About Canadian historical weather radar

Using Environment and Climate Change Canada's (ECCC) radar data can be quite beneficial when making weather sensitive decisions. To assist users, a comprehensive table of contents, detailing radar information is listed below.

• Background
• How Radar Works
• Radar Display Description
• Radar Interpretation
  • What am I looking at? (Echoes)
  • What is a DPQPE product?
  • What is a PRECIP Radar Product?
  • Common interpretation errors

Background

The term RADAR, which has been in use since the 1940’s, is an acronym formed from the term Radio Detection and Ranging.

ECCC's network is concentrated in the most populated parts of Canada, providing radar coverage to more than 95 per cent of Canadians. The network’s primary purpose is the early detection of developing thunderstorms and high impact weather, as well the tracking of precipitation.

ECCC's weather radars have a detection maximum range of 250 km around each C-Band site for reflectivity (identified by 3-letters site ID on our individual images; for example “XFT”, for the Franktown radar, near the national capital Ottawa) and 240 km for each new S-Band radar (identified by 5-letters site ID on our individual images; “CASBV” example for the Blainville radar near Montreal). As for the Doppler velocity, range is 120 km for C-Band radars and 240 km for S-Band radars. Most of the radars have now been replaced and it should be noted that by 2023, all C-band radars will be replaced by new S-band radars (more details here: RADAR replacement schedule). The Doppler data is used to detect severe weather such as tornadoes. Historical radar images are available at the National Climate Data and Information Archive.

The “How to Use” tab provides more information on how to view the images.

How Radar Works

Weather surveillance radar systems generally use a parabolic antenna (the dish type of antenna you might have at home) to focus a pulsed radio-frequency beam out into the atmosphere, a lot like a searchlight. It is important to note that radars “listen” far more than they “speak”. Radars send an incredibly brief pulse, and then listen for echoes to return. These echoes are then recorded by the very sensitive “listening” receivers. During this listening period, the radar cannot emit other pulses because it would damage the receivers, so our radars spend most of their time “listening” and generating no radio frequency energy. This narrow beam sweeps the sky for 360 degrees around the radar site, pointing at different elevation angles each time it sweeps around. ECCC's operational scan strategy for the C-band radars consists of 24 slices (also called elevation angles) which are acquired sequentially in descending order (top to bottom) while for the new S-band radars, 17 slices are acquired sequentially and always from top to bottom.

When the energy emitted by the radar antenna strikes particles of precipitation, such as drops of water, snowflakes, ice pellets, or hail, a portion of that energy is reflected back to the radar. The intensity of this energy is related to the number, size and type of the precipitation particles.

Radar Display Description

There are three views of radar images that can be seen on the Canadian Historical Weather Radar website:

1. National
2. Regional
3. Local

1. National View

This view provides at a glance, a complex, low-resolution picture of where precipitation is occurring within the complete ECCC's radar network.

The picture is interactive, and can be used to choose the geographical area you are interested in. Clicking on a province will lead you to a Regional View for that area.

From this national picture, you can refine it to view local radar data for a specific geographic region. This can be done by selecting an area near one of the black dots which represent local radar sites.

2. Regional View

This view gives you a higher resolution look at precipitation patterns. The picture is interactive, and can be used to select a more specific area. You can view the actual radar data for a specific geographic region, by selecting an area near one of the black dots representing local radar sites.

3. Local View

This is your local view of precipitation detected within the coverage of the local radar site.

Radar Interpretation

What am I looking at? (Radar Echoes)

When you look at a radar image, you are looking at a picture of precipitation distribution (called “echoes”) and its intensity.

Radar echoes are represented by a series of coloured pixels, as illustrated on the scale to the right of the radar image. The intensity scale on the right represents what is called “reflectivity in dBZ” (unit of reflectivity), and the scale on the left is the corresponding rate of fall which is an interpretation of how light or heavy the precipitation is. In the winter season, this reflectivity is linked to the snowfall rate in centimetres per hour (cm/hr), and in the summer months, the reflectivity is linked to the rainfall rate in millimeters per hour (mm/hr).

A good rule of thumb:

The higher the reflectivity value on a radar image, the heavier the precipitation rate that is being detected.

For example, the dark purple colour represents the heaviest precipitation of snow (meaning it has a snow rate of fall of 20 cm/hr), while the aqua colour is the lightest precipitation (meaning it has a snow rate of fall of 0.1cm/hr).

Reflectivity not only depends on the precipitation intensity, but also the type of precipitation. Snow generally reflects less radar energy than rain. Consequently, moderate to heavy snow can appear light in intensity. Meanwhile, ice pellets and hail are highly reflective thus light ice pellets or hail can appear as heavy precipitation.

In some cases, the radar does not distinguish between real echoes (precipitation) and non-meteorological echoes (trees, hills, tall buildings). It is also important to understand some common interpretation errors, if you are hoping to accurately interpret the radar images.

What is a DPQPE product?

DPQPE product: DPQPE stands for: Dual-Pol Quantitative Precipitation Estimation. The DPQPE product is available only for S-Band radars. It is a two-dimensional representation of the precipitation rate estimated by the lowest sweep of the radar (elevation angle of 0.4 degrees for the majority of S-Band radars). Thus, the precipitation rate is estimated as close to the earth's surface as possible. The DPQPE product is based, among other things, on a series of polarimetric processing steps (quality control) to eliminate non-meteorological artifacts from the raw data (volumetric scans). It renders in mm/h for rain and cm/h for snow. This product is calculated with a maximum coverage range of 240 km.

For more information about the Canadian Weather Radar.

What is a PRECIP-ET Product?

First, let’s mention that the type of image you are viewing is indicated at the bottom right-hand legend. The standard radar product Environment and Climate Change Canada now presents on its website is called a "PRECIP-ET" (PRECIPitation-ExTended) product replacing the previous PRECIP product (see explanation in the next section).

PRECIP-ET images:

The PRECIP-ET (PRECIPET on images) is an extension of the PRECIP product (hence the use of ET in the product name). Compared to the previous PRECIP product, the PRECIP-ET performs a suite of additional quality control (QC) tests and tracks issues better. For example, Doppler filters might be rejecting moderate ground echoes in a particular area and thus weak meteorological echoes would not be detectible. In this case, PRECIP-ET would flag the missing data area, whereas PRECIP would simply treat the area as no echoes.  This additional quality treatment is why we have migrated to PRECIP-ET in 2013, to get an improved service.

The PRECIP-ET image is designed to show the precipitation close to the ground, by using Doppler technology processing for echoes within 128 km from the radar site for the C-Band radars and 240 km for the new S-Band radars. Doppler technology allows for better resolution of the precipitation echoes and also provides the ability to detect the movement of precipitation in relation to the radar (i.e. are the raindrops or snowflakes moving towards or away from the radar and at what speed). Beyond this 128 km limit, the echoes are displayed using the more conventional CAPPI processing which is explained below.

Three factors distinguish the PRECIP-ET product from the old conventional PRECIP one (available in our radar archiving images before 2013):

1. Weather radars can receive non-meteorological echoes from ground objects such as buildings and towers; the PRECIP-ET product uses Doppler processing to edit out most of these false echoes. Doppler processing can detect these non-meteorological echoes because they are not moving in relation to the radar as the realistic precipitations as raindrops and snowflakes would. So, with the clutter caused by ground objects filtered out, the PRECIP-ET product provides a cleaner image close to the radar. In addition to the direct Doppler corrections, PRECIP-ET looks at patterns of echoes and attempts to remove echo patterns that do not appear meteorological. For example, if the radar implies very strong precipitation near the surface, then there should be a big storm above it. This process is more ambiguous than simple Doppler as some false targets look like precipitation and vice-versa, so the removal of many non-meteorological targets may lead to occasional removal of valid precipitation.
2. Trees and hills around radars can block portions or the entire radar beams, resulting in a decreased ability to detect precipitation close to the ground near the radar. This problem can be particularly obvious during winter months when a lot of precipitation forms and falls close to the ground. Therefore with some or most of the radar beam being blocked in the very low levels during the winter, the radar may not do the best job at detecting light precipitation near the radar, and why no echoes does not necessarily mean no snow in reality close to the radar.
3. Doppler processing is not as efficient, in general, at detecting weak precipitation as conventional radar processing is. Since the PRECIP-ET product is a combination of Doppler processing (first 110 km from radar) and conventional processing (beyond 110 km from the radar), there can sometimes appear to be a discontinuity in the precipitation as one goes beyond 110 km from the radar.

What is the difference between PRECIP-ET-Rain and PRECIP-ET-Snow images?

In general, rain is more easily detected by radar than snow. In order to better represent precipitation during the warm season, a certain relationship is used to relate the reflectivity to a rainfall rate (in mm/h - PRECIPET-Rain). During the cold season, a different relationship is used to relate the reflectivity to a snowfall rate (in cm/h - PRECIP-ET-Snow). However, it should be noted that just because the reflectivity is being represented in mm/h doesn’t mean that the precipitation shown on the radar is rain or if the reflectivity is being represented in cm/h that the precipitation shown on the radar is snow. The precipitation in most of the more densely populated areas of the country can still show a high degree of variability throughout a good portion of the fall, winter and spring allowing for rain, freezing rain, ice pellets and snow to occur alone or in some messy combination. Therefore, it is always important to relate the precipitation you are seeing on radar with what the current weather conditions are and what the forecast is in the coming hours to get the best sense of what may fall out of the sky in your area.

What is a CAPPI Radar Product?

Due to the curvature of the Earth, the height of a radar beam, in relation to the ground, increases as it travels further from the radar. When the radar is pointed down near the ground (a low elevation angle), the beam starts off near the ground but then its height above the ground slowly increases. By the time that same beam is 200 km from the radar, it is at a height of around 4 km above the ground. In order to get a better sense of what is happening at one approximate height above the ground (i.e. 1.5 km), a whole series of radar beams with different elevation angles (low, medium, high) are used to create one radar product. This type of radar product is called a CAPPI (Constant Altitude Plan Position Indicator). Since the CAPPI products do not use Doppler processing to filter out clutter like tall trees, hills and buildings, it can sometimes be contaminated by non meteorological echoes.   

Common Interpretation Errors

A picture may be worth a thousand words, but sometimes what you see isn't necessarily what you get. Just because something looks colourful on the radar screen doesn't mean there's rain or snow. Similarly, just because the echoes on the screen appear weak or don't appear at all, doesn't mean there isn't significant precipitation falling somewhere.

Here are some of the more common radar interpretation mistakes:

Blocking Beam

• Hills and mountains can block a radar beam and leave noticeable gaps in the pattern.
• This situation is very common in the Rockies and Newfoundland, due to the hilly terrain.
• As well, with the new S-Band radars there are cases where there is a wedge shaped gap. In some cases these are a blanked sector in the direction of a permanent or temporary obstacle. For example, there is a permanent blanked sector to the west of the Spirit River radar (254° - 266°) in the direction of a nearby tower. Our radar does not transmit in that direction. These blanked sectors may be only temporary for example if there is work being done on a nearby tower.

Beam Attenuation

• Storms closest to a radar site reflect or absorb most of the available radar energy. Only a reduced amount of this energy is available to detect more distant storms. This situation is much improved with the new S-Band radars as they have significantly reduced attenuation issues.
• Storms in the attached example’s circled area were quite intense, but were not being detected appropriately by the radar. Strong storms occurring closer to the radar kept a significant portion of the radar energy from penetrating beyond these storms.    

Overshooting Beam

• Intense precipitation such as lake effect snow squalls can be associated with clouds close to the ground. In such cases, the radar beam may overshoot most of the area of precipitation and therefore indicate only weak echoes, when in fact significant precipitation is occurring.
• The example image shows lines of precipitation on the Prairies. Although it was raining inside the radar coverage area, the radar overshoots at long range and the bands are not seen.

Virga

• Precipitation that is occurring in the air but not reaching the ground, is called virga. It happens when there are dry conditions at low levels; the dry air absorbs all the moisture before it reaches the ground.
• No precipitation was hitting the ground in the example attached.

Anomalous Propagation (AP)

• In the low levels of the atmosphere when a layer of warm air lies over a layer of much cooler air (a temperature inversion), the radar beam can't pass between the layers, and gets bent to the ground. A false strong signal is reflected back to the radar site.
• This phenomenon is most common during the early morning hours when it is clear. The false echoes generally dissipate by midday.
• In the example image, there was no precipitation occurring.

Ground Clutter

• These echoes are called "ground clutter" and they occur when a portion of the radar beam comes into contact with tall buildings, trees or hills.
• It is good to learn the common ground clutter "signature" in your area, so you can distinguish it from real precipitation.
• Radars near large bodies of water may also receive echoes from moving waves. Since waves depend on wind speed and direction, "sea clutter" is more variable than standard ground clutter and is therefore more difficult for existing processing to filter out.

Electromagnetic Interference

• The historical ECCC radar network is heterogeneous. A  frequency range of 5.6 - 5.65 GHz is used for C-band radars and 2.7 - 2.9 GHz for S-band radars. ECCC operates each weather radar at a specific frequency band specified by Innovation, Science and Economic Development Canada (ISEDC). Sometimes external sources, either at the same frequency or combined to be the same frequency, can be detected in our radar systems. Once these interference issues are detected, ISEDC is notified to find the source, which can be challenging. In moderate to severe weather, weather signals generally overshadow these interference signals, and using the information from the dual polarization technology that ECCC is deploying can help resolve some of these problems in lighter precipitation.
• Typical interference from a wireless device appears as a persistent spike on the radar going out from the radar in one direction (i.e. along a radial). A typical spike is shown in the image below.   
• Sometimes a “sun spike” can be briefly seen at sunrise or sunset when the radar has a clear view of the sun low on the horizon. This is caused by the solar radiation being picked up by the sensitive radar receiver.