Background

The term RADAR, which has been in use since the 1940s, is an acronym formed from the term RAdio Detection And Ranging. Environment Canada's RADAR network of 31 sites covers 98% of the most populated areas in Canada. Their primary purpose is the early detection of developing precipitation, thunderstorms and high impact weather. The radar has a range of 250 km around the site.

To learn more about Environment Canada's RADAR network visit http://www.msc-smc.ec.gc.ca/projects/nrp/index_e.cfm

How RADAR Works

Weather RADAR systems generally use a parabolic antenna much like a searchlight revealing objects caught in its beam. From the transmitter, a narrow beam sweeps the sky as the antenna slowly rotates while pointing at different elevation angles above the horizon. When the energy emitted by the RADAR antenna strikes particles of precipitation, such as drops of water, snowflakes, ice pellets or hail, it is reflected with intensity proportional to the number and size of the particles.

RADAR Description

There are three scales of RADAR image views on this web site.

1. National
2. Regional
3. Local

1. National View

This view provides at a glance, the latest composite picture of where precipitation is occurring in Environment Canada's RADAR network.

The picture is interactive and can be used to define your selection for a specific area. Clicking on a province leads you to a Regional view for that area. From the national composite picture you can also refine your selection to the local view of the RADAR data for a specific geographic region by selecting an area near one of the black dots which represent local radar sites.

2. Regional View

This view gives you a look at large scale 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.

Using the RADAR Images

The RADAR images can be broken down into 3 main sections.

1. Animation - Playback Controls
2. RADAR images
3. RADAR Interpretation

1. Animation - Playback Controls

How you can use the playback controls. The following is an explanation of the common playback controls for the different views of the radar images.

• The first picture loaded in your browser at any level of view is the latest image available. Any playback button which can produce an action will be in blue and clickable, while the buttons which can not produce an action will be in grey and disabled. The functionality of the buttons is determined by the position of the viewed image and the total number of images in the series.

  

The displayed image will be, by default, the latest one available in a series of 7 or 10 images. The time interval between images is either 10 or 20 minutes. To animate the series of images, you first need to click the "Play" button in the playback controls above. The animation of the images will stop after 5 minutes of uninterrupted play. Re-selecting the play button will re-activate the animation. In addition, you can select the images individually.

The Playback Controls are as follows:

 Play: Used to "play" or activate the series of images in the loop.

 Stop: Used to "stop" the series of images in the loop.

 First Image: Used to loop back to the first image in the series.

 Previous: Used to move backward one image at a time.

 Next: Used to move forward one image at a time.

 Last Image: Used to loop ahead to the last image in the series.

 Increase the speed of the animation in the loop of images.

 Decrease the speed of the animation in the loop of images.

 Reset the speed of the animation to its original setting.

2. RADAR images

Description of the three level views

1. National View

2. Regional View

3. Local View

Playback controls and Customize your Map

The Playback Controls are the same on all view levels, see Animation - Playback Controls

Customize Your Map

Selecting Customize Your Map will offer you the option of choosing several Overlays. All Overlays will appear upon selection and can be combined with other overlay layers.

The Short Animation (1 hr) will be a series of 7 frames at 10 minute intervals and the Long Animation (3 hrs) will be a series of 10 frames at 20 minute intervals.

Default selections for Overlays are Cities, Roads and RADAR Circles
Default selection for Animation is Short.

Local RADAR View

Choosing other Radar Sites in the Region
The Navigation Footer lists all the radar sites available in the province of the radar image presented as well as the radar sites in the adjacent provinces. The local view of any radar site can be accessed by clicking on the appropriate location.

3. RADAR Interpretation

What Am I Looking At? (Echoes)

Simply put, you are looking at a picture of precipitation distribution (echoes) and their 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 is reflectivity in dBZ (unit of reflectivity) and the scale on the left is the corresponding precipitation rate of fall. In the wintertime the reflectivity is correlated to snowfall rate in centimeters (cm/hr) and in the summertime the reflectivity is correlated to rainfall rate in millimeters (mm/hr).

A good rule of thumb:

The higher the reflectivity value, the heavier the precipitation rate.

For example the dark purple colour is the heaviest precipitation snow (rate of fall is 20 cm/hr) while the aqua colour is the lightest precipitation (snow rate of fall is 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 the summer months the reflectivity scale is changed to rain rates or mm/hr.

In some cases, the RADAR does not distinguish between real echoes (precipitation) and false echoes. You will need to understand some common interpretation errors to accurately interpret the RADAR images.

What is a PRECIP Radar Product?

Weather RADAR detects and locates precipitation by day or night. Meteorologists are looking to identify and locate objects that we commonly refer to as precipitation. This is what we see in the radar images that you often see on television or on a website.

The type of image you are viewing is indicated at the bottom of the right hand legend. The standard radar product Environment Canada presents on this web site is a "PRECIP" product except for the McGill radar site which is a "CAPPI" product.

PRECIP Image

The PRECIP 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. Beyond this limit, the echoes are displayed from a single elevation angle using CAPPI processing.

Three factors distinguish the PRECIP product.

1. Weather radars can receive false echoes from ground objects such as buildings and towers. The PRECIP product uses Doppler processing to edit out most of these false echoes. This Doppler correction stops at about 110km. Because of the correction, the PRECIP product is generally better.
2. Trees and hills around radars can block some or all of the radar beam, resulting in decreased echoes returned at low angles. This is particularly visible on the PRECIP product in winter when very low angles are used to detect precipitation at close range.
3. The PRECIP radar product can appear to be less sensitive in Doppler mode. A ring of discontinuity may appear at the transition to the non Doppler processing (CAPPI). This is especially evident if weak precipitation is occurring where the Doppler processing ends. Beyond this point, echoes may suddenly appear stronger as the radar's ability to detect them increases.

What is a CAPPI Radar Product ?

The CAPPI (Constant Altitude Plan Position Indicator) radar image shows precipitation at a nearly constant altitude above ground. In the case of the 1.5 km CAPPI image, it displays precipitation which is located approximately 1.5 km above the ground.

In the processing of a CAPPI image, the received radar signal is digitized with respect to the antenna position and then processed. While the CAPPI product is best to see horizontal patterns, it is more likely to be contaminated by non-meteorological echoes from the ground.

How to Estimate Motion of Precipitation in Radar Images?

By comparing a series of RADAR images, forecasters can observe the development of a disturbance, follow its trajectory, determine the intensity of precipitation, issue alerts or warnings, and fine-tune their forecasts. You too can apply a few techniques to make a short term forecast.

How to animate our RADAR images to make a short term forecast (1 to 3 hours)?

1. Select a RADAR site.
2. Click on the "Play" button to activate the controls.
3. Observe the movement of the precipitation echoes on the screen.

Note: The local view radar image is oriented such that true North is located at the top of image.

Now, you need to determine the speed at which the echo is moving. This will allow you to forecast where it's likely to be in 1 or 2 hours. For this, you need to choose 2 images with a one hour interval.

Suggested method:

• Back the pictures one hour from the time of the latest image and note where the precipitation echo is located.
• Now go back to the latest image and measure the distance between the 2 echo positions.
• Compare this to the distance between 2 of the concentric circles on the image.
• Knowing that the distance between 2 circles is 40 kilometres you can estimate the speed at which the precipitation is moving and in what direction.
• Knowing that "North" is located at the top of the image, echoes moving to the right, for example can be properly interpreted as moving eastward.

Example:

Let's say that precipitation echoes are moving from the west towards the RADAR and you want to know when the precipitation will reach the RADAR site (the centre point).

• You have noted that the echoes on the image from an hour ago touch the 4^th concentric circle (160 km from the RADAR).
• The most recent image shows that the echoes have reached the 3^rd concentric circle (120 km from the RADAR).
• This means that the precipitation is moving at 40 km/h towards the RADAR.
• If the speed of the precipitation doesn't change, you can expect that it will reach the RADAR in 3 hours.

You can also identify whether the intensity of precipitation is getting stronger or weaker by watching the color variations of the corresponding echoes. Similarly, you can identify the trend of precipitation (weakening or developing) by noting the expansion or contraction of the total coverage area in time.

Conclusion

You can now make your own short term forecast based on the movement of the RADAR echoes for any point on the RADAR screen.

All you need to do is:

1. measure the distance between the echoes with a ruler,
2. compare this with the concentric circles distance, and
3. determine the speed at which precipitation is approaching.