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Fact sheets

Weather Radar

Common Interpretation Errors

A picture is worth a thousand words but sometimes what you see isn't necessarily what you get. Canadian weather radar is now easily accessible at Environment Canada's Web site weatheroffice.ec.gc.ca. Still, 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 someplace isn't getting significant precipitation. 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.

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.
• Storms in the circled area were quite intense but were not being detected appropriately by the radar. Strong storms occurring closer to the radar keep the radar energy from reaching further.

Overshooting Beam

• Intense precipitation such as lake effect snowsqualls 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 image at right 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 aloft 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 picture to the right.

Anomalous Propagation - AP

• Strong temperature inversions low in the atmosphere can have strange effects on radar. When a layer of warm air lays over a layer of much cooler air, 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.
• There was no precipitation occurring in the image at the right.

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.
• Learn the common ground clutter "signature" in your area so you can distinguish it from real precipitation.

(A) Usual Ground Clutter Pattern  (B) Light Precipitation with Ground Clutter Pattern

For Further Information:

Contact MSC National Inquiry Response Team

This fact sheet is part of a series on air issues.
Other fact sheets in this series include:
Smog, Acid Rain, Air Toxics (Fall 1999)

Cette publication est aussi disponible en français.
Minister of Public Works and Government Services Canada, 1999.

The Green Lane^TM,
Environment Canada's World Wide Web Site.