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NEXRAD Overlay

The NEXRAD overlay paints precipitation cells onto the radar scope as colored squares — light green for sprinkles, magenta for severe. The data comes from the same FIS-B uplink stream as the textual weather products (METAR / TAF / etc.), specifically Block 63 (CONUS) and Block 64 (Regional).

Turning it on

The radar scope has a ☂ WX button across the top. Tap to toggle precip on/off. The button shows a count when grids are loaded:

  • ◯ WX — overlay disabled
  • ☂ WX — overlay enabled, no grids yet (waiting for first uplink)
  • ☂ WX 2 · <1m — 2 grids loaded, freshest is < 1 minute old
  • ☂ WX 2 · 12m — grids loaded but oldest is 12 min old (turning amber)

A status badge appears above the scope when special states occur:

Badge Meaning
☂ Waiting for FIS-B NEXRAD… Toggle is on, no uplinks received yet
☂ WX hidden — zoom out to 3+ NM At very tight ranges (<3 NM), one cell would dominate the screen; overlay auto-hides
⚠ NEXRAD stale (12m) All grids are older than 10 minutes — interpret with caution

What the colors mean

The palette is the standard NWS NEXRAD scale:

Color Intensity dBZ approx Real-world translation
(transparent) 0 < 5 No echo
Light green 1 5–20 Light precipitation
Green 2 20–30 Moderate
Yellow 3 30–40 Heavy
Orange 4 40–45 Very heavy
Red 5 45–50 Intense
Dark red 6 50–55 Extreme
Magenta 7 > 55 Severe (hail / thunderstorms)

A small color-key strip appears at the bottom of the radar when the overlay is on (Light → Yellow → Red → Magenta with a "severe" label).

Resolution

Real-world FIS-B has two resolutions:

  • Block 63 (CONUS) — coarse, ~5 NM per cell, covers the whole continental US
  • Block 64 (Regional) — fine, ~1.5 NM per cell, covers an area near you (typically ~250 NM radius)

Both are decoded automatically; Regional overlays Conus where they coincide.

The cells get bigger on screen as you zoom in because each cell represents a fixed geographic area. At a 50 NM scope on Regional data, a cell is roughly 6 px square; at 20 NM the same cell is ~15 px. Zoom out for situational awareness; zoom in only when you need to see traffic detail.

Stale data

A grid you received 5 minutes ago is still considered fresh and renders at full opacity. After 10 minutes it starts fading; by 20 minutes it's removed. The reason: storms move at 20–60 kts; old cells don't show where the storm is, they show where it was. Always re-confirm before maneuvering near weather.

Auto-hide at tight zooms

Below 3 NM scope range the overlay is auto-hidden and replaced with the "☂ WX hidden — zoom out to 3+ NM" badge. The reason: at 3 NM range a 1.5 NM NEXRAD cell would cover ~50% of the scope diameter. The result would be a giant colored blob that hides traffic — exactly when you need traffic visibility most. Zoom out and the overlay reappears.

Doesn't replace official weather products

Same disclaimer as FIS-B textual weather: NEXRAD via ADS-B is advisory only, not certified for operational use. Real radar imagery is already 5–10 minutes old by the time you receive it, and storm tops aren't included. Use it for situational awareness; route around weather using charts, controllers, and your eyeballs.