Rain gauge problems almost always fall into one of four categories: nothing is recording (stuck), too much is recording (overcounting), too little is recording (undercounting), or the gauge works but produces unreliable totals. Each has a specific diagnostic path.
Stuck at Zero — No Rain Recording
Step 1: Check the funnel screen. Remove the funnel assembly by unscrewing the top cap (on most consumer stations, this is a quarter-turn or straight pull). Look at the mesh screen. Even a thin film of pollen or a single spider web can create enough surface tension to block water flow entirely. Rinse under running water, shake out, replace. Test by pouring a small amount of water in — you should hear or see tips occur.
Step 2: Check the tipping mechanism. With the funnel removed, look at the pivoting tipping bucket. Debris, mineral deposits from hard water, or a stuck pivot can prevent tipping. Gently tilt the gauge to cause a tip manually — it should swing freely with minimal resistance. If it feels stiff or won't tip, clean the pivot point with a soft brush and distilled water. Never use oils or WD-40 on the tipping mechanism — it attracts debris and accelerates fouling.
Step 3: Check the reed switch (wired sensors). On wired rain gauges, the tip is detected by a small reed switch that closes when a magnet on the tipping bucket passes it. If the reed switch has failed or the magnet has shifted position, tips occur physically but aren't transmitted. You can test a reed switch continuity with a basic multimeter — set to continuity/resistance mode and probe the two wires from the rain gauge while manually tipping the bucket. You should get a brief resistance drop (or continuity beep) at each tip. No change = failed reed switch.
Step 4: Wireless sensor pairing (Ecowitt/Ambient). Verify the rain sensor is still showing in WSView Plus or your station app. Some sensor batteries (typically AA or AAA in the outdoor sensor unit) cause the sensor to stop communicating before the battery indicator triggers an alert. Replace batteries if the sensor hasn't been swapped in over a year.
Overcounting — Too Much Rain Recorded
Splash from hard surfaces: The most common cause. Raindrops hitting concrete, decking, or gravel bounce splash droplets into the gauge funnel, adding to the total. The fix is relocating the gauge over grass or soil, at least 5 feet from any hard surface. See our rain gauge placement guide for the full rules. Even 6 inches of clearance from pavement makes a measurable difference.
Calibration drift — bucket volume too small: Over time, mineral deposits from hard water can build up inside the tipping bucket, reducing its effective volume. The bucket tips more frequently for the same amount of water, causing overcounting. Periodically rinse the bucket mechanism with distilled water to remove mineral buildup. If overcounting persists, apply a negative calibration offset in your app (e.g., -8% if you're consistently 8% over CoCoRaHS neighbors).
Funnel that isn't level: A tilted funnel directs water preferentially to one side of the tipping bucket, causing it to fill faster and tip more frequently. Verify funnel level with a bubble level in two axes.
Undercounting — Too Little Rain Recorded
The dominant cause of undercounting is wind exposure — see our detailed placement guide. Wind deflects rain away from the gauge opening, especially during high-wind events. Gauges in wind-exposed locations on tall poles or rooftops consistently catch 10–25% less than sheltered gauges during moderate to strong wind events.
Secondary causes: partial funnel clog (water enters but drains slowly, causing some to overflow during heavy rates), calibration drift toward larger bucket volume, or gauge positioned under an overhang where roof drip adds water during some angles but direct rain is partially blocked.
Spurious Tips — Rain Recorded When It's Dry
Tips recorded without any rain — particularly common at night — can be caused by: (1) Dew accumulation in the funnel that drains when ambient temperature drops enough to trigger a brief run of water; (2) Insects, particularly small ants or gnats, that have nested in the funnel and occasionally displace the tipping mechanism; (3) Vibration from wind that rocks a loosely mounted gauge, causing the bucket to tip; (4) In freezing weather, ice forming and then melt water running suddenly. Each is identifiable by the pattern: dew tips happen at consistent nighttime hours; insect tips produce erratic single counts scattered through low-wind periods; vibration tips correlate with wind speed spikes; freeze-melt tips come in rapid bursts following warming events.
Frequently Asked Questions
Inspect the funnel screen every 6 weeks during pollen season (spring and early fall) and after any major wind event that deposits debris. In areas with heavy tree cover, monthly cleaning may be needed through growing season. Inspect the tipping mechanism annually and clean if mineral deposits are visible. Full disassembly and cleaning is a once-per-year task for most users. Stations in areas with heavy airborne pollen, agricultural dust, or near construction may need more frequent cleaning.
At a consistent 15% offset across multiple rain events in varying wind conditions, placement is the more likely cause. Try the known-volume test described above — if the mechanism calibrates correctly at 23–25 tips per cup, the systematic undercatch is happening before water reaches the bucket, which means either funnel partial clog or wind exposure. Compare your readings event by event: if the offset is larger during high-wind events and smaller during calm events, wind exposure is confirmed. If the offset is constant regardless of wind, it's more likely a calibration or funnel issue.