Atmosphere: The mass of air held close to the earth by gravity. The atmosphere is divided into four sections: the troposphere reaches an altitude of about 10 km from the earth's surface; the stratosphere which is at 10 km to 50 km from the earth’s surface; the mesosphere which is at 50 km to 80 km from the earth’s surface; and lastly the thermosphere which is anything beyond 80 km from the earth’s surface.

Atmospheric Pressure: The pressure exerted by the weight of air above a given point, sometimes expressed in millibars (mb) or inches of mercury (Hg). The internationally recognized unit for measuring atmospheric pressure is the kilopascal (kPa).

Air Mass: An extensive body of the atmosphere with comparable temperature and humidity. It may extend over an area of several million square kilometres and over a depth of several kilometers.

Astronomy: The scientific study of matter in outer space, especially the positions, dimensions, distribution, motion, composition, energy, and evolution of celestial bodies and phenomena

Aviation: The operation of aircraft to provide transportation

Backing Wind: A counter-clockwise change in wind direction. For example, from southwest to southeast, through south. It is the opposite of veering wind.

Chance of Precipitation: The chance that measurable precipitation (0.2 mm of rain or 0.2 cm of snow) will fall on any random point of the forecast region during the forecast period.

Climate: The weather in some location averaged over some long period of   time.

Climate data: The meteorological data for long period, including temperature, precipitation, and wind that characteristically prevail in a particular region.

Climatology:  The meteorological study of climates and their phenomena.

Cloud: A visible cluster of water droplets and/or ice particles in the atmosphere.

Cloud-to-cloud Lightning: A lightning discharge that occurs between areas of the same cloud (intra cloud) or from one cloud to another cloud (inter cloud).

Cloud-to-cloud flashes: are much more common than cloud to ground flashes (on average there are three to five cloud-to-cloud flashes, for every one cloud-to-ground flash).

Cloud-to-ground Lightning: A lightning discharge that occurs between a cumulonimbus cloud (thunderstorm) and the ground.

Cold Front: A transition zone where a cold air mass advances and replaces a warm air mass.

Cumulonimbus Clouds: Large, dense cloud that frequently has an anvil-shaped top. These clouds produce heavy rain showers, lightning, thunder and sometimes hail or tornadoes. The entire cloud can only be seen from a distance. (See thunderstorm)

Cumulus Clouds: Bright clouds that appear in fair weather, that have broad, horizontal bases, producing no precipitation and rarely covering more than one-half of the sky.

Cyclone: A low pressure system with a cyclonic circulation. It is also called a depression, and is generally associated with poor or stormy weather. The point of lowest atmospheric pressure marks the centre of the cyclone.

Extreme climate: Is the climate phenomena indicative of an anomalous climate that is extreme relative to historical patterns.

Severe weather: High impact unstable weather phenomena likely to endanger the safety of life or cause damage to property.

Dew Point: The temperature to which air must be cooled, in order to become saturated by the water vapour already present in the air.

Downburst: A strong convective downdraft resulting in an outward burst of often damaging winds at or near the surface. The affected area may be from less than 1 km to more than 100 km in horizontal dimension.

Easterly Wave: Also known as tropical wave or African Easterly Waves, they are a type of atmospheric low pressure trough, oriented north to south, which moves from east to west across the tropics, causing areas of cloudiness and thunderstorms. An easterly wave or tropical wave can develop into a tropical cyclone.

El Niño can be distinguished when the surface waters in the eastern tropical Pacific extending westward from Ecuador become warmer than average. The changing pattern of the Pacific Ocean causes a shift in the atmospheric circulation, which then impacts weather patterns across much of the earth. El Nino is like La Niña's brother, the totally opposite.

La Niña: An extensive cooling of the waters in the tropical eastern Pacific Ocean. It is the climatic opposite of the El Niño.

Fog which forms when a relatively moist and warm air mass moves over a colder water or land surface.

Forecast: A statement of expected meteorological and environmental conditions for a specified time or period, and for a specified area or portion of airspace.

Forecasting: To estimate or calculate in advance, especially to predict weather conditions by analysis of meteorological data

Front: The boundary between two different air masses. A cold front is the leading edge of an advancing cold air mass, while a warm front is the trailing edge of a retreating cold air mass.

Frontal Cyclone: Any cyclone associated with a front. It is often associated with extratropical cyclone (as opposed to tropical cyclone, which is non-frontal).

Geostationary Satellite: A satellite that orbits the earth at the same rate that the earth rotates, and as a result remains over a fixed place above the equator.

Hail: Precipitation in the form of lumps of ice mainly associated with thunderstorms. Hail ranges in size from that of a small pea to the size of cherries, but has been observed as large as grapefruit. Hail in Canada occurs most frequently during the summer when thunderstorm activity is at its peak.

Hazard: A type of condition (weather or environmental) that has a negative impact on the safety and security of the public.

Haze: Consists of fine particles of dust and pollution suspended in the atmosphere, and is distinguished from fog by its bluish or yellowish tinge.

High: Region of the atmosphere where the pressures are high, relative to those in the surrounding region at the same level. In the Northern Hemisphere, winds around a high move in a clockwise fashion.

Humidity (also called Relative Humidity): Humidity is the measure of water vapour content in the air. Usually, relative humidity is expressed as a percentage of total possible moisture content.

Intermittent Rain: Rain that starts and stops repeatedly, although not as abrupt or as frequent as showers.

Land Breeze: This coastal breeze blows from the land to the sea, lake or river, and usually occurs at night when the temperature of the water is often warmer than the nearby land. The water heats the air above, which rises and is replaced by cooler air from the land. (See also sea breeze)

Landfall: When the eye, or physical centre of cyclone, reaches a coastline it is said to make landfall.

Latent Heat: Heat that is stored in water vapour in the atmosphere. When water vapour rises, cools and condenses into liquid water, it releases this heat into the surrounding atmosphere.

Lead time: The difference between the time that an alert is issued, and the actual event time.

Light Wind: Wind speed of 11 knots (20 km/h) or less, with a Beaufort wind force of 0 to 4.

Lightning: Generally, any and all of the various forms of visible electrical discharge that are produced by thunderstorms; often seen as a bright flash of light in the sky.

Lightning Flash: A scientific term used to describe lightning, which consists of more than one individual stroke of lightning (and as many as 20 single lightning strokes within a single flash). This causes the “flickering” effect that is sometimes seen in lightning.

Lightning Flash Density: The number of lightning flashes detected per square unit of area (usually per square kilometre or mile) and unit of time.

Lightning Strike: Another term for cloud to ground lightning.

Lightning Stroke: A scientific term used to describe a single discharge of lightning. There can be many individual strokes of lightning within a single lightning flash.

Maritime: Of or relating to the sea, native to, inhabiting, or formed by the sea

Mesopause; Top of the mesosphere situated at about 80-85 km above Earth’s surface. It corresponds to the level on minimum temperature.

Moderate Wind: Wind speed of 12 to 19 knots inclusive, with a Beaufort wind force of 4 to 5.

Mesosphere: Region of the atmosphere, situated between the stratopause and the mesopause, in which the temperature generally decreases with height.

Meteorological information: The information on the weather conditions at a particular place be it weather forecast, meteorological data etc.

Meteorological records: The information of the meteorological historical data like rainfall amounts, temperature, sunshine hours etc.

Meteorological services: Services that are being offered to the stakeholders by the National Meteorological Service like data provision, weather forecasting etc.

Meteorologist:  A scientist who studies the weather and atmosphere.

Meteorology: Is the science of the state of the atmosphere and its interaction with ocean and land that focuses on weather processes and climate, their prediction and applications.

Precipitation: Precipitation is a liquid or solid form of water falling from the atmosphere to the earth's surface. Examples include rain, freezing rain, hail, and snow.

Prevailing Winds: The wind direction most frequently observed during a given period of time.

Radar: Radar is an acronym for Radio Detection And Ranging. It is used in the meteorological environment to detect and locate the presence of clouds and precipitation.

Rain: Moisture condensed from atmospheric vapour that falls to earth in drops.

Rainfall Warning: A warning issued for a hazardous, prolonged or intense short-duration of rainfall. It may be a major factor in the cause of disasters, such as floods, flash floods, and landslides.

Sea Breeze: This is the breeze which blows from the sea or a large lake, to the land. The breeze is set off when the temperature of the land is higher than the temperature of the water. The land heats the air above, which rises, and it is then replaced by the cooler air from over the water. (Opposite to the land breeze).

Shower: Precipitation that is characterized by the suddenness with which it starts and stops, by its rapid changes in intensity, and usually by the rapid changes in the appearance of the sky.

Storm Surge: The positive or negative difference in sea level from the predicted astronomical tide, due to the forces of the atmosphere. The two main atmospheric components that contribute to a storm surge are air pressure and wind.

Stratopause: A boundary or zone of transition separating the stratosphere and the mesosphere; it marks a reversal of temperature change with altitude. It is located at the height of roughly 50 km.

Stratosphere: The region of the atmosphere extending from the top of the troposphere (the tropopause), at height of 10-17 km to the base of the mesosphere (the stratopause), at a height of roughly 50 km.

Stratus: Generally grey cloud layer with a fairly uniform base, which may produce drizzle, ice prisms or snow grains.

Strong Wind Warning: A warning issued when a wind speed of 20 to 33 knots inclusive, is forecast or observed over coastal or inland water bodies during the recreational boating season.

Subtropical Ridge: A large belt of high pressure located around 30 degrees latitude in both hemispheres, and characterized by mostly calm winds. Air flows out from the ridge toward the upper and lower latitudes of each hemisphere, creating both the trade winds and the westerlies. 

Subtropical Storm: A subtropical storm is a cyclone that has characteristics of both a tropical storm and an extratropical cyclone.  Subtropical Cyclones can form in waters normally too cool for tropical cyclones.

Temperature: In meteorological terms, temperature refers to the degree of heat or cold of the air, as measured by a thermometer.

Temperature Anomaly: The deviation of temperature in a given region over a specified period from the long-term average value for the same region.

Thermosphere: Layer of the Earth's atmosphere, above the mesopause, in which the temperature generally increases with height.

Thunder: The sound that results from the formation of lightning. This burst of lightning expands air around it, producing an effect similar to an explosion, thus creating the noise.

Thunderstorm: A local storm, usually produced by a cumulonimbus cloud, and always accompanied by thunder and lightning.

Tornado: A tornado is a violently rotating column of air that extends from a cumuliform cloud to the surface. The pressure deficit in a tornado often results in the formation of a funnel cloud that extends fully or partially from the cumuliform cloud to the surface. A tornado is typically also made visible by rotating debris near the ground or a spray ring near the water surface.

Towering Cumulus Clouds: Cauliflower-shaped clouds associated with intermittent-type precipitation (showers). They may appear to be tall and towering, or tall and broad, and can either be isolated, or grow from a group of lower cloud called cumulus.

Trade Winds (also called Tropical Easterlies): The belts of wind on either side of the equator, blowing from the northeast in the Northern Hemisphere, and from the southeast in the Southern Hemisphere. In both hemispheres the winds become more easterly the closer they are to the equator.

Tropical Depression: A tropical cyclone with maximum sustained surface winds of 37 to 62 km/h. It does not typically have the organization or the spiral shape of the more powerful storms. When a tropical cyclone reaches this stage it is given a number so that it can be better recognized.

Tropical Disturbance: An organized region of showers and thunderstorms in the tropics, generally 200 km to 600 km in diameter, that maintains its identity for at least 24 hours but does not have a circular wind circulation.

Tropopause: The boundary between the troposphere and the stratosphere, where an abrupt change in lapse rate usually occurs.

Troposphere: The lowermost layer of the atmosphere, in which air temperature falls steadily with increasing altitude. The troposphere begins at ground level and ranges in height from an average of 11 km (at the International Standard Atmosphere) at the poles to 17 km at the equator.

Trough: An elongated area of relatively low pressure, extending from the centre of a low pressure region. It is the opposite of a ridge.

Tsunami: A gravitational sea wave produced by any large-scale, short-duration disturbance of the ocean floor. It is often caused by a shallow submarine earthquake but can also be caused by submarine earth movement, subsidence, or volcanic eruption

Turbulence: The vertical motion of the air, at times violent, which can cause the up-and-down movement of a plane.

Unstable: A turbulent, convective state in the atmosphere, resulting from a rapid decrease in air temperature with the altitude.

UTC: UTC is the international abbreviation for Coordinated Universal Time. It is the local time at the Greenwich meridian (0°), situated in United Kingdom. The World Meteorological Organization (WMO) has adopted the UTC as the standard time for use in reporting of all meteorological data. Times in UTC sometimes get the suffix "Z", i.e. 16Z. From the "Z" suffix came the mnemonic "Zulu" (as used in international marine communications standards).

Veering Wind: A clockwise change in wind direction. For example, from southeast to southwest, through south. It is the opposite of backing wind.

Vertical Wind Shear: The condition produced by a change in wind velocity (speed and/or direction) with height. Vertical wind shear can weaken or destroy a tropical cyclone, by interfering with its symmetric nature and organization.

Visibility: The greatest distance at which a black object of suitable dimensions can be seen and recognized against the horizon sky during daylight. It could also be seen and recognized during the night if the general illumination were raised to the normal daylight level.

Warm Front: A transition zone where a cold air mass retreats and is replaced by a warm air mass.

Warning: A type of alert issued where a hazardous weather or environmental event that poses a significant threat to public safety and property is certain or imminent.

Watch: A type of alert issued where conditions are favourable for the development of weather or an environmental hazard that poses a significant threat to public safety and property, but the occurrence, location, and/or timing of the expected hazardous condition(s) is still too uncertain to issue a warning. It is intended to heighten public awareness of the potential impact of the event, and serves as a lead-up to a warning.

Wavelength: The distance between two successive crests or troughs of a wave.

Weather: The condition of the troposphere (lower atmosphere) at any particular time and place.

Weather and Meteorology: Meteorology is the science that studies the processes and phenomena of the atmosphere. Weather is the state of the atmosphere at a given time and place with regard to temperature, atmospheric pressure, humidity, wind, cloudiness, and precipitation. The term weather is used mostly for conditions over short periods of time.

Weather Summary: A bulletin issued which contains a post-event summary of a weather event, especially one that has received significant media attention.

Westerlies: The dominant west-to-east motion of the atmosphere, centered over the middle latitudes of both hemispheres.

Wind: The horizontal movement of air, relative to the earth's surface.

Wind Shear: The change of wind speed and/or wind direction