Weather tools: interesting facts


The difference between a warning, a watch, a special statement, and an advisory

When severe weather threatens, Environment and Climate Change Canada, the authoritative source of Weather Alerts 24/7, is here for you, issuing special alerts that notify those in affected areas so that they can take steps to protect themselves and their property from harm.

The type of alert used depends on the severity and timing of the event:

  • Special Weather Statements are the least urgent type of alert and are issued to let people know that conditions are unusual and could cause concern.
  • Advisories are issued for specific weather events (like blowing snow, fog, freezing drizzle and frost) that are less severe, but could still significantly impact Canadians.
  • Watches alert you about weather conditions that are favorable for a storm or severe weather, which could cause safety concerns.
  • As certainty increases about the path and strength of a storm system, a watch may be upgraded to a Warning, which is an urgent message that severe weather is either occurring or will occur. Warnings are usually issued six to 24 hours in advance, although some severe weather (such as thunderstorms and tornadoes) can occur rapidly, with less than a half hour’s notice.

These alerts are updated regularly so that members of the public can stay on top of a developing situation and take the appropriate action.

To learn more about the alerts, visit: Public weather alerting program, Public weather forecast guide, and Criteria for public weather alerts.

The meaning of the different alert colours
Colours used for different alert types
Colour Meaning
 red - warning Red: region with a weather warning in effect
 yellow - watch Yellow: region with a watch in effect
 dark grey - special statement Dark grey: region with a special statement in effect
 light grey - no alert Very light grey: region with no alert in effect
 white - no forecast or alert service White: region with no forecast/alert service
 hurricane icon - Tropical Cyclone Information Hurricane icon: Tropical Cyclone statement
The different criteria for issuing warnings

Criteria for pulic weather alerts:

Criteria that trigger a warning are different from one place to another.

To learn more about the specific criteria for your region, visit: Criteria for public weather alert.

Criteria for marine weather alerts:

  • Strong wind warnings are issued when wind speeds of 20 to 33 knots, excluding gusts, are occurring or are expected to occur.
  • Gale warnings are issued when wind speeds of 34 to 47 knots, excluding gusts, are occurring or are forecast to occur.
  • Storm warnings are issued when wind speeds of 48 to 63 knots, excluding gusts, are occurring or are forecast to occur.
  • Hurricane force wind warnings are issued when winds of 64 knots or greater, excluding gusts, are occurring or are forecast to occur.
  • Freezing spray warnings are issued when moderate or severe freezing spray is occurring or is forecast to occur. Moderate or severe freezing spray implies ice build-up at rates of 0.7 cm/hr. or faster.
  • Tornado warnings come into effect when there is evidence of tornado formation (i.e. reliable observation, radar, etc.), or an existing tornado is moving from land to an adjacent marine area.
  • Tornado watches come into effect when severe thunderstorms have developed over a marine area, and the conditions are conducive for the development of one or more tornadoes within the marine area during the time specified in the watch.
  • Squall warnings come into effect when sudden, strong, gusty winds of 34 knots or greater are either forecast or observed over the marine area.
  • Squall watches come into effect when there is the possibility of sudden, strong, gusty winds of 34 knots or greater over the marine area.
  • High water level warnings come into effect in order to warn mariners and coastal populations, about the potential impacts of abnormally high water levels or waves on coastal or shoreline areas.
  • Waterspout watches come into effect when conditions for the development of cold-air waterspouts are forecast over a marine area during the time specified in the watch.
  • Special marine warnings can be issued for any other weather-related phenomena which may result in a hazardous impact on local marine operations. For example, a marine forecaster may decide to issue a Special Marine Warning when an extended period of freezing precipitation is forecast over a marine area that may result in hazardous icing conditions on marine infrastructure.
  • Special marine watches can be issued when conditions are conducive for the development of any other weather-related phenomena that may pose a hazard to local marine operations. For example, a marine forecaster may decide to issue a Special Marine Watch when a small band of freezing precipitation is occurring, or is forecast to occur, over a marine area.

To learn more, visit: Canadian marine warning program.

The Marine Warning and Watch Program

The Marine Warning and Watch Program provides advisories to mariners or marine interests to inform them of marine weather conditions that may pose a hazard to their safety, security, or operations.

Marine warnings and watches are issued year round, or during the shipping season where forecast waters may close seasonally due to the presence of ice. Marine warnings or watches are issued whenever criteria are met or expected to be met during the valid period of the regular or updated forecast.

Although warnings or watches are not issued for anticipated marine weather events which occur beyond the regular or updated marine forecast time period of coverage (i.e., beyond Day 2), marine weather statements may be issued to provide further detail regarding potentially high impact marine weather events expected to occur during the extended marine forecast time period of coverage (i.e., Day 3, 4 and 5).

Marine warnings and watches fall into two main categories: (1) synoptic warnings and (2) localized warnings and watches. Another category of warnings include the ice warnings for which the Canadian Ice Service is the responsible issuing agency.

To learn more, visit: Canadian marine warning program.

Current conditions

Why current temperature and/or weather at my home is different from the Environment and Climate Change Canada current conditions
  • The current temperature for a region is taken at a local weather station.
  • You can experience a significant temperature difference within just a few kilometers. Large differences in temperature are common especially in the early morning.
  • A difference in elevation may result in a different temperature reading. Cold air tends to go to lower ground.
  • Large bodies of water affects the air temperature along coastal sections. Usually, it can influence the temperature within 3 to 7 kilometers from the shore. The temperature may also vary depending on wind direction and speed, the water temperature and the cloud cover.
  • The make and model of a sensor, its accuracy, and its exposure to the environment can also influence the measurement of local weather.
  • Other factors that can influence the local temperature includes the type of terrain and environment, e.g. in a big city versus the open space at an airport, etc.
Why current conditions are not available (sky conditions, temperature)

The standard configuration of a Meteorological Service of Canada Surface Automatic Weather Station (AWS) will include sensors that measure temperature, relative humidity, precipitation rate-of-fall, precipitation amount, snow-on-ground, pressure, and wind speed and direction at 2m and 10m. However, these AWS are not equipped with sensors to detect sky condition (i.e. sunny, cloudy) or type of precipitation (i.e. rain, snow). Only NavCanada stations can detect these conditions.

More about the dew point

The dew point is a measure of the humidity content in the air. Dew point is short for “dew point temperature," which indicates the amount of moisture in the air. The dew point is the temperature to which the air must be cooled, keeping pressure constant, to become saturated. When the difference between the air temperature and the dew point temperature is large, the air is dry and the relative humidity is low. As the air temperature is cooled to the dew point, the relative humidity increases and reaches 100% when the two temperatures coincide.

The best way to understand the dew point notion is to visualise how dew is formed on a clear fall morning, for example. Dew occurs as a result of the air gradually cooling overnight. In the late afternoon, the air holds a certain quantity of water vapour (humidity). During a clear night, however, the earth's surface loses radiational heat rapidly and cools; consequently, the air in contact with the earth's surface is forced to cool while the atmospheric pressure remains the same. After a certain period of cooling off, the air reaches its saturation point and if it cools any further, we witness an excess of humidity that condenses and forms dew. The temperature at which condensation starts occurring is what we call the dew point.

More about the "relative humidity"

The percentage of humidity or relative humidity is the quantity of water vapour the air contains, compared to the maximum amount it can hold at that particular temperature. It is expressed as a fraction of the maximum moisture the air can hold, at the same pressure and temperature, before water droplets start forming clouds or dew (if close to the ground).

For example, a relative humidity of 60% means that the air contains 60% of the maximum moisture it could contain at the present temperature. Note that the warmer the air, the more moisture the air can hold. A relative humidity of 60% feels comfortable when it is 20 degrees, but a lot less comfortable when the temperature reaches 30 degrees. Because the air can contain a lot more moisture in 30-degree weather than in 20-degree weather, we feel the effect of humidity a lot more when the temperature reads 30 degrees, even though the relative humidity (percentage) is the same.

More about the pressure tendency

Atmospheric pressure tendency is defined as the characteristic and the amount of the change in station pressure (pressure measured at the altitude level of a given reporting observing station by opposition to the pressure measured at the sea level) in the three hours preceding the observation. The pressure tendency is usually included in weather reports every three (3) hours. The characteristic is the nature of the pressure change and can be coded accordingly. The pressure amount is the net change of pressure over a period of three (3) hours and is determined in hectopascals (hPa) to the nearest tenth.

On our Weather website the pressure tendency provided within the "Current Conditions" is simply the characteristic (rising, falling and steady) of the change in station pressure (as described above). To know how much the pressure has changed (or the amount) one needs to go to "Past 24 hour Conditions" and make the determination by subtracting the hourly pressure values for the desired period.

Converting inches of mercury (Hg) for old barometers into kilopascals (kPa) or hectopascals (hpa)

Some barometers report pressure in inches of mercury (Hg). The conversion factor is approximately 33.9 hectopascals (hPa), or 3.39 kilopascals (kPa), per inch of Hg. So divide the pressure in kPa by 3.39 to get it in inches of Hg, or multiply the value in inches of Hg by 3.39 to convert it into kPa.

Past temperatures, normals, averages and extremes

The difference in the temperature displayed in the ‘Past 24 hours’ and in the ‘Yesterday's data’

The ‘Past 24 hours’ temperatures are reported once every hour by the weather stations. As the temperature is monitored on a continuous basis, we notice that the daily maximum and minimum temperatures rarely happen exactly on the hour. So, the maximum and minimum temperatures in ’Yesterday’s data’ are usually different than the highest and lowest hourly temperatures.

The difference in the temperature displayed in the ‘Normals’ and the average in ‘Averages and extremes’

Climate Normals are calculated for 30-year periods. The Normals that we are using today are calculated from data for the period 1981-2010. They get updated every decade. 

The temperatures in the ‘Normals’ section are regional normals, for which the average temperatures for a larger region are calculated on the same 30-year average. The regional normals are based on temperatures from several locations instead of just one. They will usually differ by one to three degrees compared with the average temperatures listed for an individual location in the ‘Averages and extremes’ section.

For more information about Normals, visit Canadian Climate Normals.

The ‘Average high’ and ‘Average low’ temperatures listed in the ‘Averages and extremes’ section for one specific weather station are calculated using the temperatures recorded for that day between 1981 and 2010 (30 years).

For more information about Averages and extremes, visit Almanac Averages and Extremes.


The different weather elements (snow, rain, wind, etc.) and when they are included in the forecast

Public weather program

To learn more about weather elements and when they are included in the forecast, visit Guide to public weather forecasts: weather elements.

Marine weather program

To learn more about what is included in the marine forecasts, visit: Guide to marine weather forecasts – What information is included in the Marine Forecast?

How weather forecasts are made and how often they are updated

Environment and Climate Change Canada’s (ECCC) weather forecasts and alerts are produced using technology, including satellites, Doppler radars, a powerful supercomputer, forecaster workstations and use of weather information received from the general public.

  1. The first step in forecasting the weather involves an analysis of the current state of the atmosphere. ECCC collects data daily from thousands of weather monitoring sites across the country and the world.
  2. All the data collected are analyzed and used in ECCC’s numerical weather prediction models. These models are essentially the mathematical representations of what is present in the atmosphere. This process is done through the use of ECCC’s supercomputer.
  3. Current observations and the numerical model output are used by meteorologists in the analysis and diagnosis of current weather conditions. All of this information is then used to produce the forecasts. The meteorologists pay special attention to possible severe weather in the upcoming two days, and will issue severe weather alerts as appropriate. Forecasts are disseminated to Canadians through ECCC’s WeatherCAN app, Weather website, Weatheradio, Hello Weather (automated telephone answering devices), the media, and for display on numerous weather websites in Canada. Marine forecasts are also available over Inmarsat-C satellite and the Canadian Coast Guard’s NAVTEX service and Continuous Marine Broadcast over VHF radio.

This video explains the work of meteorologists and how a forecast is made. 

The frequency of weather forecasts updates

Issuing time of forecasts

Although there are some regional differences, the regular and extended public forecast bulletins are most commonly issued three times a day, at 5:00 a.m., 11:00 a.m., and 4:00 p.m. local time. Issue times of a few bulletins are different from the national standard due to time zone differences within a region. Please note that the forecasts can be revised at any time of the day or night if conditions are significantly different than what was originally forecasted.  

To learn more, visit: Public weather forecast bulletins.

The accuracy of the weather forecast

Public weather program

The accuracy of forecasts produced by Environment and Climate Change Canada (ECCC) depends on several factors, such as, how far in advance the forecast is for, the type of weather element (i.e. temperature, precipitation, etc.), and local effects (i.e. mountains, water bodies, etc.)

Here is an example of the accuracy of maximum temperature forecasts (within 3 degrees Celsius):

  • Day 1 - 92%
  • Day 2 - 87%
  • Day 3 - 81%
  • Day 4 - 76%
  • Day 5 - 71%
  • Day 6 - Around 65%
  • Day 7 - Around 60%

Marine weather program

ECCC tracks the accuracy of its marine Gale Warnings and presents the results as part of its marine program weather service standards. The accuracy standard is that Gale warnings will be issued at least 18 hours in advance of the event.

To learn more about the most recent results of our service standards, including the accuracy standard, visit: Meteorological service standards.

Why the forecasts produced by Environment and Climate Change Canada and other private weather organizations are sometimes different

Computer weather models are one of the main tools used to produce forecasts. There are several weather models that are in existence from other meteorological organizations from around the world. One reason that there are differences between Environment and Climate Change Canada (ECCC) forecasts and those produced by other private organizations is that they may be using a different computer model than ECCC. There may also be differences in interpretation by different forecasters.


The difference between shower, drizzle and flurry

When we mention rain in the forecast, it is associated with a large precipitation band. This is the case ahead of a warm front for example. It can last several hours or for a full day. The rain can be intermittent, meaning it can stop for short periods.

Showers are normally shorter in length lasting a few minutes to several hours with breaks in between. Showers are also convective, associated with clouds of vertical development. This is the case with cold fronts or showers or thunderstorms caused by daytime heating on a summer afternoon. Showers can occur when there is a disorganized area of precipitation over a region.

Drizzle is a water droplet that is bigger than the water droplet of fog but smaller than the water droplet of rain. Drizzle appears to float in the air but does eventually fall to the ground.

Flurries are precipitation in the form of snow from a convective cumulus-type cloud. They are characterized by the suddenness with which they start and stop, by their rapid changes in intensity, and usually by rapid changes in the appearance of the sky.

To learn more about weather and meteorological terms, visit: Weather and meteorology glossary.

How rain is measured

Rain, drizzle, freezing rain, freezing drizzle and hail are usually measured by using different gauges, depending on the network reporting the totals.

The Environment and Climate Change Canada (ECCC) networks use automated total precipitation weighing gauges which determine precipitation amounts through a change in weight. These gauges report total precipitation amounts hourly, in millimeters.

ECCC networks also use automated tipping bucket rain gauges which determine the rate-of-fall for liquid precipitation. These gauges consist of a funnel that collects and channels the precipitation into a small seesaw-like container. After a pre-set amount of precipitation falls, the lever tips, dumping the collected water and sending an electrical signal. The more frequently the lever tips, the more intense the precipitation.

Staffed reporting stations use the standard Meteorological Service of Canada Type B rain gauge, a cylindrical container 36.5 cm high and 12.5 cm in diameter. The precipitation is then funneled into a plastic measuring tube that is calibrated to show the precipitation amount in millimeters. Precipitation is typically measured twice a day and reported as a total amount for each day.

How snow is measured

At Environment and Climate Change Canada (ECCC), snow is measured by automated observing stations which register the snowfall and snow-on-ground amounts using an acoustic snow sensor (SR50, SR50A). The automated sites report snowfall amounts hourly in centimeters.

At staffed stations, the snow amount or the depth of accumulated snow-on-ground is measured using a standard Meteorological Service of Canada snow ruler or a ruler calibrated to centimeters. The measurements are made at several points which appear representative of the immediate area and then averaged. Snow is normally measured in centimeters.

Also, note that snowfall amounts are not measured at a number of ECCC, and partner's stations as the automated equipment is not capable of this measurement.

How the water equivalent of snow is calculated

To calculate the water equivalent of snow, snow captured in snow gauges is melted. At automated observation stations, Geonor and OTT Pluvio all-weather precipitation gauges can melt freezing and frozen precipitation in-situ directly with glycol, then report the Snow Water Equivalent amount in millimeters.

At staffed sites, the observer takes the gauge containing the snow indoors, melts it, and then pours the resulting liquid into a plastic measuring tube or graduated cylinder that is calibrated to show the water equivalent of the snowfall.

In many snow events, a ratio of 10 to 1 can be applied to the amount of snow to determine its water equivalent. In other words, 1 centimeter of snow is equivalent to about 1 millimeter of water once the snow is melted. This means that in many snowfall situations (on days when only snow fell), you can simply convert the units from millimeters to centimeters on the "Yesterday's Precipitation Total" for a specific location's weather page to determine an estimation of how much snow fell.

However, this 10 to 1 snow to liquid ratio is not exact. Exceptions include very fluffy snow (snow that has less water once melted) where the snow to liquid ratio could be 15 to 1 or higher (i.e. 1.5 centimeters of snow would melt to provide 1 millimeter of water). At the other extreme, the snow can be heavy and wet resulting in a snow to liquid ratio of around 5 to 1 (i.e. 0.5 cm of snow would melt to provide 1 mm of water).

We also have a map giving snow depths (snow on the ground), available on our weather website: Operational Analysis Charts.

Precipitation amounts for yesterday can vary (higher or lower than anticipated). Here are common sources of errors in measuring precipitation.

Environment and Climate Change Canada employs staffed and automated weather stations across Canada to collect temperature, rain and snowfall amounts, wind direction and speed, and barometric pressure. Considering precipitation rates constantly change over time and considering weather systems are constantly moving, measurements may differ considerably when taken at different times and/or at different locations. Consequently, precipitation amounts can vary throughout a city or region, and may be significantly different at your location compared to the weather station report.

Main sources of error in precipitation estimates include the presence of tall objects (like trees and buildings) exposed to the wind direction, which could either increase or decrease the amounts collected in the gauge. The nature of the terrain and immediate surroundings could also have some effect on accumulation on the ground (i.e. partial melting, precipitation soaking into the ground). Thus, the position of the instrument is very important in order to get the most accurate reading. In the case of snow, for example, multiple samples even over a small area must be averaged.

Despite all the precautions and a precise calibration, there are always errors related to instrument design and limitations. For example:

  • Trace amounts of precipitation, less than 0.2 mm, are not recorded by the instruments.
  • Wind that shakes the gauges can cause a false reading, including giving a precipitation measure when none has been received.
  • Strong winds can prevent rain or snow from entering the gauge, thus giving inaccurate readings.
  • Computer system malfunctions can also occur and affect data transmission.
When to include the precipitation in the forecast

Precipitation is included in the forecast when the Chance of Precipitation (COP) is equal to or greater than 30 percent.

To learn more, visit: Guide to public weather forecasts: weather elements.

The difference between Chance of Precipitation and the Likelihood of Precipitation

The Chance of Precipitation (COP) is 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.

Whenever the COP is expected to be between 30 and 70 per cent inclusive, it is indicated in the forecast. The COP values are stated in increments of 10 per cent. The use of 50 per cent is not permitted.

The term Risk is used in association with the terms Thunderstorm(s), Thundershower(s), Hail, Freezing Rain and Freezing Drizzle, when there is a 30 or 40 per cent chance of occurrence of these phenomena. In these cases, the percentage value is not stated.

To learn more, visit: Guide to public weather forecasts: weather elements: Change of precipitation.

Likelihood of Precipitation (LOP), as described in the hourly forecast, is the chance of measurable precipitation for a period of time.

  • Nil: 0%
  • Low: 40% or below
  • Medium: 60% or 70%
  • High: Above 70%

To learn more, visit: Public weather forecast bulletins: Hourly Forecasts.


When to include wind speed and direction in the forecast

Public weather program

Included in the forecast for Day 1 & 2? - Yes, if it meets the criteria.
Included in the forecast for Day 3 to 7? - Yes, if it meets the criteria.

Days one and two

The wind speed and direction are included in the forecasts when the wind speed is expected to be 20 km/h or more. Note that wind is included in the day 1 and day 2 forecasts only.

The terms Light or Calm are only used in a diminishing wind situation. For example: “Wind west at 20 km/h becoming light this evening.”

A wind speed change is indicated only when the average sustained speed is expected to change by at least 20 km/h. (i.e. “Wind south 20 km/h increasing to 40 this evening.”)

Wind shifts are included if greater than or equal to 90 degrees.

Wind is always included when wind chill is mentioned in the forecast. If the mean wind speed is less than 20 km/h and the wind chill criteria are met, then the phrase: "Wind up to 15 km/h," is used.

The following are terms used to describe wind speed in British Columbia and the Yukon only:

  • Locally windy: For wind conditions that are local in nature.
  • Gusty winds: Mean wind speed less than 30 km/h and gusts between 31 and 64 km/h.
  • Windy: Mean wind speed 30-45 km/h and gusts less than 65km/h.

Days three to seven

The term Windy is used when the mean wind speed is expected to be equal or greater than 40 km/h or when gusts are equal or greater than 50 km/h, for a period of six or more hours.

The term Very windy is used in British Columbia and the Yukon when winds meet local wind warning criteria.

To learn more, visit: Guide to public weather forecasts: weather elements: Wind.

Hourly forecast

An hourly wind forecast (even when the wind is less than 20 km/h) is available for the next 24 hours on any forecast page by clicking on Hourly forecast in the forecast section.

To learn more, visit: Public weather forecast bulletins: Hourly Forecasts.

Marine weather program

Wind is the most important weather element in the marine weather forecast. Except when the winds are light, the wind speed and direction are described in the regular marine forecast (Day 1 and 2) and the extended marine forecast (Day 3, 4 and 5).

Note: In both the regular marine forecast and extended marine forecast, the wind direction is expressed in one of eight cardinal directions of the compass, and speed is expressed in knots or, if winds are light, this will be stated - “Wind light.”

To learn more, visit: Guide to marine weather forecasts – What information is included in the Marine Forecast?

The meaning of VR5 and VR10

A wind direction of ‘VR’ indicates a variable wind direction during the hour. VR5 or VR10 indicate variable direction wind at 5 or 10 km/h. This is only for the hourly forecast.

To learn more, visit: Public weather forecast bulletins: Hourly Forecasts.

More about wind direction

In meteorology, the wind direction is the direction the wind is blowing from. It is based on true north and not magnetic north. Note that the wind speed and direction at all our weather stations are measured at a height of 10 meters.

Health related weather information


The humidex and how is it measured

The humidex is a Canadian innovation that was first used in 1965. It describes how hot, humid weather feels to the average person. The humidex combines the temperature and humidity into one number to reflect the perceived temperature. Because it takes into account the two most important factors that affect summer comfort, it can be a better measure of how stifling the air feels than either temperature or humidity alone.

Range of humidex: degree of comfort

  • 20 to 29: Little discomfort
  • 30 to 39: Some discomfort
  • 40 to 45: Great discomfort; avoid exertion
  • Above 45: Dangerous; heat stroke possible

An extremely high humidex reading is any reading over 40. In such conditions, you should reduce all unnecessary physical activity. If the reading is in the mid to high 30s, then you should tone down or modify certain types of outdoor exercise, depending on the individual age and health, physical shape, the type of clothes worn and other weather conditions.

To learn more, visit: Warm season weather hazards: Heat and humidity.

When to include the humidex in the current conditions and the forecasts

In the current conditions, the humidex values are only displayed when the air temperature is 20°C or greater and the humidex value is at least 1 degree greater than the air temperature. In the forecasts, the humidex is included when the humidex is forecast to be 25 or higher and it’s the maximum humidex value expected.

Wind chill

The wind chill and the wind chill index

A biting wind can make cold temperatures feel even colder. We call the cooling sensation that is caused by the combined effect of temperature and wind, the wind chill.

The wind chill index is expressed in temperature-like units, the format preferred by most Canadians. By equating the outdoor conditions to an equivalent temperature with no wind, the index represents the degree of "chill" that your skin senses. For example, if the wind chill is -20 while the outside temperature is only -10ºC, it means that your face will feel as cold as if it was a calm day (no wind) with a temperature of -20ºC.

The wind chill index enables Canadians to take action to avoid injuries from the cold. This includes dressing warmly to avoid frostbite and hypothermia, and making informed decisions based on accurate wind chill information, such as whether it is safe for children to play outdoors.

To learn more, visit: Wind chill index.

Why current conditions stop reporting wind chill at 0°C

The wind chill index is used to inform people of the risk of frostbite when it's windy and the temperature is below zero Celsius. When the temperature is above zero, there is no risk of frostbite no matter how windy it is, so we don't show the wind chill even though it could be calculated. However, our online wind chill calculator can be used for temperatures up to plus 5°C.

To learn more, visit: Wind chill index.

UV index

More about the UV index

The UV index was developed to help Canadians protect themselves from the sun’s damaging UV (ultraviolet) rays. The higher the UV index, the stronger the sun's rays, and the greater the need to take sun safety precautions. In Canada the UV index ranges from 0 to 11+. The maximum daily UV index is included in our forecasts year round whenever the value is expected to be 1 or more.

UV can cause sunburn, eye cataracts, skin aging and skin cancer. The amount of UV that you receive depends on the strength of the sun, as measured by the UV index, and the amount of time you spend in the sun. Protect yourself by checking the UV index and by wearing a hat, sunglasses, sunscreen, and spending less time in the sun.

To learn more, visit: UV index and sun safety.

Why the UV index is not reported in the current conditions

Our weather stations are not equipped to measure the current UV index values. However, our teams are currently working on new products that will offer real-time UV estimates in the near future.

The UV index begins at zero when the sun comes up, reaches its maximum when the sun is at its highest point in the sky (typically 1:00 PM in the summer because of daylight saving time) and falls back to zero at sunset. The time of the day to take precautions is from 11:00 AM to 4:00 PM from April to September. This is the time when most people are likely to get a sunburn.

Although the UV index is very low during the winter in Canada, skiing and other outdoor winter activities can increase your exposure. Remember that bright white surfaces like snow can double your exposure to UV. Take precautions, if you are going to be outdoors for most of the day, particularly on snow.

Air quality and the Air Quality Health Index (AQHI)

More about the Air Quality Health Index (AQHI)

The Air Quality Health Index (AQHI) is a scale designed to help you understand what the quality of the air around you means to your health. It is a tool developed by health and environmental professionals to communicate the health risk posed by air pollution.

It is designed to help you make decisions to protect your health and the environment by:

  • Limiting short-term exposure to air pollution
  • Adjusting your activity during episodes of increased air pollution and encouraging physical activity on days when the index is lower
  • Reducing your personal contribution to air pollution

The index provides specific advice for people who are especially vulnerable to the effects of air pollution as well as the general public.

The AQHI is a scale that lists a number from 1 to 10+ to indicate the level of health risk associated with air quality:

  • 1-3 = ‘Low’ health risk
  • 4-6 = ‘Moderate’ health risk
  • 7-10 = ‘High’ health risk
  • Above 10 = ‘Very high’ health risk

To learn more, visit: About the Air Quality Health Index.

How the Air Quality Health Index (AQHI) is calculated

The Air Quality Health Index (AQHI) is designed as a guide to the relative risk presented by common air pollutants which are known to harm human health. Three specific pollutants have been chosen as indicators of the overall mixture:

  1. Nitrogen Dioxide (NO2), is released by motor vehicle emissions and power plants that rely on fossil fuels. It contributes to the formation of the other two pollutants. Nitrogen dioxide is often elevated in the vicinity of high traffic roadways and other local sources.
  2. Ground-level Ozone (O3), is formed by photo-chemical reactions in the atmosphere. It can be a major component of smog during the summer, especially during hot sunny weather, but is generally low in the wintertime. Ozone can be transported long distances within a polluted air mass and can be responsible for large regional air pollution episodes.
  3. Fine Particulate Matter (PM2.5), is a mixture of tiny airborne particles that can be inhaled deep into the lungs. These particles can either be emitted directly by vehicles, industrial facilities or natural sources like forest fires, or formed indirectly as a result of chemical reactions among other pollutants. Particulate matter can reflect both local air pollution sources or widespread air pollution episodes.

All three can have serious, combined effects on human health (from illness to hospitalization to premature death), even as a result of short-term exposure. Significantly, all of these pollutants appear to threaten human health, even at low levels of exposure, especially among those with pre-existing health problems. 

In the development of the AQHI, a formula that combined these three pollutants was found to be the best indicators of the health risk of the combined impact of the mix of pollutants in the air.

To learn more, visit: Air quality: frequently asked questions.

Why the Air Quality Health Index (AQHI) scale stop at 10+

The Air Quality Health Index (AQHI) is based on health studies that link the interaction of different pollutants on health risks. The AQHI uses a scale of 1 to 10+, the higher the number, the higher the health risk. The number scale supplements the health advice. The health message associated with the 10+ value is the most extreme associated with the AQHI; higher AQHI values would not change the advice. When the AQHI is a 10+, everyone is at risk, and you should take action to protect your health.

To learn more, visit: Wildfire smoke, air quality and your health.

Why the Air Quality Health Index (AQHI) is at low risk when we can see or smell wildfire smoke

Weather determines how the smoke will spread and where it will be carried by the wind. The smoke may remain near the ground, or rise to considerable heights. When the smoke stays high in the sky, the air may appear hazy but air quality measurements on the ground may show only low levels of pollutants. In these situations, the reported Air Quality Health Index (AQHI) may be in the low risk range (AQHI 1 to 3), despite the visible smoke.

Wildfire smoke conditions can change very quickly. The AQHI observation calculation uses an average of the past 3 hours of pollutant data. In recent years, research has shown that using only PM2.5 hourly observations was more responsive since the AQHI formula is a weighted mean of the 3 pollutants with PM2.5 weighted lower than NO2 and O3. Therefore, if the smoke drifts in and out of your community quickly, the formula will change to the PM2.5 calculation when it is higher than the 3-pollutant formula. The PM2.5 formula will be used until the PM2.5 decreases lower than the 3 pollutant AQHI, indicating that smoke has moved away from the area. This method of calculating the AQHI is now able to capture the rapidly changing conditions. This calculation was developed using BC respiratory data and will be phased in to be applied nationally.

Smoke conditions may also be different in different parts of a community. Sometimes smoke levels at the air quality monitor (the equipment used to measure local air quality) may be different from those in another part of the community. In these circumstances, it is best to pay attention to your symptoms and comfort level, and adjust your outdoor activities, if you feel necessary. If you are experiencing symptoms, you should take action to protect your health.

To learn more, visit: Wildfire smoke, air quality and your health.

Where to find information on the pollen index

Environment and Climate Change Canada does not directly provide information on pollen. When it is in season, you can find pollen information for a number of Canadian cities from The Weather Network's website.


More about lightning and lightning safety

To learn more, visit: Lightning.


Learning how to use the radar

To learn more on the meaning of echoes and its intensity in the precipitation (rain, snow, etc…), and read and use the image as displaying system visit: About Canadian weather radar.

For historical single and composite radar images, visit: Canadian Historical Weather Radar.

Why some areas, particularly British Columbia, always seem to have precipitation

The radar echoes that you see are not always caused by precipitation. Sometimes they are echoes caused by "ground clutter," i.e. surface features like mountains, hills or sometimes tall buildings. Areas of ground clutter can be identified on radar animations where echoes caused by precipitation move, while those caused by ground clutter do not. We take steps to filter out ground clutter from the radar images but complete elimination is not possible.   


More on satellite and satellite images

Complete and exhaustive information on Geosynchronous Operational Environmental Satellite (GOES) and Polar-Orbiting Environmental Satellite (POES) of the National Oceanographic and Atmospheric Administration of the United States (NOAA) can be found on the NOAA Satellites Information System webpage.

The satellites have two on-board imaging sensors (visible and infrared). Each sensor "sees" the same field of view; however, they differ in their sensitivity to various wavelengths of light.

The light detector for each sensor is a charged-coupled device similar (in concept) to that found on most video cameras. Light energy (photons) hits the detector and generates an electrical current that can be measured with sensitive electronics.

Visible light falls in the wavelength region that can be detected by the eye, hence the term “optic” or “optical” often used to describe this region. Because the use of electronics is integral to the functioning of the detector, the visible-light detector is frequently called an electro-optical (EO) detector or sensor.

Infrared (IR) light occupies a large band in the light spectrum. This is the type of energy that provides heat for your home and oven. Infrared detectors can "see in the dark" by detecting the presence of heat given off by people and equipment. The detector used in infrared sensors is basically the same as that used for electro-optical sensors. However, it is sensitive to wavelengths in a different region of the spectrum. This detector must be kept cold so that its own temperature does not generate false signals.

The satellite's EO sensor can detect clouds visible to the eye. This sensor is sensitive to light with wavelengths from 0.4 to 1.1 micrometres (or microns). The IR sensor is sensitive to light with wavelengths from 10.5 to 12.5 micrometres. It can detect high clouds even when they are very thin and not visible to the EO sensor. This is possible because high clouds are also very cold (they are composed of ice crystals).

The meaning of IR and VIS and what you see in those different images

IR stands for infrared. On an image, IR is usually followed by a wavelength in micrometres (e.g. 10.7). In the IR spectrum, clouds at different heights above the ground show up very well as differences in radiances (quantity of light energy detected). Radiances can then be converted into temperatures with some calculation. What is displayed on an IR image is the distribution of temperature of the underlying surface (tops of clouds, ground or ocean) as seen by the sensor on the satellite. The legend corresponds to the temperature of whatever the satellite sensor sees (clouds at different heights, sea surface, earth surface).

VIS stands for visible. A VIS satellite image (taken in the visible spectrum) is a picture of the earth from space, just as you would see it if you were looking out the window from a spacecraft in orbit. When the satellite is over an area during the nighttime, the image is dark.

The meaning of the different colours on satellite images and where to find the legend

For an IR image: The legend indicates the relationship between the colour and the temperature, in degrees Celsius, of whatever the satellite sensor sees (clouds at different heights, sea surface or the earth’s surface).

For a VIS image: The colour on the legend at left (if present) is related to the reflectivity, i.e. the amount of light (0-100%) scattered or reflected from the Earth and clouds back towards the satellite (0-100%).

Why some satellite images are partly or all black

If it is a VIS image during nighttime over North America the image will be partly or all black. Visible images are only available during the daytime, so a nighttime image of North America will be dark because there is no visible light falling on that part of the planet. If you view the visual Satellite Images and Animation, you can watch sunrise or sunset move across the hemisphere (from east to west). At night, visual spectrum images are almost all black.

When to find satellite images north of 60° North

You can find satellite images north of 60° North on the Satellite Images and Animation page, towards the bottom of the page.

Images of areas north of 60° may look slightly different because they come from a polar orbiting satellite. The most commonly used images over southern Canada are obtained from a satellite in a geosynchronous orbit. This means they revolve around the earth in 24 hours, at a very high (34,880 km) altitude over the equator.

Due to this, these satellites remain over a fixed point of the earth (in South America for satellites that can view the Americas). Because geosynchronous satellites typically remain over the equator, the higher the latitude of the area we want to observe, the view becomes distorted due to the curvature of the earth. To obtain more useful pictures at the higher latitudes (north of 60°), we need a different satellite known as a polar-orbiting satellite.

Instead of staying high over one place, a polar orbiting satellite moves very quickly (orbits in less than two hours) at much lower altitude (around 800 km). While geosynchronous satellites take a picture of an entire hemisphere (a disk showing the planet earth), polar-orbiting satellites are so low that they only take in a small swath below the satellite at each orbit.

At present we receive data from the National Oceanographic and Atmospheric Administration (NOAA) polar orbiting satellites, and we post images of most of Canada’s northern regions including the Yukon, the Northwest Territories and Nunavut.

Where to find satellite images of Europe

For European satellite images, visit: European Meteorological Satellite Association (EUMETSAT).

The frequency at which satellite images are updated

Geosynchronous Operational Environmental Satellite (GOES) full disk images (a full Global view using all available sectors) are scanned from the satellite every three hours, while the GOES sector images are scanned from the satellite normally every half hour.

High Resolution Picture Transmission (HRPT) images are updated as they become available.

The images are available on the Weather website typically within 30 minutes of the scanned image.

Weather models

More about the forecast map title

Daily at 00 UTC and 12 UTC a worldwide sample of the atmosphere is taken by a number of upper air soundings (an atmospheric monitoring devise that provides information on winds, temperature, pressure and humidity attached in most cases to a helium filled balloon) and surface observations or reports, and then ingested into our computers. This period of time is referred to as an analysis or 00 (zero) hour.

Using the analysis and other data as a starting point, a numerical simulation or a computer program that attempts to simulate an abstract model of the atmosphere, is run on a computer, in order to predict the state of the atmosphere at various times in the future. The forecast maps are typically available about three hours after the initialization of the data (at 03 UTC and 15 UTC). Forecast maps are labeled by the simulated hour of the model, and by valid date and time. Below is a sample title from a forecast map:


This is a title from a map showing a forecast valid at 12Z (noon UTC) on Wednesday, 9 August 2000 (the second title line). The first title line means the forecast is for 12 hours after the collection time of the data on which the forecast is based. It is therefore based on data collected at 00Z, i.e. at midnight UTC on 9 August. This forecast would normally be available on the Weather website by 03 UTC.

The meaning of a "00H forecast" (a zero hour forecast)

A zero-hour forecast (00H forecast) indicates how the computer model "sees" the atmosphere at the beginning, the initial time or "zeroth" hour of a numerical simulation. A 00H forecast map shows the initial values of the meteorological elements that the model calculates.

Climate and historical weather data

More about climate and historical weather data

To learn more, visit: Historical Climate Data.

Hydrometric data (Water Survey of Canada)

More about the hydrometric data (Water Survey of Canada)


More about tropical cyclones and hurricanes

Severe weather

What to do in case of severe weather (thunderstorms, hail, blizzards, ice storms, hurricanes, storm surges, tornadoes and heavy rain)

To learn more, visit: Severe Storms - What to do?

To report severe weather

To learn more, visit: Report severe weather: overview.

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