"Yesterday's Total Precipitation", in the table on the "5 day weather page", is a total of all the types of precipitation that may have fallen on the previous day including: rain, drizzle, freezing drizzle, freezing rain, hail and snow.

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 funnelled into a plastic graduate which serves as the measuring device. The liquid precipitation is normally measured in millimetres.

Usually, the snow amount or the depth of accumulated snow is measured using a snow ruler. The measurements are made at several points which appear representative of the immediate area, and then averaged. Snow is normally measured in "centimetres".

To calculate the water equivalent of snow, we melt the snow captured in snow gauges like the Nipher snow gauge. This Nipher gauge is designed to diminish the turbulence around the opening of the gauge and positioned high enough above ground to prevent most of the blowing snow from entering the gauge.

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 centimetre of snow is equivalent to about 1 millimetre of water once the snow is melted. This means that in many snowfall situations (on days when only snow fell), you can simply change the units from millimetres to centimetres on the "Yesterday's Precip. Total" on a specific location's weather page to get a reasonably good idea 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 centimetres of snow would melt to provide 1 millimetre 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). It is available from http://weatheroffice.ec.gc.ca/analysis/index_e.html.

Environment Canada employs manned and automated weather stations located across Canada to collect temperature, rain and snow fall amounts, wind direction and speed and barometric pressure. As precipitation rates constantly change with time and because weather systems are 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 error sources 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 effects on accumulation on the ground (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 instance 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 shaking 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.

The dew point is a measure of the humidity content in the air. Dew point is short for dew point temperature. It indicates the amount of moisture in the air. It 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, 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.

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.

For more information regarding the relative humidity, consult our fact sheet on humidity.

The humidex is an index (a computed value as opposed to something measured) devised to describe how hot or humid weather feels to the average person. The humidex combines the temperature and humidity into one number to reflect the perceived temperature. It takes into account these two important factors that affect summer comfort. It is therefore a better measure of how stifling the air feels than either temperature or humidity alone. A humidex of 40 with, for example, a temperature of 30 degrees means that the sensation of heat when it is 30 degrees and the air is humid is more or less the same as when it is 40 degrees and the air is dry. We must be careful not to depend on this interpretation alone: it is a mere indication of physiological reactions, not an absolute measure

For more information regarding the relative humidity and the humidex, consult our fact sheet on humidity.

The Humidex formula is based on the work of J.M. Masterton and F.A. Richardson at the Atmospheric Environment Service (now MSC) of Environment Canada in 1979. It is a standard for Canada, but variations are used around the world. The dew point temperature should be given in kelvins (temperature in K = temperature in °C + 273.1) for the formula to work. The magic number 5417.7530 is a rounded constant; it's based on the molecular weight of water, latent heat of evaporation, and the universal gas constant.

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

h = (0.5555)*(e - 10.0);
humidex = (air temperature) + h

Note that the values associated with the discomfort of the humidex are generally for the outdoors. The Canadian Centre for Occupational Health and Safety recommends another index for indoors. For more information, visit:
http://www.ccohs.ca/oshanswers/phys_agents/humidex.html

Program designed to calculate the humidex
Environment Canada offers a program that helps calculate the humidex from temperature and dew point data. This program, called Chilldex, also allows you to calculate the wind chill from the wind speed and temperature. You may download this calculator from this page: http://www.msc.ec.gc.ca/education/windchill/calculator_e.cfm

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 according to eight (8) possible trends (such as: increasing steadily, steady, falling rapidly, etc.). 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.

In the web-site www.weatheroffice.ec.gc.ca 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.

The UV Index gives information about the level of UV rays (or ultraviolet radiation) that reach the surface. UV rays are those sun rays that can cause sunburns. Long-term exposure to UV rays has also been associated with skin cancer and cataracts.

The forecast UV Index is the maximum level of UV radiation that can be expected to reach the surface on a given day (today or, in afternoon forecasts, tomorrow). The UV Index is included in the text of the public forecast whenever it is expected to be 3 or more, i.e. when the index can reach the "moderate" category.

In Canada, the UV Index normally ranges from 0 to 10 (it can reach higher values further south), and is also expressed in categories, as follows:

For more information on UV radiation, the UV Index and the ozone layer, please check the following:

• Environment Canada's main Web site on the UV Index
• Web site on ozone and ultraviolet research and monitoring
• Environment Canada's Web site on ozone layer
• The Ozone Layer fact sheet
• Frequently asked questions on the depletion of the ozone layer and its influence on UV radiation

The probability of precipitation (POP) is the chance that measurable precipitation (0.2 mm of rain or 0.2 cm of snow) will fall on any point of the forecast region during the forecast period. For example, a 30% probability of precipitation means that the chance of you getting rained over (or snowed over in winter) is 3 in 10. In other words, there is a 30% chance that rain or snow will fall on you, and, therefore, a 70% chance that it won't. It must also be noted that a low POP does not mean a sunny day: it only means a day where the chance of rain or snow is low.

This fact sheet explains the Probability of Precipitation: http://www.msc.ec.gc.ca/cd/probability_e.cfm .

Rules for including the POP in forecasts changed in 2001. The forecast POP is only indicated when it is expected to be from 30% to 70%, inclusive. You can find more details on this page.

This fact sheet should help you understand our forecasts.

Synopses were discontinued in September 2001. We have conducted public and media opinion research and we found that 85% of the media did not even read the synopsis. In addition, pages giving the synopses had low traffic. Since we rely heavily on the media to broadcast our bulletins, if the media do not use a bulletin, this bulletin becomes a candidate for cancellation. This allows us to focus on higher-priority services.

We have not completely terminated the total functionality of the synopsis, however. When we anticipate that some significant weather conditions are going to affect certain regions of the country, we issue special weather statements to inform the concerned population of the situation. By issuing these special weather statements, we hope to satisfy the need for descriptive information on the general weather situation when circumstances call for it.

You can also look at analysed weather maps to get displays of weather systems. In particular, the maps in the "Complete" column will give you a more extensive picture of the weather features that influence the northern hemisphere.

Environment Canada does not directly provide this information. However, in association with the Lung Association of Nova Scotia and St. Mary's University, Environment Canada provides a chart of pollen and spores in the Halifax area on this page : http://www.ns.ec.gc.ca/weather/pollenenglish.html.

You can also, in season, find pollen information for a number of Canadian cities from The Weather Network's Web site at http://www.theweathernetwork.com/ .

Presently, we offer a warnings by email (e-warnings) and lightning by email (e-flash) services which are available at:
http://www.weatheroffice.pyr.ec.gc.ca/e-products.

Please follow the instructions to initiate or modify your service.

The page for e-products is
http://www.weatheroffice.pyr.ec.gc.ca/e-products.

You need to refer to the climate normals, namely the monthly and annual means derived from a variety of meteorological factors (i.e. temperature, degree days, hours of bright sunshine, rain and snow fall, humidity and winds, as available). The normals can be found at this address: http://www.climate.weatheroffice.ec.gc.ca/climate_normals/index_e.html

The tables of climatological normals also show, when available, temperature, precipitation and wind records for each month.

The Canadian Meteorological Centre has several web sites. The weather site is maintained by the Operations Branch, and provides information about the current condition of the atmosphere, and output of our models to predict its future state. What you won't find on these pages is any information about the past; this is considered climatological data. For questions on climatological topics, please consult our colleagues in the Climate and Water Information Branch.

For information regarding a region of Canada, please consult our regional climate contacts.

You may also try the Canadian Institute for Climate Studies (not part of Environment Canada).

You can find this information, and much more, at the link given below. (When you are at this link, to find historical weather information, click on "Climate Data"):

http://www.climate.weatheroffice.ec.gc.ca/prods_servs/index_e.html

If you can't find the needed data at the link above, we may be able to provide it on a cost-recovered basis. Further climatological information is available from Environment Canada's regional climate centres. For more information, contact the regional office serving your area - link to http://www.climate.weatheroffice.ec.gc.ca/contacts/index_e.html. Where available, the user-pay Climate Source service, accessible at 1-900-565-1111, gives you quick, direct access to this office.

There are six (6) time zones across Canada. These time zones can be found at the following web site: http://inms-ienm.nrc-cnrc.gc.ca/en/time_services/daylight_saving_e.php

You may also try this URL for the time zones: http://www.canadainfolink.ca/time.htm

(Reprinted from: http://www.eecis.udel.edu/~mills/leap.html)
GMT is a historic term which is now obsolete. It is today called Coordinated Universal Time (UTC). Universal time is the local time on the zero meridian which goes through the old observatory in Greenwich, London, UK.

If you are interested in knowing time more precisely than 1 second, then you have to make a difference between the following versions of universal time:

• UT0 is the precise sidereal local time on the zero meridian.
• UT1 is UT0 corrected by a number of periodic effects (UT0 has different "speeds" at different times of the year).
• UTC is a time defined not by the movement of the earth, but by a collection of atomic clocks all over the world. When UTC and UT1 drift apart for more than 0.9 s, a leap-second is inserted into UTC to correct this. The C in UTC stands for "Coordinated".
• UT2 is an even better corrected version of UT0 which is used in astronomy.

GMT is a term that was used before time was defined internationally by an atomic time reference in the late 1950s. People who talk today about GMT really mean UTC.

Daily at 00 UTC and 12 UTC, a snapshot of the atmosphere is taken, and made readable to our computers. This snapshot is called an "analysis". Using the analysis and other data as a starting point, a numerical simulation of the atmosphere is run (on a supercomputer) to predict the state of the atmosphere at various times in the future. The forecast maps are typically available about three hours after the snapshot data (at 03 UTC and 15 UTC). Forecast maps are labelled 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 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 WWW around 03 UTC.

A zero-hour 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.

Your barometer indeed reports pressure in inches of mercury (Hg). The conversion factor is approximately 33.9 hPa, or 3.39 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.

For North America, we have a map of the jet stream at http://weatheroffice.ec.gc.ca/jet_stream/index_e.html. You can get to it by clicking on Weather Maps.

For other areas of the northern hemisphere, you can get a good idea of the jet-stream's position by looking at the map of the 250 hPa analysis, which you can find on analysed maps page. The jet stream runs through the areas where the contour lines are very close together.

Try http://ww2010.atmos.uiuc.edu/(Gh)/guides/maps/home.rxml
This site gives an introduction to weather map interpretation. Or you may want to visit :
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/home.rxml

If you read French, you will also find some information on the La Girouette site; click on "Le quoi et comment de la météo". You may also try the education site InterMet. From the home page, click on Ressources éducatives.

We do not have this explanation on our Web site, but it can be found elsewhere on the Internet. You may try the following sites:

In English:
Guide to Weather Symbols

Selected Weather Map Symbols - from the American Meteorological Society

Synoptic code Table, from a British site which is to be consulted in conjunction with this other site.

In French:
Lire une carte météo (Cyberscol)

The conversion formula is °F = °C x 9/5 + 32 or, in reverse, °C = (°F - 32) X 5/9 . The formula also appears on this page.

The Canadian Hydrographic Service (CHS), Laurentian Region, web site gives tide times for most Canadian sites for which tides are calculated. Once you are in the site, click on "Canadian Tide Tables", and then on "Select region of interest".

You can order products directly from the Canadian Hydrographic Service:

Hydrographic Chart Distribution Office
Department of Fisheries & Oceans X
1675 Russell Road, P.O. Box 8080
Ottawa, Ontario K1G 3H6
Tel: (613) 998-4931
Fax: (613) 998-1217

You may also find more information through the Government of Canada's Marine Services On-Line site.

Try this web site for West Coast tides:
http://www.tides.info

You can find those times on the Web site of the Herzberg Institute of Astrophysics of the National Research Council of Canada, at http://www.hia.nrc.ca/sunrise_e.html
Second Option: http://cleardarksky.com/csk/index.html#clock_list

If you want a locality that does not appear in the list of cities, you will need to find its latitude and longitude. You can find these on the Canadian Geographical Names site of Natural Resources Canada, at http://geonames.nrcan.gc.ca/search/search_e.php

You can also use the U.S. Naval Observatory site. This will give you all the information for the sun and moon for your location.

The times posted for moonrise, moonset, sunrise and sunset are provided on this site solely as a courtesy by Environment Canada for general reference purposes. If, however, you are seeking official times, the Canadian authority for this information is the National Research Council (NRC) whose data can be accessed at: http://hia-iha.nrc-cnrc.gc.ca/sunrise_e.html. The standards used by the NRC for the generation of astronomical times conform to international practices for computing sunrise, sunset and times for related data. And these computed values may differ from the times posted on our site.

There is an aviation package at http://weatheroffice.ec.gc.ca/model_forecast/aviation_products_e.html. The METAR reports, TAF forecasts, and graphical area forecasts (GFA) for the Canadian domestic air space are available on NAV CANADA's Web site, under "Flight Planning".

While online consultation of these products can help you familiarize yourself with the expected weather conditions, it DOES NOT replace a briefing with a flight service specialist.

Complete and exhaustive information on Geosynchronous Operational Environmental Satellite (GOES) and Polar-Orbiting Environmental Satellite (POES) of the NOAA (National Oceanographic and Atmospheric Administration of the United States) can be found from the general satellite links below. (link with exit page to each)

http://noaasis.noaa.gov/NOAASIS/ml/genlsatl.html
http://ww2010.atmos.uiuc.edu/(Gh)/guides/rs/home.rxml
http://ww2010.atmos.uiuc.edu/(Gh)/guides/rs/sat/img/vis.rxml
http://ww2010.atmos.uiuc.edu/(Gh)/guides/rs/sat/img/ir.rxml
http://ww2010.atmos.uiuc.edu/(Gh)/guides/rs/sat/img/wv.rxml
http://ww2010.atmos.uiuc.edu/(Gh)/guides/rs/sat/img/cir.rxml

The satellite has 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 (non-microwave) 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. It is, however, 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; one micrometre is equal to 10-6 metre or 1/1000 of a mm). 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).

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 show up very well as differences in radiances (quantity of light energy detected) from ground level (radiances vary with cloud height). Radiances can then be converted into temperatures with some calculation. So what we see on an IR image is the distribution of temperatures as detected by the satellite's sensor, and the temperature in 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. During nighttime over the Americas, the picture is dark.

IR image: The legend indicates the relation between the colour and the temperature in degrees Celsius; please refer to the previous question.

VIS image: The colour on the legend at left (if present) is related to the reflectivity, i.e. the amount of light reflected (0-100%). Note that a visual spectrum satellite image is not a false colour one. Blue means that the particular part of the earth looks blue from where the satellite is, white means it looks white, etc. So a legend on these images is largely superfluous.

Yes; you may find them at http://meteo.ec.gc.ca/satellite/index_e.html, towards the end of the page.

However, images of areas north of 60° look different because they come from a different satellite. Most weather satellites, including those we use for images of southern Canada, are in geosynchronous orbit. This means they revolve around the earth in 24 hours, at a very high (34,880 km) altitude over the equator. These satellites, therefore, remain over a fixed point of the earth (in south America for satellites that can view the Americas). Because geosynchronous satellites stay over the equator, the higher the latitude of the area we want to observe, the worse the view. As a result, to get useful pictures at the higher latitudes (north of 60°), we need a different satellite-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, they only take in a small swath below the satellite at each orbit. So to get a full-disk, one must-using software-"stitch" the swaths together. Because the strips are photographed at different times, the result is not a true image, but a composition.

At present we receive NOAA polar orbiting satellite data, and we post images of most of Canada northern regions, including the Yukon, Northwest Territories and Nunavut.

Go the National Radar Project site, where you'll also be able to consult the Frequently Asked Questions.

For information on the basics of radars and target detection, interpreting radar imagery and exploring their applications in forecasting and severe weather prediction, go to these sites:

http://ww2010.atmos.uiuc.edu/(Gh)/guides/rs/rad/home.rxml
http://ww2010.atmos.uiuc.edu/(Gh)/guides/rs/rad/basics/cltr.rxml (clear air returns)

Please see our Careers in Meteorology web site.

You may also consult the list of jobs open to the public of the Public Service Commission of Canada.

Environment Canada with headquarters in Ottawa-Hull, is a scientific Department of the Government of Canada. It has a network of WWW servers known as The Green Lane. The Department is divided into "Services." The Meteorological Service of Canada (MSC) is the one providing Canadians with weather and climate information to enhance their safety and quality of life. The service has a national headquarters (located partly in Toronto and partly in the National Capital Region), a national weather centre, the Canadian Meteorological Centre or CMC (located partly in Toronto, partly in Dorval, QC), and Regional Centres across Canada.

The CMC takes care of the data which needs to be collected from across the country and around the world. The CMC uses this data in its computer models of the atmosphere to produce predictions of what the atmosphere will look like in the future (up to six days). The Regional Centres produce and issue weather warnings and regional routine forecasts like those for aviation, the public, marine users, travel or recreation. They monitor the weather in the region, produce special weather statements and advisories for weather of interest or potential risks, provide a broad range of commercial services for private clients, and provide briefings to the local media. In addition, they collect and redistribute local data.

Browser caching options on your computer control how and when your browser displays or downloads images from our web server. The animations on the Environment Canada (EC) "weatheroffice" web site are shown as a series of individual images, like a slide show. If you set your browser's cache options to download images ¨on every visit¨, then each run through the slide show will trigger a new transfer of every image in the series.

With Microsoft Internet Explorer browser, look in Tools/Internet Options/ General /Settings. Under ¨Check for newer versions of stored pages¨ a selection of ¨automatically¨ should solve the problem. This change would also substantially accelerate your web activities by reducing the amount of data downloaded to your computer from the web sites you visit. Users of the mozilla-firefox browser will not encounter this problem, since the faulty setting has been removed as this browser ¨automatically¨ defaults to the proper setting.

There is usually nothing wrong with our images. We can download them on the systems we have at the CMC without problems. If you have difficulties viewing some elements at the site, we are unlikely to be able to help you unless it is a problem on our server. Your best bet is to consult internet resources on the various types of software available, e.g. Netscape Help or Internet Explorer Support.

If you still think you should ask us about a problem viewing a given image or animation, please include the exact URL of the image you are having difficulty with, and a description of your system, namely the hardware (how much RAM), the operating system (name and version, for instance Windows 95b), the browser (name and version), the plug-ins and the helpers (name and version) you are using. This will help us solve the problem.