How Weather Radar Works
What radar actually sees, what the colours mean, and why it can miss weather that’s right over Lethbridge.
In Simple Terms
Radar sends pulses of radio energy into the sky and listens for the echo. Rain, snow and hail bounce some of that energy back. From the echo, the radar works out where the precipitation is, roughly how heavy it is, and whether it’s moving toward or away from the radar.
Clouds, temperature, wind at the ground, or how much snow is piling up on your driveway. For those you need surface stations, like the one at West Lethbridge Weather.
Our nearest radars
There is no radar in Lethbridge itself. Southern Alberta is covered by two Environment Canada radars:
- Strathmore (CASSM), east of Calgary, roughly 150 km north-northwest of us.
- Schuler (CASSU), north of Medicine Hat, roughly 200 km to the east-northeast.
Both were replaced in 2019 and 2020 with new S-band, dual-polarization radars that reach out about 240 km. That’s good coverage, but it also means Lethbridge sits a long way from either radar, and that matters (see beam height, below).
Reflectivity: what the colours mean
The usual radar map shows reflectivity: how much energy came back. Bigger or more numerous drops, flakes and hailstones send back more, so brighter colours usually mean heavier precipitation.
Reflectivity doesn’t tell you how bad it feels on the ground, whether it’s rain, snow or freezing rain, how much snow will build up, or how strong the wind is. Wet, melting snowflakes can light up the radar more than heavy dry snow.
Beam height: why distance matters
The radar beam goes out in a straight line, but the Earth curves away underneath it. The farther from the radar, the higher above the ground the beam is.
- By the time the Strathmore beam reaches Lethbridge it is roughly 2 km above the ground, even at its lowest angle.
- Anything below that, like shallow snow, drizzle or blowing snow, can be missed completely.
- What the radar sees at long range is often ice and snow high in the cloud, not what is actually reaching the ground.
This is the main reason radar can look clear over Lethbridge while it’s snowing lightly outside, or look busy while nothing reaches the ground.
Ground clutter and false echoes
Radar energy also bounces off things that aren’t weather: hills, buildings, wind turbines, and even birds and insects. Modern radars filter most of this out, and dual-polarization helps a lot, but some clutter still sneaks through, especially on calm summer evenings. A fuzzy ring or speckle around a radar site on a clear night is usually not rain.
Velocity: seeing motion
Doppler radar also measures whether precipitation is moving toward or away from the radar. That’s called velocity.
- It only shows motion along the beam, not the full wind direction.
- It isn’t the wind at the ground, and it isn’t gust strength.
- Forecasters use it to spot rotation in thunderstorms, wind shear and gust fronts, which is why it matters for tornado and severe storm warnings.
Dual-polarization, simply
The new radars send pulses that are both horizontal and vertical. Comparing the two echoes tells the radar about the shape of what it hit: round raindrops, flat snowflakes, tumbling hail, or birds and debris. That helps forecasters tell rain from snow from hail, and filter out non-weather echoes. It adds confidence, not certainty, especially when rain is changing to snow.
Why radar can miss weather here
- The precipitation is very light or very shallow.
- The beam passes over the top of it (common this far from the radars).
- The mountains block part of the beam to the west.
- Blowing snow and ground blizzards happen right at the surface, far below the beam.
Upslope snow from the east, and light snow in the cold air after a chinook ends, are often shallow. Radar can show little or nothing while roads are getting slick. “Clear radar” doesn’t always mean clear weather.
Why radar can look worse than it is
- Virga: precipitation falling from high cloud that evaporates in our dry air before it reaches the ground. Very common here.
- Strong echoes high up in a storm that never make it down.
- Melting snow (a “bright band”) that shows up stronger than the rain or snow below it.
Reading radar the right way
1. Watch the loop, not one picture. Movement, speed and growth matter more than a single frame.
2. Check the ground. Compare radar with a nearby station: is it actually raining, is the temperature dropping, did the wind shift?
3. Know the limits. Far from the radar, low-level weather can hide under the beam.
4. Use it with the warnings. Radar shows you where; Environment Canada’s watches and warnings tell you how serious.
Radar is great for answering where is it, where is it going, and is it getting stronger? It’s not good at how much snow, are the roads slippery, or how bad will it feel here?
Where to see it
- Environment Canada radar on weather.gc.ca or the WeatherCAN app.
- Live surface readings from the west side at West Lethbridge Weather.
- More apps and tips on our Forecasts, warnings & radar and Weather apps pages.