Glossary

A

atmospheric pressure/barometric pressure

Atmospheric Pressure, also called barometric pressure, is the force per unit area exerted by the atmosphere as a consequence of its weight. It is equal to the weight of a vertical column of air of a unit area, extending from a given elevation or level to the outer limit of the atmosphere. Measurement units for pressure are called hectopascals (hPa).

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B

blowing dust

Dust that is raised by the wind to moderate heights above the ground. Visibility at eye level may be reduced to 1 km.

blowing snow

Snow particles violently stirred up by wind to sufficient heights above the ground to reduce visibility to 10 km or less.

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C

calm

An absence of wind flow or any other air motion.

celsius

A temperature scale where water at sea level has a freezing point of 0°C (Celsius) and a boiling point of +100°C. Commonly used in countries that observe the metric system of measurement. Created by Anders Celsius in 1742. In 1948, the Ninth General Conference on Weights and Measures replaced "degree centigrade" with "degree Celsius."

To convert from degrees Celsius to degrees Fahrenheit, (C * 9/5) + 32

climate

The historical record and description of average daily and in seasonal weather events that help describe a region. Statistics are generally drawn over several decades. Climatology, or the study of climate, includes climatic data, the analysis of the causes of the differences in climate, and the application of climatic data to the solution of specific design or operational problems. It differs from weather, which is concerned with short term or instantaneous variations in the state of the atmosphere at a specific time.

climate ID, station ID, station number

Also referred to as station ID or station number, the Climate ID is a 7 digit number assigned by the Meteorological Service of Canada to a site where official weather observations are taken, and serves as a permanent, unique identifier.

The first digit assigned identifies the province where the second and third digits identify the climatological district within the province.

When observations are discontinued at a site, the number is not used for subsequent stations (which may, or may not, differ in name) unless it is judged that the records from the earlier and subsequent stations may be combined for most climatological purposes.

cloud

A visible collection of minute particle matter, such as water droplets and/or ice crystals, in the air. A cloud forms in the atmosphere as a result of condensation of water vapour. Condensation nuclei, such as smoke or dust particles, form a surface upon which water vapour can condense.

Code for Normals

Normals for some elements are derived from less than 30 years of record but can still be considered useful. The minimum number of years used are indicated by a "code" defined as:

  • "A": No more than 3 consecutive or 5 total missing years between 1971 to 2000.
  • "B": At least 25 years of record between 1971 and 2000.
  • "C": At least 20 years of record between 1971 and 2000.
  • "D": At least 15 years of record between 1971 and 2000.

cooling degree-days

Cooling degree-days for a given day are the number of degrees Celsius that the mean temperature is above 18°C. If the temperature is equal to or less than 18°C, then the number will be zero. For example, a day with a mean temperature of 20.5°C has 2.5 cooling degree-days; a day with a mean temperature of 15.5°C has zero cooling degree-days. Cooling degree-days are used primarily to estimate the air-conditioning requirements of buildings.

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D

data interval

Depending on individual climate observing programs and climate element, weather observations at a climate station are recorded at three basic intervals:

  • Hourly: indicates data values for observations taken on an hourly basis.
  • Daily: indicates data values for observations taken once in a 24-hour period.
  • Monthly: are averages for each month, derived from daily data values.

Daylight Saving Time

In Canada, time zones and daylight saving time (DST) are the responsibility of provincial and territorial governments. Clocks are moved ahead one hour on the second Sunday in March, and back one hour on the first Sunday of November. At present, all of Canada except Saskatchewan, and parts of British Columbia and Quebec, follows daylight saving time.

degree-days

Degree-days for a given day represent the number of Celsius degrees that the mean temperature is above or below a given base temperature. For example, heating degree-days are the number of degrees below 18°C. If the temperature is equal to or greater than 18, then the number will be zero. Values above or below the base of 18°C are used primarily to estimate the heating and cooling requirements of buildings. Values above 5°C are frequently called growing degree-days, and are used in agriculture as an index of crop growth.

dew point temperature (°C)

The dew point temperature in degrees Celsius (°C), a measure of the humidity of the air, is the temperature to which the air would have to be cooled to reach saturation with respect to liquid water. Saturation occurs when the air is holding the maximum water vapour possible at that temperature and atmospheric pressure.

direction of maximum gust (10's deg/tens of degrees)

The direction of the maximum gust (true or geographic, not magnetic) from which the wind blows. Expressed in tens of degrees (10's deg), 9 means 90 degrees true or an east wind, and 36 means 360 degrees true or a wind blowing from the geographic north pole. This value is only reported if the maximum gust speed for the day exceeds 29 km/h.

drizzle

Fairly uniform precipitation composed of fine drops of water (diameter < 0.5 mm). Drizzle drops are too small to cause appreciable ripples on the surface of still water. The drops appear almost to float in the air, thus making visible even slight movements of air.

dry bulb temperature

The dry bulb temperature is the value taken from a thermometer when the bulb of the thermometer is dry. It reflects the ambient temperature independent on the moisture content of the air.

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E

elevation (m)

The elevation in metres (m) refers to the elevation of the observing location above mean sea level. The elevation of each site is given to the nearest metre and is generally the height of ground on which the instruments are exposed. Prior to April 1, 1986, the elevation at principal stations located at airports was generally the established by the elevation of the aerodrome. For principal stations not located at airports the elevation was established by the elevation of the barometer cistern.

evaporation

The physical process by which a liquid, such as water is transformed into a gaseous state, such as water vapour. It is the opposite physical process of condensation.

Evaporation refers to the calculated lake evaporation occurring from a small natural open water-body having negligible heat storage and very little heat transfer at its bottom and sides. It represents the water loss from ponds and small reservoirs but not from lakes that have large heat storage capacities. Lake Evaporation is calculated using the observed daily values of pan evaporative water loss, the mean temperatures of the water in the pan and of the nearby air, and the total wind run over the pan.

extreme maximum temperature (°C)

The highest daily maximum temperature in degrees Celsius (°C) reached at that location for that month.

extreme minimum temperature (°C)

The lowest daily minimum temperature in degrees Celsius (°C) reached at that location for that month.

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F

fog

A visible aggregate of minute water droplets suspended in the air at or near the surface of the earth, reducing horizontal visibility to less than 1 km. It is created when the temperature and the dew point of the air have become the same, or nearly the same. It is rarely observed when the temperature and dew point differ by more than 2°C.

freezing drizzle

Drizzle, the drops of which freeze on impact with the ground or with objects at or near the ground.

freezing rain

Rain, the drops of which freeze on impact with the ground or with objects at or near the ground.

frost

Frost is the condition that exists when the temperature of the air near the earth or earth-bound objects falls to freezing or lower (0°C).

Alternately, frost or hoar frost describes a deposition of ice crystals on objects by direct sublimation of water vapour from the air.

frost free days

Number of frost free days is calculated based on the last occurrence of frost in spring and the first occurrence of frost in autumn. This is an especially important parameter for agriculture, because the variability in the number of frost free days is crucial for many agricultural activities such as planting and harvesting, but the impact of a strong frost can become an economic problem that affects the prices.

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G

gusts

Gusts are sudden, rapid and brief changes in the wind speed. They are characterized by more or less continual fluctuations between the high (peak) and low (lull) speed. The extreme gust speed is the instantaneous peak wind observed from the anemometer dials, abstracted from a continuous chart recording, or from a data logger.

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H

hail

Precipitation of small balls or pieces of ice with a diameter ranging from 5 to 50 mm or more. Hail is generally observed during heavy thunderstorms.

heating degree-days

Heating degree-days for a given day are the number of degrees Celsius that the mean temperature is below 18°C. If the temperature is equal to or greater than 18°C, then the number will be zero. For example, a day with a mean temperature of 15.5°C has 2.5 heating degree-days; a day with a mean temperature of 20.5°C has zero heating degree-days. Heating degree-days are used primarily to estimate the heating requirements of buildings.

hourly weather conditions

Normally includes observations of sky condition, visibility (in kilometers), present weather conditions, obstructions to vision, atmospheric pressure (kPa) temperature (degrees C), humidity (%), wind speed (km / h) and direction (tenths of degrees), cloud cover (tenths of sky) and / or obscuring phenomena (tenths).

humidex

Humidex is an index to indicate how hot or humid the weather feels to the average person. It is derived by combining temperature and humidity values into one number to reflect the perceived temperature.
For example, a humidex of 40 means that the sensation of heat when the temperature is 30 degrees and the air is humid feels more or less the same as when the temperature is 40 degrees and the air is dry.

The standard Humidex formula used by Environment Canada is:

Humidex = (air temperature) + h

Where
h = (0.5555)*(e - 10.0);
e = vapour pressure in hPa (mbar), given by:
e = 6.11 * exp[5417.7530 * ( (1/273.16) - (1/dewpoint) ) ]
exp = 2.71828

Dewpoint is expressed in kelvins (temperature in K = temperature in °C + 273.16) and 5417.7530 is a rounded constant based on the molecular weight of water, latent heat of evaporation, and the universal gas constant.

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I

ice

The solid form of water. It can be found in the atmosphere in the form of ice crystals, snow, ice pellets, and hail for example.

ice crystals

Precipitation in the form of slowly falling, singular or unbranched ice needles, columns, or plates. They make up cirriform clouds, frost, and ice fog. Also, they produce optical phenomena such as halos, coronas, and sun pillars. May be called "diamond dust." Precipitation of ice crystals in the form of needles, columns or plates sometimes so tiny, they seem suspended in air. They are mainly visible when they glitter in sunshine and occur only at very low temperatures and stable air masses.

ice fog

A type of fog composed of suspended particles of ice or ice crystals 20 to 100 microns resulting from the freezing of tiny supercooled water droplets. Ice fog occurs in clear, calm, stable air when temperatures are < -30°C.

ice pellets

Precipitation of transparent or translucent pellets of ice, which are spherical or irregular shaped, having a diameter of 5 mm or less. They are classified into two types: hard grains of ice consisting of frozen rain drops or largely melted and refrozen snowflakes; pellets of snow encased in a thin layer of ice which have formed from the freezing of droplets intercepted by pellets or water resulting from the partial melting of pellets. Ice pellets usually bounce when hitting hard ground and make a sound on impact. They can fall as continuous precipitation or in showers.

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L

latitude

Latitude co-ordinates for climate stations are generally for the instrument site; however prior to April 1, 1986 at principal stations (airports) the locations given were normally that of the official airport locations. The accuracy of these locations depended on the quality of the reference maps available at the time. The latitude of each site is given to the nearest second or to the nearest 0.003 of a degree. All locations in Canada are north of the equator.

Local Standard Time

The Local Standard Time is used for observation purposes and is that of the standard time zone in which the station is located, whether or not "daylight saving time" is adopted for other purposes. In Canada, Local Standard Time is commonly used for archiving surface weather observations.

longitude

Longitude co-ordinates for climate stations are generally for the instrument site; however prior to April 1, 1986 at principal stations (airports) the locations given were normally that of the official airport locations. The longitude of each site is given to the nearest second or to the nearest 0.003 of a degree. The accuracy of these locations depends on the quality of the reference maps available at the time. Negative values of longitude denote degrees west of the Greenwich Meridian. All locations in Canada have negative values of longitude.

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M

maximum relative humidity (%)

The maximum percentage (%) value of all hourly relative humidity values observed at a specified location for a specified time interval.

maximum temperature (°C)

The highest temperature in degrees Celsius (°C) observed at a location for a specified time interval.

mean maximum temperature (°C)

The average of the maximum temperature in degrees Celsius (°C) observed at the location for that month.

mean minimum temperature (°C)

The average of the minimum temperature in degrees Celsius (°C) observed at the location for that month.

mean sea level pressure

The atmospheric pressure at mean sea level usually determined from the observed station pressure. Mean sea level pressure is computed from the station pressure and reported so that the barometric pressures at stations of different elevations can be compared at a common level for analysis purposes.

mean temperature (°C)

The mean temperature in degrees Celsius (°C) is defined as the average of the maximum and minimum temperature at a location for a specified time interval.

minimum relative humidity (%)

The minimum percentage (%) value of all hourly relative humidity values observed at a specified location for a specified time interval.

minimum temperature (°C)

The lowest temperature in degrees Celsius (°C) observed at a location for a specified time interval.

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N

number of days with specified parameters

These tables give the average number of days per month or year on which a specific meteorological event occurs.
For example:

  • In the case of rainfall and precipitation, 0.2 mm or more must occur before a "day with" is counted. The corresponding figure for snowfall is 0.2 cm.
  • A day with freezing precipitation is counted if there is an occurrence of 0.2 mm or more of rain or drizzle that turns to ice on contact with the underlying surface.
  • Fog for this purpose is defined as a suspension of very small water droplets reducing the horizontal visibility to less than 1 km.
  • A day with thunderstorms occurs if thunder is heard.

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O

occurrence of weather and obstructions to vision

Observations of atmospheric phenomenon including the occurrence of weather and obstructions to vision have been taken at many hourly reporting stations. The current standard for these observations is in the Manual of Surface Weather Observations, Chapter 3, Atmospheric Phenomena.

The phenomena that are reported are:

  • Tornado
  • Waterspout
  • Funnel Cloud
  • Thunderstorms
  • Heavy Thunderstorms
  • Rain*
  • Rain Showers*
  • Drizzle*
  • Freezing Rain*
  • Freezing Drizzle*
  • Snow*
  • Snow Grains*
  • Ice Crystals
  • Ice Pellets*
  • Ice Pellet Showers*
  • Snow Showers*
  • Snow Pellets*
  • Hail*
  • Fog
  • Ice Fog
  • Smoke
  • Haze
  • Blowing Snow
  • Blowing Sand
  • Blowing Dust
  • Dust
  • Freezing Fog
  • Virga

Precipitation types marked with an asterisk (*) are observed in three intensities: light, moderate and heavy. If the precipitation is listed in the WEATHER column without a modifier then the intensity is light. Otherwise it will appear with a modifier of moderate or heavy.

At some stations observations are made by automatic aviation weather reporting systems. The types of phenomena reported from these systems are not as numerous and are limited to:

  • Rain
  • Snow
  • Drizzle
  • Hail
  • Freezing Rain
  • Freezing Drizzle
  • Unknown Precipitation

Currently, the observation of Unknown Precipitation is not reported in the WEATHER column. The other precipitation types are reported.

When no weather or obstructions to visibility occur, sky conditions are provided reflecting the observation of total cloud amount. The following terms are used, based on the amount (in tenths) of cloud covering the dome of the sky:

  • Clear (0 tenths)
  • Mainly clear (1 to 4 tenths)
  • Mostly cloudy (5 to 9 tenths)
  • Cloudy (10 tenths)

These observations of cloud amounts are not available from aviation automatic weather stations.

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P

peak gust

The highest instantaneous wind speed observed or recorded.

precipitation

Any and all forms of water, liquid or solid, that falls from clouds and reaches the ground. This includes drizzle, freezing drizzle, freezing rain, hail, ice crystals, ice pellets, rain, snow, snow pellets, and snow grains. Types of precipitation that originate aloft are classified under Liquid Precipitation, Freezing Precipitation and Frozen Precipitation. The measurement of precipitation is expressed in terms of vertical depth of water (or water equivalent in the case of solid forms) which reaches the ground during a stated period. The millimetre (mm) is the unit of measurement of liquid precipitation and the vertical depth of water or water equivalent is express to the nearest 0.2 mm. Less than 0.2 mm is called a "Trace". Snow depth is measured to the nearest 0.2 cm. Less than 0.2 cm is called a "Trace".

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R

radiation

The process by which energy is propagated through any medium by virtue of the wave motion of that medium. Electromagnetic radiation, which emits heat and light, is one form. Sound waves are another.

Radiation values are recorded hourly in Local Apparent Time and the units are expressed in megajoules per square metre, except daylight illumination is in 1000 lumen-hours per square metre.

  • Global solar radiation RF1
  • Sky radiation RF2
  • Reflected solar radiation RF3
  • Net radiation RF4
  • Daylight illumination RF7
  • Global solar radiation RF1 and sky radiation RF2
  • Global solar radiation RF1 and Reflected solar radiation RF3
  • Global solar radiation RF1 and Net radiation RF4
  • Global solar radiation RF1, Sky radiation RF2 and Reflected solar radiation RF3
  • Global solar radiation RF1, Sky radiation RF2, Reflected solar radiation RF3 and Net radiation RF4
  • Global solar radiation RF1, Reflected solar radiation RF3 and Net radiation RF4
  • Global solar radiation RF1, Sky radiation RF2, Reflected solar radiation RF3, Net radiation RF4 and Daylight illumination RF7
  • Global solar radiation RF1, Sky radiation RF2 and Net radiation RF4

rain

Precipitation in the form of liquid water droplets greater than 0.5 mm. If widely scattered, the drop size may be smaller. The intensity of rain is based on rate of fall. "Very light" means that the scattered drops do not completely wet a surface. "Light" means it is greater than a trace and up to 2.5 mm an hour. "Moderate" means the rate of fall is between 2.6 mm to 7.5 mm per hour. "Heavy" means 7 mm per hour or more.

rainfall, snowfall, and precipitation

Rain, drizzle, freezing rain, freezing drizzle and hail are usually measured using the standard Canadian rain gauge, a cylindrical container 40 cm high and 11.3 cm in diameter. The precipitation is funneled into a plastic graduate that serves as the measuring device. Snowfall is the measured depth of newly fallen snow, measured using a snow ruler. Measurements are made at several points which appear representative of the immediate area, and then averaged. "Precipitation" in Canadian Climate Normals tables is the water equivalent of all types of precipitation.

At most ordinary stations the water equivalent of snowfall is computed by dividing the measured amount by ten. At principal stations it is usually determined by melting the snow that falls into Nipher gauges. These are precipitation gauges designed to minimize turbulence around the orifice, and are high enough above the ground to prevent most blowing snow from entering. The amount of snow determined by this method normally provides a more accurate estimate of precipitation than using the "ten-to-one" rule. Even at ordinary climate stations the normal precipitation values will not always be equal to rainfall plus one tenth of the snowfall. Missing observations is one cause of such discrepancies.

Precipitation measurements are usually made four times daily at principal stations. At ordinary sites they are usually made once or twice per day. Rainfall, snowfall and precipitation amounts given in the tables represent the average accumulation for a given month or year.

relative humidity (%)

Relative humidity in percent (%) is the ratio of the quantity of water vapour the air contains compared to the maximum amount it can hold at that particular temperature.

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S

saturation point

The point when the water vapour in the atmosphere is at its maximum level for the existing temperature.

smoke or haze

A suspension in the air of small particles produced by combustion. Viewed through smoke, the sun appears very red at sunrise and sunset. When high in the sky, smoke is tinged with orange. Smoke from nearby cities may be brown, dark gray or black. Smoke in extensive layers originating from forest fires give the sky a greenish-yellow hue. Evenly distributed smoke from distant sources is generally light gray or blue. In large quantities, smoke may be distinguished by its smell. Plumes of smoke of local origin are not reported as an atmospheric phenomenon.

snow

Frozen precipitation in the form of white or translucent ice crystals in complex branched hexagonal form. It most often falls from stratiform clouds, but can fall as snow showers from cumuliform ones. At temperatures > than -5° C, the crystals generally cluster to form snowflakes.

snow depth

Snow depth is the depth of accumulated snow on the ground, measured at several points that appear representative of the immediate area and then averaged.

snow grains

Frozen precipitation in the form of very small, white opaque grains of ice. The solid equivalent of drizzle. Their diameter is generally < 1 mm. When grains hit hard ground, they do not bounce or shatter. They usually fall in very small quantities, mostly from Status clouds or fog and never in the form of a shower.

snow on the ground (cm)

The depth of snow in centimetres (cm) on the ground. The total depth of snow on the ground at the time of the observation is determined in whole centimetres by making a series of measurements and taking the average.

snow on the ground on the last day (cm)

The depth of snow in centimetres (cm) on the ground. The monthly value displayed is for the final day of the month.

snow pellets

Frozen precipitation of particles of either spherical or conical ice; their diameter is about 2 to 5 mm. They are brittle, easily crushed, and unlike hail, when they fall on hard ground, they bounce and often break up. Snow pellets always occur in showers and are often accompanied by snow flacks or raindrops when the surface temperature is around 0° C.

snow shower saturation point

Frozen precipitation in the form of snow, characterized by its sudden beginning and ending.

snow squall

A heavy snow shower accompanied by sudden strong winds.

snow survey

Snow surveys by designated stations are made at regular intervals during the winter months to determine the water equivalent (mm) and depth of the snow pack (cm).
- 5 points (30 m apart), measurements taken on the 1st, 8th, 15th and 23rd of each month.
- 10 points (30 m apart), measurements taken on the 1st and 15th days of each month.

soil temperature

Soil temperature measurements provide a climatology of soil thermal characteristics such as the depth of frost penetration into the soil and the duration that the soil remains frozen. It is of interest to hydrologists because it affects surface runoff, infiltration and snowmelt and to agriculturists because it affects seed germination. Measurements of soil temperature are made in accordance with the World Meteorological Organization (WMO) recommendations at the standard depths of 5, 10, 20, 50, 100, 150 and 300 cm. They are measured daily as close as possible to 08:00 LST and again at the shallowest depth at 16:00 LST.

solar radiation

Solar radiation is the measurement of radiant energy from the sun, on a horizontal surface. There are several standardized components of independent measurements. Each component is assigned a different identifying number referred to as Radiation Fields (RF). The standard metric unit of radiation measurement is the megajoule per square metre (MJ m-2).

Components measured and used by MSC:

RF1: Global Solar Radiation: the total incoming direct and diffuse short-wave solar radiation received from the the whole dome of the sky on a horizontal surface.

RF2: Sky Radiation (Diffuse): the portion of the total incoming short-wave solar radiation received on a horizontal surface that is shielded from the direct rays of the sun by means of a shade ring.

RF3: Reflected Solar Radiation: the portion of the total incoming short-wave radiation that has been reflected from the Earth's surface and diffused by the atmospheric layer between the ground and the point of observation onto a horizontal surface.

RF4: Net Radiation: the resultant of downward and upward total (solar, terrestrial surface, and atmospheric) radiation received on a horizontal surface

speed of maximum gust (km/h)

The speed in kilometres per hour (km/h) of the maximum wind gust during the day. The gust is the maximum or peak instantaneous or single reading from the anemometer (the instrument used to observe wind speed) during the day. The duration of a gust typically corresponds to an elapsed time of 3 to 5 seconds.

station ID, station number, climate ID

Also referred to as climate ID or station number, the station ID is a 7 digit number assigned by the Meteorological Service of Canada to a site where official weather observations are taken, and serves as a permanent, unique identifier (ID).

The first digit of the number indicates the province in which the observing site is located; the second and third digits identify the climatological district within the province.

When observations are discontinued at a site, the number is not used for subsequent stations (which may, or may not, differ in name) unless it is judged that the records from the earlier and subsequent stations may be combined for most climatological purposes.

station name

The station name is the official name of any meteorological station in the Climate National Archive as administered by the Meteorological Service of Canada (MSC).

station pressure (kPa)

The atmospheric pressure in kiloPascal (kPa) at the station elevation. Atmospheric pressure is the force per unit area exerted by the atmosphere as a consequence of the mass of air in a vertical column from the elevation of the observing station to the top of the atmosphere.

station status

Indicates whether a station is currently collecting and/or transmitting weather data. An "active" or "open" station is considered operational and is expected to provide ongoing information. An "inactive" or "closed" station no longer collects or transmits data.

surface weather observation

A surface weather observation is an evaluation of meteorological elements, visually and/or by measurement at a specified location on the earth's surface, usually a weather observing station.

synoptic observations

Synoptic observations consist of sky condition, wind speed and direction; visibility, weather and obstructions to vision, atmospheric pressure, temperature, dew point, precipitation amount, special phenomena and maximum and minimum temperature. Observations are taken at specified times (usually at 0000 UTC, 0600 UTC, 1200 UTC and 1800 UTC) simultaneously around the globe and collectively give a comprehensive "synoptic" picture of weather conditions.

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T

TC ID

The TC ID is the identifier assigned by Transport Canada to identify meteorological reports from airport observing sites transmitted in real time in aviation formats.

temperature (°C)

The temperature of the air in degrees Celsius (°C). At most principal stations the maximum and minimum temperatures are for a day beginning at 0600 Greenwich (or Universal) Mean Time, which is within a few hours of midnight local standard time in Canada.

thunder

The sound emitted by rapidly expanding gases along the channel of a lightning discharge. Most of lightning's electrical discharge is used to heat atmospheric gases around the visible channel. Temperatures can rise to over 10,000 °C in microseconds, resulting in a violent pressure wave, composed of compression and rarefaction. The rumble of thunder is created as one's ear catches other parts of the discharge, the part of the lightning flash nearest registering first, then the parts further away.

thunderstorm

A thunderstorm is a local storm produced by a cumulonimbus cloud. It is an event of relatively short duration and is always accompanied by lightening and thunder and lightning, usually with strong gusts of wind, heavy rain and sometimes hail. Officially, a thunderstorm is reported when:

  1. thunder is heard with the past 15 minutes
  2. lightning is observed within the past 15 minutes and the local noise level is such as might prevent hearing thunder. In this case, hail may also be an indicator of a thunderstorm in progress.

total precipitation (mm)

The sum of the total rainfall and the water equivalent of the total snowfall in millimetres (mm), observed at the location during a specified time interval.

total rain (mm)

The total rainfall, or amount of all liquid precipitation in millimetres (mm) such as rain, drizzle, freezing rain, and hail, observed at the location during a specified time interval.

total snow (cm)

The total snowfall, or amount of frozen (solid) precipitation in centimetres (cm), such as snow and ice pellets, observed at the location during a specified time interval.

trace

An unmeasurable or insignificant quantity. A precipitation amount of less than 0.2 mm.

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U

Universal Time Coordinates (UTC)

UTC is the local time on the zero meridian (0°) which goes through the old observatory in Greenwich, London, UK. The World Meteorological Organization (WMO) has adopted the UTC as the standard time for reporting all meteorological data internationally. Times in UTC sometimes get the suffix Z, i.e. 16Z. From the mnemonic "Zulu", as used in international marine communications standards.

upper air

Readings of pressure (kPa), altitude (m), air temperature (C), relative humidity (%), wind speed (m/s), and wind direction (degrees) for standard pressure surfaces at 00 and 12 (UTC) at..

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V

vapour pressure

The pressure exerted by the molecules of a given vapour. In meteorology, it is considered as the part of total atmospheric pressure due to the water vapour content. It is independent of other gases or vapours.

visibility (km)

Visibility in kilometres (km) is the distance at which objects of suitable size can be seen and identified. Atmospheric visibility can be reduced by precipitation, fog, haze or other obstructions to visibility such as blowing snow or dust.

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W

weather

The state of the atmosphere at a specific time. It is the short term or instantaneous variations of the atmosphere, as opposed to the long term, or climatic, changes.

weather observing station

A weather observing station is any site from which official weather observations are made. It is normally equipped with instruments for measuring meteorological elements.

wet bulb temperature

The wet bulb temperature differs from the dry bulb temperature by an amount dependent on the moisture content of the air and is normally the same as or lower than the dry bulb temperature.

wind

Wind is defined as air in motion. It represents the horizontal flow of air at a height of 10 meters. Wind information includes direction, speed and character. Wind in the first ten's of metres above the ground tends to increase in speed and veer with height.
Winds are normally measured at level, open sites removed as much as possible from obstacles to wind flow such as trees, buildings, or hills. At most principle stations, wind is usually measured by taking a one- or two-minute mean at each observation, from an anemometer. At other wind-measuring sites, values may be obtained from autographic records of anemometers. Averaging periods may vary from one minute to an hour. The extreme gust speed is the instantaneous peak wind observed from the anemometer dials, or abstracted from a continuous chart recording.

wind chill

Wind chill is an index to indicate how cold the weather feels to the average person. It is derived by combining temperature and wind velocity values into one number to reflect the perceived temperature.
For example, if the outside temperature is -10°C and the wind chill is -20, it means that your face will feel more or less as cold as it would on a calm day when the temperature is -20°C.

There are two Wind Chill formulas used by Environment Canada. The first equation is used when the temperature of the air is ≤ 0&°C and the reported wind speed is ≥ 5km/h. The second equation is used when the temperature of the air is ≤ 0°C and the reported wind speed is > 0km/h but < 5km/h.

The standard Wind Chill formula for Environment Canada is:

1. W = 13.12 + 0.6215 × Tair - 11.37 × V10m0.16 + 0.3965 x Tair × V10m0.16

2. W = Tair + [(-1.59+0.1345 × Tair)/5] × V10m

Where
W is the wind chill index, based on the Celsius temperature scale
Tair is the air temperature in degrees Celsius (°C), and
V10m is the wind speed at 10 metres (standard anemometer height), in kilometres per hour (km/h).

wind direction (10's deg/tens of degrees)

The direction (true or geographic, not magnetic) from which the wind blows. It represents the average direction during the two minute period ending at the time of observation. Expressed in tens of degrees (10's deg), 9 means 90 degrees true or an east wind, and 36 means 360 degrees true or a wind blowing from the geographic north pole. A value of zero (0) denotes a calm wind.

wind speed (km/h)

The speed of motion of air in kilometres per hour (km/h) usually observed at 10 metres above the ground. It represents the average speed during the two-minute period ending at the time of observation. In observing, it is measured in nautical miles per hour or kilometers per hour.

Conversion factors: 1 nautical mile = 1852 metres or 1.852 km
therefore 1 knot = 1.852 km/h
and 1 km/h = 0.54 knot.

WMO ID

A 5-digit number permanently assigned to Canadian Stations by the World Meteorological Organization to identify the station internationally. The WMO ID is an international identifier assigned by the Meteorological Service of Canada to standards of the World Meteorological Organization for stations that transmit observations in international meteorological formats in real time.

World Meteorological Organization (WMO)

Since weather systems and climatic conditions extend beyond international boundaries, it is necessary to exchange weather information freely throughout the world. This requires coordination and standardization of practices and procedures for efficient exchange of weather transmissions. To promote these services and to further the application of meteorology to aviation, shipping, agriculture and other human activities, the World Meteorological Organization was established by the United Nations in 1951. Its weather reporting codes are called International Codes.

WMO standards for "Climate Normals"

"Climate averages", "climate means" or "climate normals" are all interchangeable terms. They refer to arithmetic calculations based on observed climate values for a given location over a specified time period and are used to describe the climatic characteristics of that location. Real-time values, such as daily temperature, are compared to the "climate normal" to determine how unusual or how great the departure from "average" they are.

There are many ways to calculate "climate normals"; the most useful ones adhere to accepted standards. The WMO considers thirty years long enough to eliminate year-to-year variations. Thus the WMO climatological standard period for normals calculations are "averages of climatological data computed for consecutive periods of 30 years as follows: 1 January 1901 to 31 December 1930, 1 January 1931 to 31 December 1960, etc." and should be updated every decade. In addition, the WMO established that normals should be arithmetic means calculated for each month of the year from daily data.

To qualify, temperature data, soil temperatures and evaporation must fit the following rule: "If more than 3 consecutive daily values are missing or more than 5 daily values in total in a given month are missing, the monthly mean should not be computed and the year-month mean should be considered missing." This is referred to as the "3/5" rule. For total precipitation, degree-days, and "days with" calculations, no missing days are allowed.

Once the months that qualify are determined, a similar "3/5" rule is also applied to the number of monthly average or total values in the thirty-year period. For instance, to meet this WMO standard, the "normal" value of a monthly element, such as the normal rainfall amount for May, can have no more than 3 consecutive, or 5 in total, missing rainfall values in any month of May between 1971 to 2000. For the purposes of the Canadian Climate Normals, calculated for 1971 to 2000, locations or climate stations, which meet to these WMO standards, are referred to as Class "A".