. Irregularities caused by uneven terrain, drifting, footsteps before sampling, etc., tend to introduce unavoidable errors in the measurements. Therefore, classify amounts as estimated if they are not considered representative.

Table 12.1. Conversion of Inches to Millimeters.

12.3.5. Measurement of Precipitation Amounts (Water Equivalent). Wa t e r e q u i v a l e n t i s a n expression used to reflect the amount of liquid produced by the melting of solid forms of precipitation. Obtain precipitation amounts (or water equivalent) using the following procedures as a guide:

12.3.5.1. Under normal circumstances, obtain liquid precipitation amounts and the water equiva- lent of frozen/freezing precipitation using the collection in the rain gauge.

Inches 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 Millimeters 0.00 0.0 0.3 0.5 0.8 1.0 1.3 1.5 1.8 2.0 2.3 0.10 2.5 2.8 3.1 3.3 3.6 3.8 4.1 4.3 4.6 4.8 0.20 5.1 5.3 5.6 5.8 6.1 6.4 6.6 6.9 7.1 7.4 0.30 7.6 7.9 8.1 8.4 8.6 8.9 9.1 9.4 9.7 9.9 0.40 10.2 10.4 10.7 10.9 11.2 11.4 11.7 11.9 12.2 12.5 0.50 12.7 13.0 13.2 13.5 13.7 14.0 14.2 14.5 14.7 15.0 0.60 15.2 15.5 15.8 16.0 16.3 16.5 16.8 17.0 17.3 17.5 0.70 17.8 18.0 18.3 18.5 18.8 19.1 19.3 19.6 19.8 20.1 0.80 20.3 20.6 20.8 21.1 21.3 21.6 21.8 22.1 22.4 22.6 0.90 22.9 23.1 23.4 23.6 23.9 24.1 24.4 24.6 24.9 25.2 1.00 25.4 25.7 25.9 26.2 26.4 26.7 26.9 27.2 27.4 27.7 1.10 27.9 28.2 28.5 28.7 29.0 29.2 29.5 29.7 30.0 30.2 1.20 30.5 30.7 31.0 31.2 31.5 31.8 32.0 32.3 32.5 32.8 1.30 33.0 33.3 33.5 33.8 34.0 34.3 34.5 34.8 35.1 35.3 1.40 35.6 35.8 36.1 36.3 36.6 36.8 37.1 37.3 37.6 37.9 1.50 38.1 38.4 38.6 38.9 39.1 39.4 39.6 39.9 40.1 40.4 1.60 40.6 40.9 41.2 41.4 41.7 41.9 42.2 42.4 42.7 42.9 1.70 43.2 43.4 43.7 43.9 44.2 44.5 44.7 45.0 45.2 45.5 1.80 45.7 46.0 46.2 46.5 46.7 47.0 47.2 47.5 47.8 48.0 1.90 48.3 48.5 48.8 49.0 49.3 49.5 49.8 50.0 50.3 50.6 2.00 50.8 51.1 51.3 51.6 51.8 52.1 52.3 52.6 52.8 53.1 2.10 53.3 53.6 53.9 54.1 54.4 54.6 54.9 55.1 55.4 55.6 2.20 55.9 56.1 56.4 56.6 56.9 57.2 57.4 57.7 57.9 58.2 2.30 58.4 58.7 58.9 59.2 59.4 59.7 59.9 60.2 60.5 60.7 2.40 61.0 61.2 61.5 61.7 62.0 62.2 62.5 62.7 63.0 63.3 2.50 63.5 63.8 64.0 64.3 64.5 64.8 65.0 65.3 65.5 65.8 2.60 66.0 66.3 66.6 66.8 67.1 67.3 67.6 67.8 68.1 68.3 2.70 68.6 68.8 69.1 69.3 69.6 69.9 70.1 70.4 70.6 70.9 2.80 71.1 71.4 71.6 71.9 72.1 72.4 72.6 72.9 73.2 73.4 2.90 73.7 73.9 74.2 74.4 74.7 74.9 75.2 75.4 75.7 76.0 3.00 76.2 76.5 76.7 77.0 77.2 77.5 77.7 78.0 78.2 78.5 3.10 78.7 79.0 79.3 79.5 79.8 80.0 80.3 80.5 80.8 81.0 3.20 81.3 81.5 81.8 82.0 82.3 82.6 82.8 83.1 83.3 83.6 3.30 83.8 84.1 84.3 84.6 84.8 85.1 85.3 85.6 85.9 86.1 3.40 86.4 86.6 86.9 87.1 87.4 87.6 87.9 88.1 88.4 88.7 3.50 88.9 89.2 89.4 89.7 89.9 90.2 90.4 90.7 90.9 91.2 3.60 91.4 91.7 92.0 92.2 92.5 92.7 93.0 93.2 93.5 93.7 3.70 94.0 94.2 94.5 94.7 95.0 95.3 95.5 95.8 96.0 96.3 3.80 96.5 96.8 97.0 97.3 97.5 97.8 98.0 98.3 98.6 98.8 3.90 99.1 99.3 99.6 99.8 100.1 100.3 100.6 100.8 101.1 101.4 4.00 101.6 101.9 102.1 102.4 102.6 102.9 103.1 103.4 103.6 103.9 4.10 104.1 104.4 104.7 104.9 105.2 105.4 105.7 105.9 106.2 106.4 4.20 106.7 106.9 107.2 107.4 107.7 108.0 108.2 108.5 108.7 109.0 4.30 109.2 109.5 109.7 110.0 110.2 110.5 110.7 111.0 111.3 111.5 4.40 111.8 112.0 112.3 112.5 112.8 113.0 113.3 113.5 113.8 114.1 4.50 114.3 114.6 114.8 115.1 115.3 115.6 115.8 116.1 116.3 116.6 4.60 116.8 117.1 117.4 117.6 117.9 118.1 118.4 118.6 118.9 119.1 4.70 119.4 119.6 119.9 120.1 120.4 120.7 120.9 121.2 121.4 121.7 4.80 121.9 122.2 122.4 122.7 122.9 123.2 123.4 123.7 124.0 124.2 4.90 124.5 124.7 125.0 125.2 125.5 125.7 126.0 126.2 126.5 126.8

NOTE: To convert inches to centimeters, multiply the inches column by 10 and read direct from table; e.g., 4.6

12.3.5.2. If the rain gauge collection is not considered representative, disregard the catch and clas- sify the amount of precipitation as undeterminable when it consists entirely of liquid types. If pos- sible, obtain water equivalent by means of core sampling or an estimation when precipitation during the period consisted entirely of solid forms.

12.3.5.3. To estimate water equivalent of solid forms of precipitation, first obtain a measurement of the snowfall. Convert the actual depth to its water equivalent on the basis of a 1:10 ratio (or other ratio if known to be representative for the station or the snowfall). For example, if 1.6 inches of snow has fallen, the water equivalent is approximately .16 inch (1.6 divided by 10 = .16) in using a 1:10 ratio. For 4 cm (40 mm) of snowfall, the water equivalent is approximately 4 mm in using a 1:10 ratio.

12.3.5.3.1. The following table may be used to help determine the water equivalent of new snowfall. Table 12.2. will not be used to determine the water equivalency of old snowfall because packing and melting/freezing will have a substantial effect on the density of the snow pack are not accounted for in this table.

Table 12.2. New Snowfall to Water Equivalent Conversion.

12.3.6. Measurement of Snowfall (Solid Precipitation). For the purpose of snowfall measure- ments, the term snow also includes other types of freezing and frozen precipitation which fell during the measurement period. Obtain snowfall amounts using the following procedures as a guide.

12.3.6.1. Using a standard ruler (graduated in inches) or other suitable measuring device, measure the depth in several locations, preferably at points where the snow has fallen and is undisturbed by the wind. If practical, make these measurements using snowboards or a surface which has been cleared of previous snowfall. If the previous snowfall has crusted, the new fall may be measured by permitting the end of the ruler to rest on the crust.

12.3.6.1.1. If a suitable spot is not available and snowboards are not in place, the snowfall amount may be obtained by measuring the total depth of snow and subtracting the depth pre- M e l t Wa t e r

E q u i v a l e n t (WE) in Inches

New Snowfall (inches)

Temperature (oC) 01-M02 M03-M07 M07-M09 M10-M12 M13-M18 M18-M29 M30-M40 Trace Trace 0.1 0.2 0.3 0.4 0.5 1.0 .01 0.1 0.2 0.2 0.3 0.4 0.5 1.0 .02 0.2 0.3 0.4 0.6 0.8 1.0 2.0 .03 0.3 0.5 0.6 0.9 1.2 1.5 3.0 .04 0.4 0.6 0.8 1.2 1.6 2.0 4.0 .05 0.5 0.8 1.0 1.5 2.0 2.5 5.0 .06 0.6 0.9 1.2 1.8 2.4 3.0 6.0 .07 0.7 1.1 1.4 2.1 2.8 3.5 7.0 .08 0.8 1.2 1.6 2.4 3.2 4.0 8.0 .09 0.9 1.4 1.8 2.7 3.6 4.5 9.0 .10 1.0 1.5 2.0 3.0 4.0 5.0 10.0 .11 1.1 1.7 2.2 3.3 4.4 5.5 11.0 .12 1.2 1.8 2.4 3.6 4.8 6.0 12.0 .13 1.3 2.0 2.6 3.9 5.2 6.5 13.0 .14 1.4 2.1 2.8 4.2 5.6 7.0 14.0 .15 1.5 2.3 3.0 4.5 6.0 7.5 15.0 .16 1.6 2.4 3.2 4.8 6.4 8.0 16.0 .17 1.7 2.6 3.4 5.1 6.8 8.5 17.0 .18 1.8 2.7 3.6 5.4 7.2 9.0 18.0 .19 1.9 2.9 3.8 5.7 7.6 9.5 19.0 .20 2.0 3.0 4.0 6.0 8.0 10.0 20.0 .21 2.1 3.1 4.2 6.3 8.4 10.5 21.0 .22 2.2 3.3 4.4 6.6 8.8 11.0 22.0 .23 2.3 3.4 4.6 6.9 9.2 11.5 23.0 .24 2.4 3.6 4.8 7.2 9.6 12.0 24.0 .25 2.5 3.8 5.0 7.5 10.0 12.5 25.0 .30 3.0 4.5 6.0 9.0 12.0 15.0 30.0 .35 3.5 5.3 7.0 10.5 14.0 17.5 35.0 .40 4.0 6.0 8.0 12.0 16.0 20.0 40.0 .45 4.5 6.8 9.0 13.5 18.0 22.5 45.0 .50 5.0 7.5 10.0 15.0 20.0 25.0 50.0 .60 6.0 9.0 12.0 18.0 24.0 30.0 60.0 .70 7.0 10.5 14.0 21.0 28.0 35.0 70.0 .80 8.0 12.0 16.0 24.0 32.0 40.0 80.0 .90 9.0 13.5 18.0 27.0 36.0 45.0 90.0 1.00 10.0 15.0 20.0 30.0 40.0 50.0 100.0 2.00 20.0 30.0 40.0 60.0 80.0 100.0 200.0 3.00 30.0 45.0 60.0 90.0 120.0 150.0 300.0 WE Ratio 1:10 1:15 1:20 1:30 1:40 1:50 1:100

NOTE: For temperatures above 34oF (1oC) or for slushy, wet snow, a 1:8 ratio may be appropriate, e.g., 0.10” WE = 0.8” snowfall, 0.15” WE = 1.2” snowfall.

viously measured. This will normally be an estimation due to the effects of melting, sublima- tion, etc.

12.3.6.1.2. When melting or settling occurred between measurements, estimate the depth of new snow which would have collected if the melting or settling had not occurred. For instance, if several snow showers occur between observations and each melts before the fol- lowing one occurs, the total snowfall for the period will be the sum of the maximum depth (measured or estimated) for each occurrence.

12.3.6.2. Obtain an average of the several measurements, to the nearest 0.1 inch. Consider the amount as estimated if there is any doubt as to its accuracy; e.g., due to melting, drifting, etc. 12.3.6.3. When an accurate water equivalent of frozen precipitation has been obtained, the snow- fall amount may be estimated on the basis of a 1:10 ratio (or other ratio if known to be representa- tive for the station or the snowfall); i.e., as an alternative to procedures in 12.3.6.1. For example, if the water equivalent of snowfall from the rain gauge is 0.16 inch, the actual amount of snowfall is approximately 1.6 inches (.16 times 10 = 1.6) using a 1:10 ratio. If the water equivalent is 4 mm, the actual amount of snowfall is approximately 40 mm (or 4 cm) using the 1:10 ratio.

12.3.7. Measurement of Total Snow Depth. For the purpose of snow depth measurements, the term snow also includes other types of frozen precipitation (e.g., ice pellets, hail) and sheet ice formed directly or indirectly from precipitation. Obtain total depth of snow in conjunction with snowfall measurements using the following procedures as a guide.

12.3.7.1. Using a standard ruler or other suitable measuring device, measure the total depth in several locations, preferably at points where the snow is undisturbed by the wind.

12.3.7.1.1. If the ground is covered with ice, cut through the ice with some suitable implement and measure its thickness. Add the thickness of the ice to the depth of snow above the ice. 12.3.7.1.2. When the snow has drifted, include the greatest and least depths in measurements from the representative area. For example, if spots with no snow are visible, use zero as one of the values.

12.3.7.1.3. Obtain an average of the several measurements, to the nearest whole inch.

12.3.7.2. Estimates of total depth may be obtained using snow stakes at stations where this method is considered necessary and practical. In such cases, place several stakes in the most rep- resentative area available; i.e., where the snow is least likely to be disturbed within a few feet (or meters) of the stakes. Obtain an average depth by reference to graduated markings on the stakes.

Chapter 13

MISCELLANEOUS TERMS AND OBSERVATIONS

13.1. General Information . This chapter contains definitions for miscellaneous terms and instructions for miscellaneous observations unique to individual weather observing units.