NATIONAL WEATHER SERVICE TERMINOLOGY

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NATIONAL WEATHER SERVICE ( NWS ) TERMINOLOGY PRECIPITATION TERMINOLOGY PROBABILITY of PRECIPITATION (POP) is the likelihood of occurrence (expressed as a percent) of a precipitation event at any given point in the forecast area. The NWS uses two different methods to indicate the
chance of precipitation for a specific area: numerical or in non- numerical terms. The "Expression of
Uncertainty" category is used for widespread precipitation and the "Equivalent Areal Coverage" for
convective (i.e., showery) events. Below is a table of these two methods with the corresponding POP.
POP Expression of Uncertainty Equivalent Areal Coverage 0% None Used None Used 10% Slight Chance (seldom used) Isolated or few 20% Slight Chance Widely Scattered 30-50% Chance Scattered 60-70% Likely Numerous 80-100% None Used None Used

There are other qualifying terms which are used with the above non-numerical expressions. For example:
For duration - brief, occasional, intermittent, frequent. For intensity - very light, light, heavy, very heavy. VERY LIGHT <.01 inches LIGHT .01 to .10 inch per hour MODERATE .10 to .30 inch per hour HEAVY .30 inch per hour NATIONAL WEATHER SERVICE file:///W|/termsprint/national_weather_servicep1.htm [7/23/2000 12:55:46 PM] WIND TERMINOLOGY WIND DIRECTION and SPEED A forecast wind (direction and speed) is included in the first two periods of the forecast. The wind is included in the third and/or fourth period if considered significant.

1. Wind direction is the direction where the wind is coming FROM and is based on an 8-point compass (NE,
E, SE, etc.). Light wind (usually 5 mph or less) will be handled in the following ways: LIGHT SOUTH WINDS (if direction is known),
LIGHT AND VARIABLE WINDS , or
LIGHT WINDS (where "light" implies a variable wind direction). 2. Wind speed will be given in miles per hour. Following is a list of terms sometimes used to describe the
wind speed.

Speed range Terms 0-5 mph Light or Light and Variable 5-15 mph None used 15-25 mph Breezy (usually for mild weather)
Brisk (usually for cold weather) 20-30 mph Windy 30-40 mph Very Windy 40 mph or greater Strong, Damaging, Dangerous, High
Note: A forecast can contain a peak wind speed in gusty situations. For example, "NORTHWEST WIND 20 TO 30 MPH WITH OCCASIONAL GUSTS TO 40 MPH .") TEMPERATURE TERMINOLOGY Numerical temperature values are represented in NWS forecasts in four ways: 1. "Near," "around," or "about" a specific value rounded to the nearest five zero. Above 100°F or below 10°F,
any number will be used. For example: NEAR 40, AROUND 15, ABOUT 85, or NEAR 106. 2. A general range where the terms are defined by the following:
LOWER 50's (50 - 54) MID 50's (53 - 57) UPPER 50's (56 - 59) 50's (50 - 59) 3. A specific range rounded to the nearest five or zero (except ranges below 10°F or above 100°F, any number
may be used). For example, 70 to 75 or 102 to 108. 4. Specific numbers for site-specific locations: TRI-CITIES 70/50/72 (NWS) WIND TERMINOLOGY file:///W|/termsprint/wind_terminologyp2.htm [7/23/2000 12:57:49 PM] SKY COVER TERMINOLOGY Term Opaque Coverage Aviation Clear or Sunny < 1/10 Clear Mostly Clear/Mostly Sunny 1/10 to 2/10 Scattered Partly Cloudy/Partly Sunny 3/10 to 6/10 Scattered Mostly Cloudy 7/10 to 8/10 Broken Cloudy 9/10 to 10/10 opaque clouds Overcast WIND CHILL INDEX
The wind chill index provided below shows the effective cooling on exposed skin. When the wind blows
across the skin, it removes the insulating layer of warm air adjacent to the skin. When all factors are the same,
the faster the wind blows, the greater the heat loss, which results in a colder feeling.
Temperature (° F) mph W I N
D S P E
E D 35 30 25 20 15 10 5 0 -5 -10 -15 -20 -25 -30 -35 -40 5 32 27 22 16 11 6 0 -5 -10 -15 -21 -26 -31 -36 -42 -47 10 22 16 10 3 -3 -9 -15 -22 -27 -34 -40 -46 -52 -58 -65 -71 15 16 9 2 -5 -11 -18 -25 -31 -38 -45 -51 -58 -65 -72 -78 -85 20 12 4 -3 -10 -17 -24 -31 -39 -46 -53 -60 -67 -74 -81 -88 -95 25 8 1 -7 -15 -22 -29 -36 -44 -51 -59 -66 -74 -81 -88 -96 -103 30 6 -2 -10 -18 -25 -33 -41 -49 -56 -64 -71 -79 -86 -93 -101 -109 35 4 -4 -12 -20 -27 -35 -43 -52 -58 -67 -74 -82 -89 -97 -105 -113 40 3 -5 -13 -21 -29 -37 -45 -53 -60 -69 -76 -84 -92 -100 -107 -115 45 2 -6 -14 -22 -30 -38 -46 -54 -62 -70 -78 -85 -93 -102 -109 -117 (NWS) SKY COVER TERMINOLOGY file:///W|/termsprint/sky_cover_terminologyP3.htm [7/23/2000 12:58:36 PM] HEAT INDEX
The NWS has devised the "HEAT INDEX" (HI), sometimes called the "apparent temperature." The HI is the
temperature the body feels when the heat and humidity are combined. The table below is the Heat Index
Chart. (Note: This chart is based upon shady, light wind conditions. Exposure to full sunshine can increase HI
values by up to 15° F.)
T
E M P Relative Humidity (%) 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 120 107 111 116 123 130 139 148 115 103 107 111 115 120 127 135 143 151 110 99 102 105 108 112 117 123 130 137 143 150 105 95 97 100 102 105 109 113 118 123 129 135 142 149 100 91 93 95 97 99 101 104 107 110 115 120 126 132 138 144 95 87 88 90 91 93 94 96 98 101 104 107 110 114 119 124 130 136 90 83 84 85 86 87 88 90 91 93 95 96 98 100 102 106 109 113 117 122 85 78 79 80 81 82 83 84 85 86 87 88 89 90 91 93 95 97 99 102 105 108 80 73 74 75 76 77 77 78 79 79 80 81 81 82 83 85 86 86 87 88 89 91 75 69 69 70 71 72 72 73 73 74 74 75 75 76 76 77 77 78 78 79 79 80 70 64 64 65 65 66 66 67 67 68 68 69 69 70 70 70 70 71 71 71 71 72 Heat Index Possible Heat Disorder 80°F to 90°F Fatigue possible with prolonged exposure and/or physical activity. 90°F to 105°F Sunstroke, heat cramps and heat exhaustion possible with prolonged exposure and/or
physical activity. 105°F to 130°F Sunstroke, heat cramps, or heat exhaustion likely, and heatstroke possible with
prolonged exposure and/or physical activity. 130°F or greater Heat stroke/sunstroke highly likely with continued exposure. STATION INFORMATION Copyright © 1999-2000 Michael Lees All Right Reserved World Wide. Send mail to mike@lfstore.com with questions or comments about this web site. DISCLAIMER All graphics on this site by Michael Lees, LFS Site Updated Using Weather View 32 Custom Software This Site LFS Westtown, PA Weather Station http://www.lfsweather.com MEMBERS OF CASI C entral A tlantic S torm I nvestigators Format updated Sunday, July 23, 2000 01:16:20 PM ( NWS ) HEAT INDEX file:///W|/termsprint/heat_indexP4.htm [7/23/2000 1:17:16 PM]