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National Weather Service - San Francisco/Monterey Bay Area - glossary




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 Isolated or none used
20% Slight Chance Isolated
30-40% Chance Scattered
60-70% Likely Numerous or none used
80-100% None 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 < .01 inch per hour
Light .01 to .10 inch per hour
Moderate .10 to .30 inch per hour
Heavy greater than .30 inch per hour


WIND DIRECTION and SPEED A forecast wind (direction and speed) is included in the first three periods of the forecast. The wind is included in the 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 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
0-5 mph light or light and variable
5-15 mph none used
15-25 mph none used or breezy for mild weather
  brisk for cold weather
20-30 mph breezy (mild weather), brisk (cold weather)
25-35 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.")

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

Predominate Sky Condition
Daytime Terminology
Day or Night Terminology
0/8 Opaque Clouds
1/8 - 2/8 Opaque Clouds
Mostly Sunny
Mostly Clear
3/8 - 5/8 Opaque Clouds
Partly Sunny
Partly Cloudy
6/8 - 7/8 Opaque Clouds
Mostly Cloudy
Mostly Cloudy
8/8 Opaque Clouds


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.

  • Fuel Moisture:
    The quantity of moisture in fuels expressed as a percentage of the weight when thoroughly dried at 212 degrees Fahrenheit. Also referred to as fuel moisture content.. Fuel moisture observations are generally for the 10-hour time lag fuels (medium-sized roundwood 1/4" to 1" in diameter)


US Dept of Commerce
National Oceanic and Atmospheric Administration
National Weather Service
San Francisco/Monterey Bay Area Weather Forecast Office
21 Grace Hopper Ave, Stop 5
Monterey, CA 93943-5505

Tel: (831)-656-1725

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