MCC Weather

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Listed below are numerous terms common to this and other weather sites.

For a comprehensive list of weather terms, consult the glossaries of the

National Weather Service, or

Environment and Climate Change Canada.

Alerts - warning system providing information to ensure public safety for meteorological and related events. The primary alerts in order of precedence from lowest to highest:

Environment Canada - Statement, Advisory, Watch, Warning

US NWS -Statement, Advisory, Watch, Warning

MCC Weather - Note, Advisory

Anemometer - instrument for measuring wind speed.

Arctic Outflow - large movement of the cold air mass, normally residing in the interior regions of Canada, towards the more temperate coastal regions. This is the most common cause of wintry conditions in the Vancouver area. 

Atmosphere - held in place by gravity, it is the layered gaseous body enveloping the Earth.  It's composition is primarily nitrogen (78%), oxygen (21%), argon, carbon dioxide, and other trace gases. The layer closest to Earth's surface is the troposphere, next is the stratosphere.  The troposphere makes up about 75% of the atmosphere and is where most weather occurs.

Baro - abbreviation of Barometer. see Barometer

Barometer - instrument used to measure atmospheric pressure.  A primary tool of meteorology used to develop a weather forecast. Common units of measure include millibars, pascals, and inches of mercury.  As the unit of measure increases in value, pressure increases, and the barometer is said to be rising.  In the case of decreases, the barometer is said to be "falling". 


Barometric Trends - general terms to describe changing weather. With any change in the weather, better or worse, the actual conditions will vary.  The change itself introduces instability, with the result being weather similar to a storm even when overall conditions may be improving.                   

Rising rapidly - a break in the weather soon

Rising - conditions will improve

Rising slowly - expect mildly better conditions

Steady - little to no change 

Falling slowly - expect mildly worse conditions

Falling - conditions will worsen

Falling rapidly - significantly worse soon, stormy

Chandler Burning Index - uses temperature and humidity to create a numerical index of fire danger.  This number doesn't factor in fuel moisture content.  Helpful as a relative tool for gauging fire danger from day to day.

CoCoRaHS - a citizen science organization of volunteers dedicated to collecting data on precipitation and related phenomena.  It began in Colorado in 1998, it now counts more than 20,000 observers across North America, the Bahamas, Puerto Rico, and the US Virgin Islands.  CoCoRaHS is acronym for Community Collaborative Rain, Hail, & Snow Network. 

Convection - rising air currents.  There are numerous types, or sources, of convection.  Weather is influenced by convection and can contribute to its creation as well.  


Coriolis - the combined effect of gravity and rotation of the Earth.  Weather systems and their movement are impacted by this factor.     

Cumulus (cloud) - the cotton ball like clouds which form at all altitudes.  Cumulus is a general term, there are numerous sub-species of cumulus clouds.

Cumulonimbus - abbreviated as Cb, these are the massive clouds which form as mature thunderstorms.  The precursor to cumulonimbus is the Towering Cumulus.  

Citizen Weather Observer Program (CWOP) - a global volunteer network of privately operated Personal Weather Stations.  These Weather Observers continuously report their observations to the US National Weather Service via internet connection and are quality checked for accuracy as the data is compiled.  Besides the public forecast, more than 800 organizations utilize the data, including numerous universities and NASA.  There are more than 7000 stations in North America alone.    

Dew Point - the point at which water vapour condenses taking visible form as fog, clouds, precipitation, and dew.  Humidity reaches 100% as temperature and dew point converge. 

Doppler Radar - a specific type of radar.  Primarily measures movement within the atmosphere.  This allows meteorologists to observe and predict precipitation and wind patterns.  

Environment Canada (EC) - federal agency of the Government of Canada.  Canada's official and primary agency responsible for weather prediction, atmospheric science, environmental regulation, and public information of environmental concerns.  Similar to NOAA in the US, with some added responsibilities for land sciences. 

Evaporation - also known as vaporization, is a phase transition of a substance.  In this case, from liquid to gas.  The opposite of this is condensation.

Fall Streak Hole - a large gap, usually circular or elliptical, that can appear in cirrocumulus or altocumulus clouds.  The holes are formed when the water temperature in the clouds is below freezing, but the water droplets have not yet frozen due to a lack of the required condensation nuclei.

Fire Weather Index - a system for rating potential fire intensity for use in forested areas.  It factors in fuel and weather conditions.  While intended for use by fire services, the numeric values can be translated to word.  Allowing for the public to be notified of the fire danger with a series of risk levels.

Forecast - a prediction of future weather conditions based on current available data, satellite and radar imagery, known patterns, and intuition. 

Fog - a form of cloud in contact with the ground, it is the presence of visible water droplets suspended in the air. Fog forms when the difference between air temperature and dew point is less than 2.5°C.  Humidity will normally be at or close to 100%.  There are numerous forms of fog, each caused by different effects.  When visibility is greater than 1 km, it is referred to as mist, and if humidity is less than 95%, it becomes haze.

Freezing - is a phase transition of a substance.  In this case, from solid to liquid.  The opposite of this is melting.

Frost - when humid air which is above freezing comes in contact with surfaces below freezing a thin layer of ice crystals can form on that surface.  There are a number of different types of frost.  Each the result of differing local effects.  Frost can damage or kill vegetation.  

GDD - abbreviation of the term Growing Degree Days.

Growing Degree Days - using a base temperature to determine when plants will germinate and grow.  A plant with a GDD of 10° Celsius will only grow at and above this temperature. Using GDD, one can estimate the day a plant will reach each stage of growth, when it will need tending, right through to maturity or harvest.

Gust - a temporary and noticeable change in the velocity of the wind.

Halo (22° Halo) - is an optical phenomenon resulting from ice crystals suspended in the atmosphere.  It appears as a ring or halo around the Sun or Moon with an approximate 22° radius.  When visible around the moon, it is called a moon ring or winter halo.  When around the Sun, it can be accompanied by another optical effect commonly called a Sundog.

High / High Pressure - an area of increased atmospheric pressure.  Essentially a heavier column of air.  Weather generally flows out of a high and into a low. 

Humidity - a measurement of the amount of water vapour contained in the atmosphere.  It is expressed as a percentage of the total amount of water vapour a given body of atmosphere can hold.  Condensation occurs at 100% humidity.  Air is said to be dry when humidity is near or below 40%.

Humidex - a Canadian weather term used to describe how hot the weather feels to an average person.  It's determined using temperature and humidity.

Lightning - an electrical discharge in the atmosphere.  It is static energy built from the friction between moving particles.  Usually found in a storm, some lightning reaches the ground, and some travels between clouds.  

Low / Low Pressure - an area of decreased atmospheric pressure relative to the surrounding atmosphere.  Disruptive weather generally flows towards and collects in low pressure area's.  Wind is the movement of air between high and low pressure area's as the atmosphere attempts to equalize the difference.      

MCC - abbreviation for Mesoscale Convective Complex. A large, powerful, and sustained storm, forming nocturnally, from numerous daytime convective storms. This storm type exhibits specific characteristics as seen by satellite.

MESOnet - a network of weather stations spaced closer together than normal, usually within 5 to 40 kms of each other.  Their purpose is to collect instantaneous data on current weather conditions allowing for insight into patterns and trends of smaller scale, short term (see meso) events often missed or too small to see within the larger synoptic scale. There can be significant differences in observations taken by stations of the same mesonet.  MCC Weather operates a MESOnet consisting of 6 stations in Metro Vancouver, the 7th station in Sunshine Valley being too far away to be considered within the network.

Mesoscale - a meteorological term to describe medium scale events and phenomena.  These can often have short term, short range effects, yet be significant to the area's in which they occur. The Mesoscale Convective Complex, being at the large end of these phenomena, is in fact an amalgamation of numerous mesoscale events.

Meteorology - a science of atmospheric study. Weather is a major component. 

Meteorologist - a person who works the science of meteorology. 

Meteorological Service of Canada - a sub unit of Environment Canada. Responsible for weather prediction and related atmospheric sciences as well as public notification of weather phenomena.  Equivalent to NWS of the US. 

National Oceanic and Atmospheric Administration (NOAA) - a scientific agency within the Department of Commerce of the United States government.  NOAA has a mandate to study all aspects of oceanic and atmospheric sciences. The National Weather Service is one part of NOAA. 

National Weather Service (NWS) - federal agency of the United States Government.  The official and primary agency responsible for weather prediction and public notification of weather phenomena in the United States.  A sub-unit of NOAA.  The NWS observes and forecasts weather for all area's on Earth. 

Nimbostratus - a thick, grey looking cloud found at altitudes under 10000 feet.  Sometimes referred to as ”rain clouds”, they are a common source of precipitation.                                                                                                     

Observations - common term used when referencing meteorological data.                                            

Offshore Wind - a wind blowing from land outward to a large body of water.                             

Onshore Wind - a wind blowing from a large body of water onto land. 

Personal Weather Station (PWS) - small, reasonably inexpensive, clustered instrument packages used by hobbyists to measure and predict weather. Most measure temperature, humidity, and pressure; many add other capabilities such as wind, rain, UV, lightning, and more. 

Precipitation - any kind of liquid or solid water falling from the sky towards the surface, including rain, drizzle, snow, sleet, and hail. 

Pressure - the force exerted on the Earth's surface by atmosphere. In meteorology, pressure is termed to be either High or Low.  These terms are relative to each other within an air mass.  Generally, weather systems are forced out of the High and into the Low pressure zones as the atmosphere attempts to equalize.            

Radar - an invented device which emits and receives radio waves.  Radar can detect objects like precipitation particles.  Depending on the radar used, things like the type of precipitation and wind patterns can be determined.   

Rainbow - a phenomenon that is caused by the reflection, refraction and 

dispersion of light in water droplets.  It takes the form of a multicoloured arc. Rainbows caused by sunlight always appear in the section of sky directly opposite the sun.  They can be half or full circles.  Double rainbows can also be seen.  Caused by the light being refracted twice inside the water droplet, the second rainbow will appear with a reverse colour pattern.

Ridge - a linear section of air at the point of highest atmospheric pressure.  

Satellite - an important observation and communication tool used to monitor and investigate meteorological and atmospheric events.  Satellites used for these purposes can be at low or high orbit and be in geosynchronous or geostationary orbit. 

Sheer (Wind Sheer) - a sudden and noticeable change in wind direction.

Stn - abbreviation for Station.

Storm - from small to massive, these are body's of disturbed air.  They can be comprised of cloud, precipitation, wind, lightning, tornadoes and can cause damage to structures and natural objects.   The Earth's largest and most powerful storms are known as hurricanes or typhoon's.

Stratus (cloud) - when water vapour forms a smooth sometimes fog like cloud, this is a stratus cloud.  There are numerous sub-species of stratus cloud, they form at all altitudes.

Sublimation - the phase transition of a substance.  In this case, directly from solid to gas without passing through the liquid phase.  The opposite of this is deposition.

Sun Dog - an optical effect caused when sunlight is refracted by ice crystals suspended in the atmosphere.  Formally called a parhelion, it consists of a bright spot to one or both sides of the Sun.  These bright spots are usually around 22° to the left and/or right of the Sun, and at the same altitude above the horizon. 

Thunder - the audible result of lightning super heating the atmosphere.

Thunderstorm - a powerful and massive storm which contains and produces numerous types of precipitation, high wind, lightning, and thunder.  These storms can build to altitudes as high as 40,000 feet, even higher in the tropical regions.  

Trough - a linear section of air at the point of lowest atmospheric pressure.

Turbulence - the physical result when unstable air moves.  Turbulence is easily felt in an airplane as bumpy, unpredictable movement.  Similar to waves on the ocean, there are numerous sources capable of producing or affecting  turbulence.


UV Index - a standard scale measuring the amount of ultra violet (UV) radiation to which a person can be exposed.  The higher the number, the more precautions needed to prevent sunburn.                                                                                                                                            

Virga - precipitation which falls towards the surface but evaporates or sublimates before reaching it.                                                                                         

Wind Chill - a combination of temperature and wind factoring in the cooling effect caused by the wind.                                                                                                     

Wind Direction - expressed in compass degrees or by cardinal, it is always the direction the wind is coming from at the point where the observation is made. ie A westerly wind would be originating from the west and blows to the east. 


Weather Station - professional, amateur, and scientific sites dedicated to observing and measuring weather at their location.  They can be of any size, stationary or mobile, as in the case of ships, and may or may not share their data with outside sources. Some are equipped with expensive instruments, others use rudimentary devices. Official weather stations follow international quality control standards. 

Wx - abbreviation for Weather