|
|
 |
Following are common terms used in meteorology. To obtain a definition of the term you can either click on the term in the table below or scroll down through this page to view all definitions in alphabetical order.
- Anemometer
- An instrument used to measure the strength of the wind.
- Cold Front
- The boundary between a mass of warm air and a mass of cold air that underruns and displaces it.
- Dew Point
- Temperature at which condensation begins on a surface; relative humidity is 100%.
- Drizzle
- Small (conventionally less than 0.5 mm) water droplets that (unlike fog) fall slowly toward the ground.
- Fog
- A cloud that touches the surface of the earth.
- Freezing Rain
- Rain that freezes upon contact with an object.
- Frost
- A deposit of ice crystals that forms on an object, such as the ground, by sublimation.
- Funnel Cloud
- Tornado or waterspout cloud, usually used to describe clouds not touching the surface of the earth.
- Graupel
- A type of precipitation consisting of frozen cloud droplets that have clumped and formed pellets. They are also called snow pellets.
Top of Page
- Hail
- Precipitation in the form of balls or irregular lumps of ice. It is always produced by convective clouds, usually thunderstorms. By convention, hail has a diameter of greater than 5mm, and the smaller particles are called graupel.
- Hygrometer
- An instrument that measures the amount of water vapor in the air.
- Inversion
- Reversal of the normal temperature change with altitude, where temperature rises rather than falls as altitude increases.
- Jet Stream
- Relatively narrow bands of high velocity winds in the atmosphere. The term is usually applied to high winds in the upper troposphere associated with a polar front.
- Psychrometer
- An instrument for determining the wet bulb temperature.
Top of Page
- Probability of Precipitation (POP)
- The likelihood of occurrence (expressed as a percent) of a precipitation event at any given point in the forecast area. The National Weather Service 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 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 |
<0.01 inches |
| Light |
0.01 to 0.10 inch per hour |
| Moderate |
0.01 to 0.30 inch per hour |
| Heavy |
0.30 inch per hour |
- Relative Humidity
- The amount of water in the air, expressed as a percentage of the maximum amount that the could hold at a given temperature.
- Sea Level Pressure
- The atmospheric pressure at sea level. If the station is at sea level, it can be measured directly; but if the station is at some higher (or lower) level, the sea level pressure must be calculated based on the station pressure, temperature, and height above (or below) sea level.
Top of Page
- Squall Line
- A line along which converging air currents bring about cumulonimbus activity, with resulting heavy, squally precipitation.
- Sky Cover Terminology
| Term |
Opaque Coverage |
| Clear or Sunny |
1/10 |
| Mostly Clear/Mostly Sunny |
1/10 to 2/10 |
| Partly Cloudy/Partly Sunny |
3/10 to 6/10 |
| Mostly Cloudy |
7/10 to 8/10 |
| Cloudy |
9/10 to 10/10 |
- Temperature Terminology
- Numerical temperature values are represented in NWS forecasts in four ways:
- "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.
- 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)
- 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.
- Specific numbers for site-specific locations: Tri-Cities 70/50/72
Top of Page
- Virga
- Wisps or streaks of water or ice falling out of a cloud but evaporating before reaching the ground.
- Warm Front
- A weather front along which an advancing mass of warm air rises over a retreating mass of cold air.
- Wind Shear
- A rapid change in wind velocity as altitude changes, as in horizontal or vertical wind shear.
- 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.
- 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).
- 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".
Top of Page
|
|
|
|
