| 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 can be expressed in several ways. One is in millibars. Another is in inches or millimeters of mercury (Hg). 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. Virtual 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 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. |
| MESONET | Mesonet is a combination of the words "mesoscale" and "network". In meteorology, "mesoscale" refers to weather events that range in size from a few miles to a few hundred miles. Mesoscale events last from several minutes to several hours. Thunderstorms and squall lines are two examples of mesoscale events. A "network" is an interconnected system. Thus, the Weather Underground Mesonet is a system designed to measure the environment at the size and duration of mesoscale weather events. Your weather station is an important tool for meteorologists and hobbyists to predict local weather conditions. |
| METAR REPORTS | The METAR acronym roughly translates from the French as Aviation Routine Weather Report. A special report, SPECI, is merely a METAR formatted report which is issued on a non-routine basis as dictated by changing meteorological conditions. Since METAR is the reporting standard for weather observations, VWS generates METAR reports so you can share weather data. VWS also includes an interpreter, so you can interpret other METAR reports. The main reason users put their information on the internet is to share data. VWS provides a conduit for sharing weather information. |
| 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. Virtual Weather 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 usually expressed in percentage. |
| SEA LEVEL PRESSURE | The atmospheric pressure at mean sea level either directly measured by stations 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. |
| SOLAR RADIATION | Radiant energy emitted by the sun, particularly electromagnetic energy (wavelengths of 200 to 1,100 nanometers). About half of the radiation is in the visible short-wave part of the electromagnetic spectrum. The other half is mostly in the near-infrared part, with some in the ultraviolet part of the spectrum |
| 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. 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. |
| UV INDEX |
Overexposure to the sun's ultraviolet (UV) radiation can cause immediate effects such as sunburn and long-term problems such as skin cancer and cataracts. The UV Index, which was developed by the National Weather Service and the U.S. Environmental Protection Agency (EPA), provides important information to help you plan your outdoor activities to prevent overexposure to the sun’s rays.
UV Index Number Exposure Level 0 to 2 - Low 3 to 5 - Moderate 6 to 8 - High 8 to 10 - Very High 11+ - Extreme |
| 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 or length of flow of the air past a point during a given interval of 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 United States is miles per hour. |