Weather tools: frequently asked questions
Frequently Asked Questions
- Can you give me some general information on satellite images?
- What do IR and VIS mean? What do we see on IR and VIS images?
- What do the colours mean on satellite images? Is there a legend?
- Why is the satellite image partly or all black?
- Are there images north of 60° North on your site?
- Where can I find images of Europe?
- How often are the images updated?
- How is rain measured?
- How is snow measured?
- How do you calculate the water equivalent of snow?
- Why is the precipitation amount for yesterday so high or so low? What are common sources of errors in measuring precipitation?
- What is the dew point?
- What is the "relative humidity"?
- What is the pressure tendency?
- I have an old barometer that gives readings that range from approximately 28 to 31. Are these inches of mercury (Hg)? What is the factor to convert kilopascals (kPa) or hectopascals (hpa) into inches of Hg?
Questions and Answers
On radar images, why does it seem that there's always some precipitation over certain areas, particularly in British Columbia?
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.
Can you give me some general information on 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 from the general satellite link below.
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).
What do IR and VIS mean? What do we see on IR and VIS 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.
What do the colours mean on satellite images? Is there a 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 is the satellite image 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 download the visual animation, you can watch sunrise or sunset move across the hemisphere (from east to west). At night, visual spectrum images are almost all black.
Are there images north of 60° North on your site?
Yes, you may find them at http://weather.gc.ca/satellite/index_e.html towards the end 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 can I find images of Europe?
You can try the European Meteorological Satellite Association (EUMETSAT) website at: http://www.eumetsat.int/Home/Main/Image_Gallery/Real_Time_Imagery/index.htm
How often are the images 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.
I don't understand the forecast map title. Can you explain its meaning to me?
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:
12 H FORECAST - PREVISION 12 h
12Z WED-MER 09 AUG-AOU 00
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 Weatheroffice website by 03 UTC.
What's a "00H forecast" (a zero hour forecast)?
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.
Where can I find information on the pollen index?
Environment 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 at http://www.theweathernetwork.com/.
How is rain 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 Canada networks use automated total precipitation weighing gauges (Geonor & Pluvio) which determine precipitation amounts through weight change. These gauges report total precipitation amounts hourly, in millimetres.
Staffed reporting stations use the standard Canadian Type B rain gauge, a cylindrical container 40 cm high and 11.3 cm in diameter. The precipitation is then funnelled into a plastic measuring tube that is calibrated to show the precipitation amount in millimetres. Precipitation is typically measured every 6 hours with a total amount reported at the end of the day.
The NAV CANADA Automated Weather Observing Stations (NC-AWOS) located at most airports use a heated Met One Tipping Bucket Rain Gauge. A few Automated Weather Observing Stations (AWOS) in use at airports use Fisher-Porter automatic rain gauges. These gauges will be taken out of service by late 2012.
How is snow measured?
At Environment Canada, snow is measured by automated observing stations which register the snowfall and snow-on-ground amounts, using an acoustic snow sensor (SR-50). The automated sites report snowfall amounts hourly in centimetres.
At staffed stations, the snow amount or the depth of accumulated snow-on-ground is measured using a snow ruler or a ruler calibrated to centimetres. The measurements are made at several points which appear representative of the immediate area, and then averaged. Snow is normally measured in "centimetres".
Also note that snowfall amounts are not measured at a number of Environment Canada and partner’s stations as the automated equipment is not capable of this measurement.
How do you calculate the water equivalent of snow?
To calculate the water equivalent of snow, we melt the snow captured in snow gauges. The Geonor, Pluvio, and Fischer-Porter gauges melt freezing and frozen precipitation directly with glycol, then report the Snow Water Equivalent amount in millimetres.
At staffed sites the observer takes the gauge containing the snow indoors, melts it, then pours the resulting liquid into a plastic measuring tube 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 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 Precipitation 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 Weather.gc.ca Operational Analysis Charts.
Why is the precipitation amount for yesterday so high or so low? What are common sources of errors in measuring precipitation?
Environment Canada employs staffed and automated weather stations across Canada to collect temperature, rain and snowfall 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 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 (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.
What is 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.
What is 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.
What is 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.gc.ca 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.
I have an old barometer that gives readings that range from approximately 28 to 31. Are these inches of mercury (Hg)? What is the factor to convert kilopascals (kPa) or hectopascals (hpa) into inches of Hg?
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.
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