The 5 cm radars used in Canada are best at seeing targets in the mm/cm range (and larger). Micron-sized smoke is almost invisible. Targets need to be reflective (K²) at radar wavelength. In addition, material from the fire needs to intersect the radar beam and needs to fill a large volume. 

Range is a big factor. Signals are weak to moderate and fall below minimum detectable signal at long range. Standard displays may hide weaker echoes. (To better correspond to public's conception of "real" rain.) In addition, the radar beam rises with range: measurable particles will not intersect the beam at long range. However, because fire plumes are relatively shallow and the Earth curves, it is really rare to see fire plumes beyond 80km, and reliable coverage probably only extends to 50km.

 

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Types of Fires

• Forest fires: lots of quickly consumed fuel per square meter = lots of convection. Lots of junk that can be wafted aloft. Large area.
• Building fires: Relatively contained: not so much convection. Less loose stuff to carry aloft - finer smoke. (Often occur at night: more atmospheric stability?)
• Grass fires: intermediate amount of loose material (on ground) intermediate fuel and heat.

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Acknowledgments:  Norman Donaldson, King City Radar