Sensor calibration

Every station drifts. A barometer reads 2 hPa low, a thermometer catches an hour of afternoon sun, a tipping bucket under-catches in heavy rain. Pro Weather already discards impossible readings, but a reading that is merely wrong looks plausible and gets stored like any other.

Calibration is where you correct that. It lives under your site → Calibration in the dashboard, and it is set per station.

How a correction works

Each reading is adjusted as:

corrected = raw x multiplier + offset

• Offset shifts every reading by a fixed amount. Use it for sensors that read consistently high or low: temperature, humidity, pressure, wind direction.
• Multiplier scales every reading. Use it for sensors that mis-scale rather than mis-read: a rain gauge catching 4% too little needs a multiplier of 1.04, not an offset.

Leave a field blank to change nothing. Values are entered in your site's unit system, so an imperial site types offsets in °F and inHg.

What you can correct

Derived values are recalculated from the corrected readings, so nothing contradicts itself: correct the air temperature and your dew point, heat index, wind chill, humidex, apparent temperature and cloud base all follow.

Finding the right numbers

• Temperature: compare against a reference thermometer in a properly shielded, ventilated spot, over a full day rather than one moment.
• Pressure: compare against a nearby official station or an airport METAR at the same minute. Pressure is the most commonly mis-set sensor and the easiest to verify.
• Humidity: ageing capacitive sensors usually read low. A salt test (a saturated salt solution in a sealed jar reads 75% at room temperature) gives you a reference point.
• Rainfall: pour a measured volume through the funnel slowly and compare the total. Set the multiplier to the ratio of what you poured over what was recorded.

Start small. A large correction usually means something else is wrong, such as a sensor in direct sun or a blocked rain funnel, and no amount of arithmetic fixes a badly sited sensor.

Spike limits

A spike limit is the largest change you accept between two consecutive readings. Radio interference, a dying battery or a loose connector produce single readings that jump far beyond anything the weather can do. A reading that fails its limit is dropped rather than corrected, exactly like an impossible value: it is treated as if the station never reported it.

• Temperature, humidity, pressure, wind and gust limits cap the change between readings.
• The rain limits are absolute caps on the rate and on the amount recorded in a single reading, because a phantom bucket tip has no previous value to be compared against.

Two readings more than an hour apart are never compared, so a gateway coming back online after a quiet night is not mistaken for a spike.

Keep limits generous. A real cold front can move the temperature several degrees in minutes, and a limit that is too tight quietly deletes real weather. If the page shows that readings are being dropped on calm days, raise the limit. The "a reading was rejected as a spike" event alert tells you when a limit fires.

What calibration does and does not change

• Corrections apply the moment data arrives, before anything is shown or stored. Your public site, your charts, your records and your CSV exports therefore always agree with each other.
• Older history is not rewritten. A correction changes new readings, plus roughly the last hour, which your station's archive still hands back on every refresh and which is re-saved each time. Everything before that keeps the values it was stored with, so your charts show a small step at the moment you saved the correction. This is deliberate: months of stored observations are not rewritten behind your back. To fix what is already stored, use the data editor.
• Imported history is not corrected. Data brought in from weewx or CumulusMX was already calibrated by that software.
• Calibration is per station, so a site with more than one station calibrates each separately.