Weather Glossary: P's

The rate of flow of a fluid through a cross section of a porous mass under a unit hydraulic gradient, at a temperature of 60°F (16°C). The standard coefficient of permeability used in hydrologic work in Meinzer's Units is defined as the rate of flow of water at 60°F (16°C), in gallons per day, through a cross section of 1 ft², under a hydraulic gradient of 100%.

A related coefficient, which may be called the Field Coefficient of Permeability, is defined as the rate of flow of water, in gallons a day, under prevailing conditions, through each foot of thickness of a given aquifer in a width of 1 mile, for each foot per mile of hydraulic gradient.

• Location and/or extent of reported weather phenomenon,
• Type of aircraft (only with reports turbulence or icing).

A device for measuring the velocity of flowing water using the velocity head of the stream as an index of velocity. It consists essentially of an orifice held to a point upstream in the water, connected with a tube in which the rise of water due to velocity head may be observed and measured.

It also may be constructed with an upstream and downstream orifice, with two water columns, in which case the difference in height of water column in the tubes is the index of velocity.

1. The holding back of water for later release for power development above the dam of a hydroelectric plant to equalize daily or weekly fluctuations of streamflow, or to permit irregular hourly use of water by the wheels to care for fluctuations in the load demand.
2. In general, the holding back of water for later releases.
3. The storage capacity available for the use of such water.

Slang for showers and thunderstorms that form on a scattered basis with little or no apparent organization, usually during the afternoon in response to diurnal heating.

Individual thunderstorms typically are of the type sometimes referred to as air-mass thunderstorms. These thunderstorms are small, short-lived, very rarely severe, and they almost always dissipate near or just after sunset.

1. The ratio of pore volume to total volume of the formation. Sandy soils have large pores and a higher porosity than clays and other fine-grained soils.
2. An index of the void characteristics of a soil or stream as pertaining to percolation; degree of perviousness.

1. The process where water vapor condenses in the atmosphere to form water droplets that fall to the Earth as rain, sleet, snow, hail, etc.
2. As used in hydrology, precipitation is the discharge of water, in a liquid or solid state, out of the atmosphere, generally onto a land or water surface.

It is the common process by which atmospheric water becomes surface, or subsurface water. The term "precipitation" is also commonly used to designate the quantity of water that is precipitated. Precipitation includes rainfall, snow, hail, and sleet, and is therefore a more general term than rainfall.

It is often the first serious indication that a hurricane is approaching. It is a generally a straight line and resembles a squall-line that occurs with a mid-latitude cold front.

It is as much as 50 miles (80 kilometers) or even more before the first ragged rain echoes of the hurricane's bands and is usually about 100 to 200 miles (160 to 320 kilometers) ahead of the eye, but it has been observed to be as much as 500 miles (800 kilometers) ahead of the eye in monstrous size hurricanes.

It is defined as the likelihood of occurrence (expressed as a percent) of a measurable amount of liquid precipitation (or the water equivalent of frozen precipitation) during a specified period of time at any given point in the forecast area. Measurable precipitation is defined as equal to or greater than 0.01" (0.25 mm).

In short, it is the chance that you will receive at least 0.01" (0.25 mm) at your location at some point during the forecast period. Some examples:

• 20%: 1-in-5 chance you will receive at least 0.01" (0.25 mm) of precipitation at some point during the forecast period.
• 50%: Either you will or won't receive at least 0.01" (0.25 mm) of precipitation. It is a coin toss.
• 100%: The forecaster guarantees you will get wet at some point during the forecast period. It may rain all day or for just a minute. You may get flooded or only receive 0.01" (0.25 mm) of precipitation.

What the PoP is NOT:

• It is NOT a percentage of coverage of precipitation for any time period.
• A higher PoP is NOT an indicator of intensity of any precipitation.
• It is NOT an indicator of percentage of time precipitation will occur.
• It is NOT an indicator of how many times precipitation has occurred in the past with this same weather pattern.

Normally, the period of time is 12 hours, unless specified otherwise. The forecast area, or zone, is generally considered to be a county. In some geographically unique areas (mountains), the forecast area/zone may consist of a portion of a county or two or more counties.

At times, some NWS forecasters will use "occasional" or "periods of" to describe a precipitation event that has a high probability of occurrence, i.e., they expect any given location in a forecast zone area to most likely have precipitation, but it will be of an on and off nature. Usually, away from the mountains, each and every county is a forecast zone area itself.

This Weather Prediction Center discussion may include analysis of numerical and statistical models, meteorological circulation patterns and trends, and confidence factors.

Reference is usually made to the manually produced 6- to 10-day Northern Hemisphere prognoses for mean 500 millibar heights and mean 500 millibar height anomalies. Discussions may also refer to the method of operational ensemble predictions.

This type of derecho typically occurs in the warm season (May through August) and is most frequent in a zone extending from eastern South Dakota to the upper Ohio Valley. The severe wind storms typically begin during the afternoon and continue into the evening hours. Several hours typically pass between initial convection and the first wind damage report.

A boundary between a supercell's inflow region and the rear-flank downdraft (or RFD). It extends outward from the mesocyclone center, usually toward the south or southwest (but occasionally bows outward to the east or southeast in the case of an occluded mesocyclone), and is characterized by advancing of the downdraft air toward the inflow region.

It is a particular form of gust front. See also pseudo-warm front.

A boundary between a supercell's inflow region and the forward-flank downdraft (or FFD).

It extends outward from at or near the mesocyclone center, usually toward the east or southeast, and normally is either nearly stationary or moves northward or northeastward ahead of the mesocyclone. See pseudo-cold front and beaver tail.

• Unusual atmospheric phenomena such as sun dogs, halos, rainbows, aurora borealis, lenticular clouds, and stories about a long-term dry/cold/wet/warm spell,
• Public educational information and activities, such as storm safety rules, awareness activities, storm drills, etc., or
• Information regarding service changes, service limitations, interruptions due to reduced or lost power or equipment outages, or special information clarifying interpretation of NWS data.

These are issued when the Storm Prediction Center in Norman, Oklahoma anticipates an especially significant and/or widespread outbreak of severe weather.

This outlook will stress the seriousness of the situation, defines the threat area, and provides information on the timing of the outbreak. The lead time on this outlook is normally less than 36 hours prior to the severe weather event.

1. The act of compacting earth, soil clay, etc., by mixing them with water and rolling or tamping the mixture.
2. A compact mass of earth, soil, clay, or a mixture of material, which has been compacted through the addition of water, rolling and tamping. This makes the material less permeable.
3. A small pool of water, usually a few inches in depth and from several inches to several feet in it greatest dimension.

The linear distance in range occupied by an individual pulse from a radar. h = c * t , where t is the duration of the transmitted pulse, c is the speed of light, h is the length of the pulse in space.

Note, in the radar equation, the length h/2 is actually used for calculating pulse volume because we are only interested in signals that arrive back at the radar simultaneously. This is also called a pulse width.