Glossary of Terms Used on The Buxton Weather Site:Term Definition

AIR DENSITY The ratio of the mass of a substance to the volume it occupies. In oceanography, it is equivalent to specific gravity and represents the ratio of the weight of a given volume of sea water to that of an equal volume of distilled water at 4.0 degrees Celsius or 39.2 degrees Fahrenheit.

ATMOSPHERIC PRESSURE See barometric pressure

BAROMETRIC PRESSURE The pressure exerted by the atmosphere as a consequence of gravitational attraction exerted upon the "column" of air lying directly above the point in question. The measurement is expressed in millibars. Also known as atmospheric pressure.

BEAUFORT WIND SCALE   A system of estimating and reporting wind speeds. It is based on the Beaufort Force or Number, which is composed of the wind speed, a descriptive term, and the visible effects upon land objects and/or sea surfaces. The scale was devised by Sir Francis Beaufort (1777-1857), hydrographer to the British Royal Navy.

CALIBRATION ERROR The inaccuracy that the manufacturer permits when the unit is calibrated in the factory.

CLOUD BASE For a given cloud or cloud layer. The lowest level in the atmosphere at which the air contains a perceptible quantity of cloud particles. The weather station estimates the cloud base on temperature and humidity readings, using the following equation: 

Cloud Base (ft) = 250(Temperature - Dew Point)

COOLING DEGREE DAY A cooling degree day is given for each degree that the daily mean temperature departs above the baseline of 75 degrees a given temperature It is used to estimate the energy requirements, and is an indication of fuel consumption for air conditioning or refrigeration. Refer to degree day or heating degree day.

DEW POINT The temperature to which a sample of air must be cooled, while the mixing ratio and barometric pressure remain constant, in order to attain saturation by water vapor. When this temperature is below O°C, it is sometimes called the frost point.

GUST A sudden significant increase in or rapid fluctuations of wind speed. Peak wind must reach at least 16 knots (18 miles per hour) and the variation between peaks and lulls is at least 10 knots (11.5 miles per hour). The duration is usually less twenty seconds.

HEAT INDEX The combination of air temperature and humidity that gives a description of how the temperature feels. This is not the actual air temperature.

HEATING DEGREE DAY One heating degree day is given for each degree that the daily mean temperature is below 65 degrees a given temperature. It is used as an indication of fuel consumption. Refer to degree day or cooling degree day.

HUMIDITY The amount of water vapor in the air. It is often confused with relative humidity or dew point. Types of humidity include absolute humidity, relative humidity, and specific humidity.

LATITUDE The location north or south in reference to the equator, which is designated at zero (0) degrees. Parallel lines that circle the globe both north and south of the equator. The poles are at 90 degrees North and South latitude.

LONGITUDE The location east or west in reference to the Prime Meridian, which is designated as zero (0) degrees longitude. The distance between lines of longitude are greater at the equator and smaller at the higher latitudes, intersecting at the earth's North and South Poles. Time zones are correlated to longitude. See Greenwich Mean Time.

MOON PHASE The moon phase is caused by sun rays reflecting off the moon's surface while it moves around the earth. The sun illuminates half of the moon at any time while the moon orbits around the earth. The variation in the angle made by the earth-moon line with respect to the earth-sun line causes changing phase of the moon. The moon completes one revolution around the earth in 27.322 days with respect to the background stars. This is called the SIDERIAL period of the moon. During this same time the earth moves about 27 degrees along its orbit around the sun. As a result, the moon takes about two extra days to complete the cycle with respect to the sun-earth line. This longer cycle of the moon that takes about 29.57 days is called SYNDONIC period of the moon. The longer cycle is considered as Lunar month.

PRESSURE ALTITUDE Atmospheric or barometric pressure expressed in terms of altitude which corresponds to that pressure in the standard atmosphere.

RATE OF CHANGE The derivative or change in a parameters value with respect to time. The station calculates the rate of change by calculating the derivative of a parameter, and then filtering it over one hour. Thus, the rate of change equation factors all of the measurements taken in the last hour, and may not exactly match the change in one hour.

RELATIVE HUMIDITY A type of humidity that considers the ratio of the actual vapor pressure of the air to the saturation vapor pressure. It is expressed in percentage.

SEA LEVEL PRESSURE The atmospheric pressure at mean sea level either directly measured by stat ions at sea level or empirically determined from the station pressure and temperature by stations not at sea level. Used as a common reference for analyses of surface pressure patterns.

SUNRISE The daily appearance of the sun on the eastern horizon as a result of the earth's rotation. In the United States, it is considered as that instant when the upper edge of the sun appears on the sea level horizon. In Great Britain, the center of the sun's disk is used instead. Time of sunrise is calculated for mean sea level. See sunset for comparison.

SUNSET The daily disappearance of the sun below the western horizon as a result of the earth's rotation. In the United States, it is considered as that instant when the upper edge of the sun just disappears below the sea level horizon. In Great Britain, the center of the sun's disk is used instead. Time of sunset is calculated for mean sea level. See sunrise for comparison.  

SUNSHINE HOURS - Daily and Cumulative  Are collected by a digital sun duration sensor which is equivalent to a Campbell Stokes recorder.  It has a resolution to 0.1 hours daily.  As records build Monthly and Annual hours will be posted

VAPOR PRESSURE The pressure exerted by water vapor molecules in a given volume of air

 VIRTUAL TEMPERATURE Virtual temperature is a fictitious temperature that takes into account moisture in the air. The formal definition of virtual temperature is the temperature that dry air would have if its pressure and specific volume were equal to those of a given sample of moist air. Virtual temperature allows meteorologists to use the equation of state for dry air even though moisture is present.

WIND CHILL INDEX The calculation of temperature that takes into consideration the effects of wind and temperature on the human body. Describes the average loss of body heat and how the temperature feels. This is not the actual air temperature.

WIND DIRECTION The direction from which the wind is blowing. For example, an easterly wind is blowing from the east, not toward the east. It is reported with reference to true north, or 360 degrees on the compass, and expressed to the nearest 10 degrees, or to one of the 16 points of the compass (N, NE, etc.).

WIND RUN The distance the Wind has travelled. i.e. the Wind Run for a constant wind speed 20mph for 2 hours is 40 miles.   Wind Run = Wind Speed X Time

WIND SPEED The rate of the motion of the air on a unit of time. It can be measured in a number of ways. In observing, it is measured in knots, or nautical miles per hour. The unit most often used in the UK  & United States is miles per hour. 

Beaufort ScaleThe Beaufort scale is a system of recording wind velocity (speed) devised in 1806 by Francis Beaufort (1774–1857). It is a numerical scale ranging from 0 for calm to 12 for a hurricane as follows:r 

Kph / mph

0 calm smoke rises vertically; water smooth 0–2 0–1

1 light air smoke shows wind direction; water ruffled 2–5 1–3

2 light breeze leaves rustle; wind felt on face 6–11 4–7

3 gentle breeze loose paper blows around 12–19 8–12

4 moderate breeze branches sway 20–29 13–18

5 fresh breeze small trees sway, leaves blown off 30–39 19–24

6 strong breeze whistling in telephone wires; sea spray from waves 40–50 25–31

7 near gale large trees sway 51–61 32–38

8 gale twigs break from trees 62–74 39–46

9 strong gale branches break from trees 75–87 47–54

10 storm trees uprooted; weak buildings collapse 88–101 55–63

11 violent storm widespread damage 102–117 64–73

12 hurricane widespread structural damage above 118 above 74 

Heat Stress Index The Heat Stress Index calculates the body's reaction to heat and humidity, or the Heat Index. High temperatures and humidity stress the body's ability to cool itself, and heat illness becomes a special concern during hot weather .Do not base important decisions on this heat stress index! Everybody's reaction to heat is different. :Scale Condition

Heat Index <26 °C

Caution 26 °C < Heat Index < 32 °C

Extreme Caution 32 °C < Heat Index < 40 °C

Danger 40 °C < Heat Index > 55 °C

Extreme Danger Heat Index > 55 °C 

Comfort Index The comfort index calculates the body's reaction to heat, cold, humidity and wind chill. High temperatures and humidity stress the body's ability to cool itself, and low temperature can stress the body's ability to heat itself. Do not base important decisions on this comfort index! Everybody's reaction to heat and cold is different.Scale

Extreme Cold Wind chill < -20 °C

Uncomfortably Cold -20 °C < Wind chill < 0 °C

Cool 0 °C < Wind Chill < 16 °C

Comfortable 16 °C < Temperature < 27 °C

Warm 27 °C < Temperature < 32 °C

Uncomfortably Hot Temperature > 32 °C and Heat Index < 38 °C