When a summer storm builds west of Lethbridge, most of us pull up the radar on our phones. But not all radar is the same. The radar that watches our weather, the radar on a ship’s mast, and the radar gun a police officer points at your truck all work on the same idea, yet they use very different “colours” of radio wave. Here’s why, and why the radar picture over southern Alberta is better today than it was a few years ago.
How radar works, in one minute
- Send and listen. A radar sends out a short burst of microwave energy and listens for the echo.
- How far: the time the echo takes to come back tells it the distance.
- How much: the strength of the echo shows how heavy the rain, snow or hail is.
- Which way it’s moving: this is the Doppler shift. If the raindrops are moving toward the radar, the echo comes back at a slightly higher frequency; moving away, slightly lower. It’s the same thing you hear when a train horn drops in pitch as it passes. Forecasters use it to spot winds and rotation inside a storm.
The radar bands: long waves and short waves
Radars are grouped into “bands” by their wavelength. The trade-off is simple: long waves punch through heavy rain and hail but need a big, expensive dish, while short waves show fine detail with a small antenna but get soaked up by heavy precipitation.
- S-band (about 10 cm (3.9 in)). The heavyweight. It sees through heavy rain and hail with very little fading, so it can see the storm behind the storm. The catch: large antennas and high power.
- C-band (about 5 cm (2 in)). Cheaper and still very good, but heavy thunderstorms soak up part of its signal, so cells behind a big storm can look weaker than they really are.
- X-band (about 3 cm (1.2 in)). Sharp, detailed pictures from a small antenna, but heavy rain weakens it quickly. Great for short range.
- K-band and shorter (about 1 cm (0.4 in) and less). Too easily absorbed for storm watching, but useful up close: police speed radar uses K-band, and research “cloud radars” use very short waves to see tiny cloud droplets.
Canada switched to S-band, and that matters here
Most of Canada’s older weather radars were C-band. Environment and Climate Change Canada has since rebuilt the whole network with 33 new S-band, dual-polarization radars, all transmitting by August 2023. Each one sees precipitation out to about 330 km (205 mi), with Doppler wind information out to about 240 km (149 mi).
Southern Alberta is covered by two of them: Strathmore, east of Calgary, which switched from C-band to S-band in November 2019, and Schuler, near Medicine Hat, which switched in September 2020. There isn’t a weather radar at Lethbridge itself; we sit between those two. Because the Earth curves away under a radar beam, the farther a storm is from the radar, the higher above the ground the beam is passing. That’s one reason a radar picture can’t tell you everything happening right at ground level here, and why ground stations like ours still matter.
“Dual-polarization” means the radar sends out both horizontal and vertical pulses. Raindrops, hailstones and snowflakes have different shapes, so comparing the two echoes helps forecasters tell hail from heavy rain, and rain from snow, which is a big deal in hail country.
Why big ships carry two radars
Ships face the same trade-off, and the international safety rules for ships (SOLAS) reflect it. Ships of 300 gross tonnage and up must carry an X-band (9 GHz) radar, and ships of 3,000 gross tonnage and up add an S-band (3 GHz) radar.
- X-band at sea gives the sharpest picture: small boats, buoys, harbour entrances and coastlines, with fine detail. It is also the only band that shows a SART, the search-and-rescue transponder carried in lifeboats, which works at 9 GHz.
- S-band at sea keeps working in heavy rain and rough weather, when X-band gets cluttered, and is better for spotting targets at longer range.
So the bridge crew uses X-band for detail in good weather and switches to S-band when the rain comes in. It’s the same logic as Canada’s weather radars: when you need to see through bad weather, longer waves win.
A note for the hams
Amateur radio has microwave bands close to these radar bands: the 13 cm (5.1 in) band sits near S-band, and the 3 cm (10 GHz) band sits in X-band territory. The same physics applies. Rain that weakens X-band radar will also weaken a 10 GHz contact, which is something microwave operators watch for. There’s more on bands and propagation over on VE6DOK’s PropMap.
Watch: radar in action
Sources
- Canada’s New Weather Radars, Environment and Climate Change Canada
- About Canadian weather radar, Environment and Climate Change Canada
- Canadian weather radar network (Strathmore and Schuler details)
- Weather radar frequency bands, Radartutorial
- SOLAS shipborne navigational carriage requirements
- Dual-frequency radar: X-band and S-band at sea, Riviera Maritime Media
- Radar techniques for navigators (SART and X-band), Marine Insight