Weather Terms and Definitions

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Some basic weather terms and definitions which appear on this site.

Advisories   Barometric Pressure   Cloud Height   Dew Point   Heat Index   Humidity   Mesomap   Evapotranspiration METAR   Weather Station   Wind Chill   Heating/Cooling days  Fire Index Codes PENNSYLVANIA Records   Further Reading

ADVISORIES

A statement generally provides additional or follow up information to an existing weather condition.

An advisory is for less serious conditions that cause significant inconvenience and, if caution is not exercised, could lead to situations that may threaten life and/or property.

A watch is used when the risk of a hazardous weather event has increased significantly, but its occurrence, locations, and/or timing is still uncertain. It is intended to provide advance notice of possible inclement weather.

A warning is used for conditions posing an immediate threat to life or property. Depending on the type of warning, you should take immediate, appropriate action.

NOAA advisory types

National Weather Service Glossary

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BAROMETRIC PRESSURE
The air that makes up our atmosphere exerts a pressure on the surface of the earth. This pressure is known as atmospheric pressure. Generally, the more air above an area, the higher the atmospheric pressure. Barometric pressure changes with local weather conditions, making barometric pressure an important and useful weather forecasting tool. High pressure zones are generally associated with fair weather, while low pressure zones are generally associated with poor weather. For forecasting purposes, the absolute barometric pressure value is generally less important than the change in barometric pressure. In general, rising pressure indicates improving weather conditions, while falling pressure indicates deteriorating weather conditions.
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CLOUD HEIGHT
The cloud height on this site is an estimate of cumulus clouds using a formula based on temperature and dew point. Actual measurements of cloud height are made with a ceilometer. This device fires a laser into the sky and measures the backscattered signal. Costs for such a device are beyond the scope of weather hobbyists.
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DEW POINT
Dew point is the temperature to which air must be cooled for saturation to occur. The dew point is an important measurement used to predict the formation of dew, frost, and fog. If dew point and temperature are close together in the late afternoon when the air begins to turn colder, fog is likely during the night. Dew point is also a good indicator of the air's actual water vapor content, unlike relative humidity, which takes the air's temperature into account. High dew point indicates high vapor content; low dew point indicates low vapor content. In addition a high dew point indicates a better chance of rain and severe thunderstorms. You can even use dew point to predict the minimum overnight temperature. Provided no fronts or other weather pattern changes are expected overnight, the afternoon's dew point gives you an idea of what minimum temperature to expect overnight.

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HEAT INDEX
The Heat Index is a measure of relative discomfort due to combined heat and high humidity. It was developed by R.G. Steadman (1979) and is based on physiological studies of evaporative skin cooling for various combinations of ambient temperature and humidity. As temperatures climb above 90 F and humidity goes above 40 percent, conditions are ripe for heat-related illnesses.

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heat index chart

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Humidex

The humidex is a measurement used by Canadian meteorologists to reflect the combined effect of heat and humidity. It differs from the heat index used in the United States in using dew point rather than relative humidity.  According to the Meteorological Service of Canada, a humidex of at least 40 causes "great discomfort" and above 45 is "dangerous." When the humidex hits 54, heat stroke is imminent.  Humidex is a unit less number.

THSW INDEX

Parameters Used: Temperature, Humidity, Solar Radiation, Wind Speed, Latitude & Longitude, Time and Date

Like Heat Index, the THSW Index uses humidity and temperature to calculate an apparent temperature. In addition, THSW incorporates the heating effects of solar radiation and the cooling effects of wind (like wind chill) on our perception of temperature.

Formula: The formula was developed by Steadman (1979). The following describes the series of formulas used to determine the THSW or Temperature-Humidity-Sun-Wind Index. Thus, this index indicates the level of thermal comfort including the effects of all these values. This Index is calculated by adding a series of successive terms. Each term represents one of the three parameters: (Humidity, Sun & Wind). The humidity term serves as the base from which increments for sun and wind effects are added.

HUMIDITY FACTOR The first term is humidity. This term is determined in the same manner as the Heat Index. This term serves as a base number to which increments of wind and sun are added to come up with the final THSW Index temperature.

WIND FACTOR The second term is wind. Depending upon your version of firmware or software, this term is determined in part by a lookup table (for temperatures above 50F) and in part by the wind chill calculation, or uses an integrated table that is used both for calculation of this term and for wind chill.

SUN FACTOR The third term is sun. This term, Qg, is actually a combination of four terms (direct incoming solar, indirect incoming solar, terrestrial, and sky radiation). The term depends upon wind speed to determine how strong an effect it is. The value is limited to between −20 and +130 W/m2.

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HUMIDITY
Humidity or relative humidity measures the amount of water vapor in the air relative to the temperature. It is important in weather because humidity affects how humans feel. A hot, humid day feels hotter because we cannot sweat as effectively. A cool, dry day feels colder because moisure evaporates more easily.

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MESOMAP
Mesomap is a combination of the terms "mesonet" and "map".

In meteorology, "mesonet" is a network of weather stations used to monitor weather over a regional area. Such stations gather data like temperature, pressure, humidity, wind speed/direction, cloud conditions, and precipitation.

A "map" is a graphical representation of some area.

Thus, a weather mesomap is a graphical representation of observing stations (usually surface stations) designed to display regional scale weather information. This allows an observer to gain a perspective of weather phenomena otherwise unobtainable.

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Evapotranspiration
Evapotranspiration (ET) is a term used to describe the sum of evaporation and plant transpiration from the earth's land surface to atmosphere. Evaporation accounts for the movement of water to the air from sources such as the soil, canopy interception, and waterbodies. Transpiration accounts for the movement of water within a plant and the subsequent loss of water as vapor through stomata in its leaves. Evapotranspiration is an important part of the water cycle. An element (such as a tree) that contributes to evapotranspiration can be called an evapotranspirator.
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METAR
Acroymn for METeorological Aerodrome Report. It is the primary observation code used in the United States to satisfy requirements for reporting surface meteorological data. Minimum reporting requirments includes wind, visibility, runway visual range, present weather, sky condition, temperature, dew point, and altimeter setting. Latrobe Airport (KLBE) 40-17N 079-24Wis this site's primary METAR. Latrobe is also the weather most often quoted by weather sources for the Greensburg area (but can be over an hour old!).

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WEATHER STATION
A weather station is a facility with instruments and equipment to make weather observations by monitoring atmospheric conditions to study the weather. This weather station has a thermometer for measuring temperature; barometer for measuring changes in air pressure; hygrometer for measuring humidity; anemometer for measuring wind speed and wind direction; and rain gauge for measuring precipitation. Other non-traditional equipment is used like a webcam for visual weather observation and an electro-magnetic pulse counter for lightning detection.

More on Weather Station

More on Anemometer

More on Thermometer

More on Hygrometer

More on Rain Gauge

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WIND CHILL
The wind chill temperature is what the temperature "feels like" to people and animals during cold weather. Wind chill is based on the rate of heat loss from exposed skin caused by wind and cold. As the wind increases, it draws heat from the body, driving down skin temperature and eventually the internal body temperature. Once temperatures drop below 10 F and the wind is gusting, conditions are ripe for cold-related illnesses. Below -5 F, any wind is a major factor in frostbite and hypothermia.

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wind chill chart

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UV Index
The UV index is an international standard measurement of how strong the ultraviolet (UV) radiation from the sun is at a particular place on a particular day. It is a scale primarily used in daily forecasts aimed at the general public. Its purpose is to help people to effectively protect themselves from UV light, of which excessive exposure causes sunburns, eye damage such as cataracts, skin aging, and skin cancer (see the section health effects of ultraviolet light). Public-health organizations recommend that people protect themselves (for example, by applying sunscreen to the skin and wearing a hat) when the UV index is 3 or higher; see the table below for complete recommendations.

Recommendations for protection when the day's predicted UV index is at various values are:

UV Index Description Media Graphic Color Recommended Protection
02 No danger to the average person Green Wear sunglasses; use sunscreen if there is snow on the ground, which reflects UV radiation, or if you have particularly fair skin.
35 little risk of harm from unprotected sun exposure Yellow Wear sunglasses and use sunscreen, cover the body with clothing and a hat, and seek shade around midday when the sun is most intense.
67 High risk of harm from unprotected sun exposure Orange Wear sunglasses and use sunscreen having SPF 15 or higher, cover the body with sun protective clothing and a wide-brim hat, and reduce time in the sun from two hours before to three hours after solar noon (roughly 10:00 AM to 4:00 PM during summer in zones that observe daylight saving time.
810 Very high risk of harm from unprotected sun exposure Red Wear sunscreen, a shirt, sunglasses, and a hat. Do not stay out in the sun for to long.
11+ Extreme risk of harm from unprotected sun exposure Violet Take all precautions, including: wear sunglasses and use sunscreen, cover the body with a long-sleeve shirt and trousers, wear a very broad hat, and avoid the sun from two hours before to three hours after solar noon.
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Heating degree day and Cooling degree day

Heating degree day (HDD) and cooling degree day (CDD) are quantitative indices designed to reflect the demand for energy needed to heat or cool a home or business. These indices are derived from daily temperature observations, and the heating (or cooling) requirements for a given structure at a specific location are considered to be directly proportional to the number of heating degree days at that location.

More specifically, the number of heating degrees in a day is defined as the difference between a reference value of 65F (18C) and the average outside temperature for that day. The value of 65F is taken as a reference point because experience shows that if the outside temperature is this value then no heating or cooling is normally required. Occupants and equipment within a building usually add enough heat to bring the temperature up to a more comfortable level.

Suppose, for example, that the average temperature for a given day is 55F. Since this value is ten degrees lower than the reference point of 65F then one would say this is a ten degree-day. Obviously, the outside temperature is not always constant, so one needs a method to determine the average temperature. A simple way to do this is to compute the arithmetic mean of the high and low temperatures for the day. While not always correct, this is sufficiently accurate for most purposes and is done for practicality because these temperatures are always recorded by the weather bureau. Thus, in the previous example, if the high temperature were, say, 65F and the low 45F, then the average would still be 55F for a ten degree-day.

Heating and cooling degree days can be added over periods of time to provide a rough estimate of seasonal heating and cooling requirements. In the course of a year, for example, the number of heating degree-days for New York City is around 5,000 whereas that for Barrow, Alaska is over 20,000. Thus, one can say that, for a given home of similar structure and insulation, four times the energy would be required to heat that home in Barrow than in New York.

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Forest Fire Index Codes

The Canadian Forest Fire Weather Index (FWI) System consists of six components that account for the effects of fuel moisture and wind on fire behavior.

The first three components are fuel moisture codes and are numerical ratings of the moisture content of litter and other fine fuels, the average moisture content of loosely compacted organic layers of moderate depth, and the average moisture content of deep, compact organic layers.

The remaining three components are fire behavior indexes which represent the rate of fire spread, the fuel available for combustion, and the frontal fire intensity; their values rise as the fire danger increases.

Fine Fuel Moisture Code

The Fine Fuel Moisture Code (FFMC) is a numerical rating of the moisture content of litter and other cured fine fuels. This code is an indicator of the relative ease of ignition and flammability of fine fuel.

Duff Moisture Code

The Duff Moisture Code (DMC) is a numerical rating of the average moisture content of loosely compacted organic layers of moderate depth. This code gives an indication of fuel consumption in moderate duff layers and medium-size woody material.

Drought Code

The Drought Code (DC) is a numerical rating of the average moisture content of deep, compact, organic layers. This code is a useful indicator of seasonal drought effects on forest fuels, and amount of smouldering in deep duff layers and large logs.

Initial Spread Index

The Initial Spread Index (ISI) is a numerical rating of the expected rate of fire spread. It combines the effects of wind and the Fine Fuel Moisture Code on rate of spread without the influence of variable quantities of fuel.

Buildup Index

The Buildup Index (BUI) is a numerical rating of the total amount of fuel available for combustion that combines the Duff Moisture Code and the Drought Code.

Fire Weather Index

The Fire Weather Index (FWI) is a numerical rating of fire intensity that combines the Initial Spread Index and the Buildup Index. It is suitable as a general index of fire danger throughout the forested areas of Canada.

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PENNSYLVANIA AVERAGES and RECORDS (Pittsburgh)
 

High: 60.4F average, 103F Max (Jul 16, 1988)
Low: 41.5F average,  -22F Min (Jan 19, 1994)
Mean:  50.9F
Rainfall: 3.56" Max daily (Oct 15, 1954) , 11.05" Max monthly total (Nov 1985)
Rainfall (Yearly): 37.85" annual
Snowfall: 40.3" annual
The greatest 24 hour snowfall in:  23.6" (Mar 13, 1993)

Data: NCDC: Climatic Extremes

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Suggest a new "FURTHER READING" link and have your school receive credit for the new find!!!

December 27, 2012 W.B. Goodwin Community Center. -> Weather Central for Kids! - A Guide to Weather Phenomenons :)

May 8, 2013 Lexington Middle School. -> What is Air? :)