Weather Glossary

Weather Terminology Explained


AIRPRESSURE: Air in the atmosphere is made up of a number of gases. These gases press down on the Earth?s surface, exerting a force that we call atmospheric pressure or air pressure. Changes in air pressure bring changes in the weather and make winds blow. Air usually moves from areas of high pressure to areas of low pressure, and this produces winds. Changes in air pressure are measured on an instrument called a barometer. Most barometers give a reading in millibars (mb). Readings over 1010 mb indicate high pressure.


ALTIMETER: An Altimeter is an instrument used to measure the altitude of an object above a fixed level. Altitude can be determined based on the measurement of atmospheric pressure. The greater the altitude the lower the pressure. When a barometer is supplied with a nonlinear calibration so as to indicate altitude, the instrument is called a pressure altimeter or barometric altimeter.


ANENOMETER: An Anemometer is a device for measuring wind speed, and is a common weather station instrument. The term is derived from the Greek word anemos, meaning wind, and is used to describe any airspeed measurement instrument used in meteorology or aerodynamics. Anemometers can be divided into two classes: those that measure the wind's speed, and those that measure the wind's pressure.


APPARENT TEMPERATURE: Apparent Temperature is the general term for the perceived outdoor temperature, caused by the combined effects of air temperature, relative humidity and wind speed.


BAROMETER: Barometric pressure is the pressure exerted by the weight of the column of air above a given point, expressed in kilopascals. Generally speaking, when the barometric pressure is high, the air is sinking, usually resulting in fair weather. When the barometric pressure is low or falling, air is rising, usually resulting in cloudy skies and precipitation.


BEAUFORT SCALE: The Beaufort Scale is a scale for measuring wind speeds. It is based on observation rather than accurate measurement. It is the most widely used system to measure wind speed today. There are twelve levels, plus 0 for "no wind". (See also:Beaufort Scale for complete Scale indication).


CUMULUS CLOUDS: Besides the Software Engine behind this Weather Station (Cumulus) it has another meaning as well. Cumulus clouds look like white fluffy balls of cotton. They usually have flat bases and lumpy tops. Cumulus clouds can form on fair weather days with daytime heating or with the passage of the front. Most of these clouds form below 18-hundred meters and are relatively thin and associated with fair weather.


COLD FRONT: A cold front is the leading edge of colder air. In front of it, you usually have warmer, more humid air. Behind the front lies much cooler or colder and drier air. The cause for cold fronts is colder air masses migrating southward from the polar regions.


COOLING DEGREE DAYS: Cooling Degree Days is the cumulative number of degrees in a month or year by which the mean temperature is above 18.3C. Cooling degree days are also based on the day's average minus 18.3C. They relate the day's temperature to the energy demands of air conditioning. For example, if the day's high is 32C and the day's low is 21C, the day's average is 26.5. 26.5 minus 18.3 is 8.2 cooling degree days.


DEW POINT: The Dew Point is a measure of atmospheric moisture. It is the temperature at which air must be cooled in order to reach saturation (assuming that air pressure and moisture content are constant). As the surface of the earth cools at night, warm moist air near the ground is chilled and water vapour in the air condenses into droplets on the grass and other objects. Dew is particularly heavy on clear nights, when the earth cools rapidly. When a blanket of cloud insulates the earth, the cooling rate is slower. The greater the difference between the temperature and the dew point, the drier the air.


FEELS LIKE: Refers to how the outdoor air is expected to feel in degrees Celsius when actual temperature and relative humidity are combined (see Humidex) or when actual temperature and wind are combined (see Windchill).


GUST: A sudden significant increase in, or rapid variation of wind speed. Usually a gust lasts less than twenty seconds.


HEAT INDEX: The Heat Index is an index that combines Air Temperature and Relative Humidity in an attempt to determine the human-perceived equivalent temperature ? how hot it feels. The result is also known as the "Apparent Temperature". For example, when the temperature is 32 C with very high humidity, the heat index can be about 41 C.


HEATING DEGREE DAYS: Heating Degree Days is the cumulative number of degrees in a month or year by which the mean temperature falls below 18.3C. To calculate the heating degree days for a particular day, find the day's average temperature by adding the day's high and low temperatures and dividing by two. If the number is above 18.3C, there are no heating degree days that day. If the number is less than 18.3C, subtract it from 18.3C to find the number of heating degree days.


HEAT WAVE: A Heat Wave is three (3) or more consecutive days with temperatures above 32C. Heat waves occur when the jet stream rides far to the north, creating a large ridge of high pressure to the south. The sinking air within that high pressure system warms and produces high temperatures and clear skies over a wide area.feels.


HIGH PRESSURE CENTRE: Indicates an area of high pressure. In a high, air will slowly descend and flow out in a clockwise direction at the ground. Normally a high will bring mainly sunny skies to an area.


HUMIDEX: The Humidex is an index number used by British meteorologists to describe how hot the weather feels to the average person, by combining the effect of heat and humidity. The Humidex is a unit-less number based on the Dew Point, but it is equivalent to dry temperature in degrees Celsius. For example, if the temperature is 30 C , and the calculated Humidex is 40, then it indicates the humid heat feels approximately like a dry temperature of 40 C . The index is widely used in UK weather reports during summer.


HYGROMETER: A Hygrometer is an instrument used for measuring the moisture content in the environment. Humidity measurement instruments usually rely on measurements of some other quantity such as temperature, pressure, mass or a mechanical or electrical change in a substance as moisture is absorbed. By calibration and calculation, these measured quantities can lead to a measurement of humidity.


JET STREAM: The Jet Stream is like a current or river of air in the upper atmosphere, 7,000 to 13,000 metres up. It?s created when cold and warm air masses come together. In the mid-latitudes of the Northern Hemisphere winds will tend to move from the west to the east. The jet stream, quite simply put, is the apex of these winds in the upper atmosphere. The minimum criterion for jet stream speed is 93 km/h . The location and orientation of the jet stream changes from day to day. Weather patterns are influenced by the position, strength and orientation of the jet stream.


LOW PRESSURE CENTRE: Indicates an area of low atmospheric pressure. In a low, air is flowing counterclockwise into the centre of the Low. The air will rise and cool often resulting in clouds and precipitation.


PRECIPITATION: Any and all forms of water, either in liquid or solid state, that falls from clouds and reaches the ground. Deposits of dew, fog and frost are not included in this category.


RAIN GAUGE: A Rain Gauge is a type of instrument used to gather and measure the amount of liquid precipitation over a set period of time. Most Rain Gauges generally measure the precipitation in millimeters. Types of Rain Gauges include graduated cylinders, weighing gauges, tipping bucket gauges, and simple buried pit collectors. The one used at N.C.I. Rossall Point is a "tipping bucket" Rain Gauge with measurement / display in 0.1mm accuracy.


UV INDEX: Ultraviolet Radiation (UV) from the sun is a higher energy form of radiation which can cause damage to the skin and eyes depending on the dose. The UV Index ranges from 0 to 11+ and represents the intensity of the sun?s ultraviolet radiation. Higher UV Index values relate to higher doses of UV radiation and a greater potential for sunburn. The numbers indicating UV Index are related to the amount of UV radiation reaching the surface of the earth, measured in W/m. (See also UV Info on this Web Site)


WET-BULB TEMPERATURE The Wet-Bulb Temperature is a type of temperature measurement that reflects the physical properties of a system with a mixture of a gas and a vapor, usually air and water vapor. Wet-Bulb Temperature is the lowest temperature that can be reached by the evaporation of water only. It is the temperature one feels when one's skin is wet and is exposed to moving air. Unlike dry bulb temperature, wet bulb temperature is an indication of the amount of moisture in the air.


WINDCHILL: Windchill is the cooling our body feels when the impact of temperature and wind are combined. Normally, on a relatively calm day our body is able to provide some protection from the outside temperature by heating up a thin layer of air that lies close to the skin. This added insulation is known as a boundary layer. On windy days, however, this insulating layer gets taken away, leaving our skin more exposed to the outside temperature. It takes time and energy for our body to warm up another layer of air, and if this layer continually gets blown away, eventually our skin's temperature will fall and our body will feel colder.


WIND RUN: Wind Run is a meteorological term used to categorize or determine the total distance (or amount) of the traveled wind over a period of time. The readings are collected using an anemometer (usually part of a weather station).


WIND SPEED: The Average Wind Speed taken during an hourly ten-minute window, and given in kilometers per hour.