12 Reflectivity
Reflectivity is the product most people picture when they think “radar”: the green-to-red blobs of precipitation. It measures how much transmitted energy was scattered back to the radar, which depends on the number and especially the size of the targets in the beam.
12.1 Units: dBZ
Reflectivity is reported in dBZ (decibels of reflectivity factor Z). It’s a logarithmic scale, so each step represents a large change in returned power. Rough guide:
| dBZ range | Typical meaning |
|---|---|
| 5–20 | Light drizzle, mist, very light snow, or “clear-air” returns (insects, dust) |
| 20–35 | Light to moderate rain |
| 35–45 | Moderate to heavy rain |
| 45–55 | Heavy rain; possible small hail |
| 55–65 | Very heavy rain / hail likely |
| 65+ | Large hail (and/or beam hitting debris or non-meteorological targets) |
Because returned power scales with the sixth power of droplet diameter, a few large drops or hailstones light up the radar far more than a swarm of tiny ones. High dBZ means “big targets,” not necessarily “lots of water.”
12.2 How to read it
- Find the cores. The reddest/most intense pixels mark the heaviest precipitation and updraft regions. Tight, high-dBZ cores often indicate strong storms.
- Watch the gradients. A tight reflectivity gradient, colors jumping from green to red over a short distance, frequently marks the leading edge of a strong updraft or a developing supercell.
- Look at the shape. Lines of cells suggest squall lines; isolated, persistent cells with sculpted shapes suggest supercells; broad, uniform returns suggest stratiform (steady) rain or snow.
12.3 Hook echoes and other shapes
Classic severe-storm shapes show up first in reflectivity:
- Hook echo: a curl of reflectivity on the rear-right flank of a supercell, wrapping around the mesocyclone. A textbook tornado precursor, but confirm it with velocity (see Storm Signatures).
- Bow echo: a line of storms bulging outward, associated with damaging straight-line winds.
- Bounded weak echo region (BWER): a notch of low reflectivity caused by an updraft so strong it lofts precipitation before it can fall; visible on higher tilts.
12.4 Limitations to keep in mind
- Reflectivity alone cannot tell you rotation, wind, or whether a tornado is on the ground. You need velocity for that.
- High dBZ doesn’t always mean weather. Birds, insects, chaff, wind-farm clutter, and ground returns can all produce returns. Dual-pol products (Dual-Polarization) are how you tell precipitation from junk.
- At long range the beam is high and wide, so it underestimates and smears shallow or distant precipitation.
Reflectivity tells you where and how heavy. To learn what the air is doing, move on to Velocity.