As an Amazon Associate, Akermin earns from qualifying purchases. Product links on this page are affiliate links.
A hygrometer measures water vapour already in the air and reports it as a percentage that can never exceed one hundred. A rain gauge measures liquid water that has fallen out of the air and reports it as an accumulated depth that keeps rising for as long as the rain continues. One is an instantaneous state with a ceiling, the other is a running total without one, and no relationship lets you predict either reading from the other.
Air Moisture Is a Ratio, Rainfall Is a Depth
Start with the units, because the units carry the whole distinction. Relative humidity is dimensionless. It is the vapour present divided by the vapour the air could hold at its current temperature, so the answer is a fraction and the display shows it as a percentage. Nothing accumulates. Read it twice an hour apart and the second reading replaces the first.
Rainfall is a length. Millimetres or inches of depth, meaning the layer of water that would stand on a flat impermeable surface if none of it drained, evaporated or soaked away. It accumulates by definition, which is why a rain total always comes attached to a period: a depth with no duration attached is meaningless, while a humidity figure with no duration attached is perfectly sensible.
That difference in kind is why instruments that do both keep them in separate channels rather than combining them. The Tempest weather station states that it monitors temperature, humidity, barometric pressure, wind speed and direction, dew point, solar radiation, UV index, lightning strikes and rainfall from one device, with no moving parts and updates every three seconds. Humidity there is a live percentage, rainfall is a total, and the console presents them differently because they are different sorts of number.
Why the Hygrometer Saturates and the Gauge Keeps Counting
During steady rain, the air near the ground usually sits very close to saturation. A hygrometer reads something in the high nineties and then stops being informative, because it has reached the top of its scale and cannot distinguish light drizzle from a downpour. Both look the same to it. Meanwhile the gauge is separating those two cases cleanly, because one is adding depth slowly and the other quickly.
The reverse case is just as instructive. A dry sunny afternoon can produce a rising humidity reading as the air cools toward evening, with the rain total unchanged at zero for days. Humidity varies on a daily cycle driven mostly by temperature, and that cycle happens whether or not any weather is arriving.
Practically, this means the hygrometer is the wrong instrument for answering how wet the weather has been, and the gauge is the wrong instrument for answering whether anything indoors is at risk. For the indoor question, a dew point capable display helps more than a bare percentage, because dew point is an absolute statement of vapour content rather than a ratio. The ALLOSUN digital thermometer hygrometer states that it displays temperature, relative humidity and dew point together, with a settable humidity alarm and a sensor probe about 1.64 feet long that can be extended away from the meter body.
Collector Area, Resolution and the Tipping Bucket
The classic gauge is a funnel of known area feeding a small seesaw with two chambers. Water fills one side, the weight tips it, a magnet passes a reed switch, the chamber empties and the other side starts filling. Each tip is a fixed volume, and a fixed volume over a fixed funnel area is a fixed depth, so the electronics only have to count.
The arithmetic is worth doing once, because it explains why a funnel is large and a bucket is small.
| Quantity | Working | Result |
|---|---|---|
| Depth to volume on open ground | 1 millimetre over 1 square metre | 1 litre, which is 1 kilogram of water |
| Volume per tip on a 200 square centimetre funnel | 0.02 centimetres depth multiplied by 200 square centimetres | 4 millilitres per tip |
| Tips in a heavy hour | 20 millimetres divided by 0.2 millimetres per tip | 100 tips, so intensity is visible from tip timing |
| Load on a flat roof | 10 millimetres over 50 square metres | 500 litres, about half a tonne of water |
| Catch error from a 2 degree tilt | Effective aperture shrinks with the cosine of the tilt | Small for small angles, but it never reads high, only low |
The last row is the one that costs people accuracy. A gauge that is not level under reads, and so does one sited where turbulence around a wall or a hedge carries drops past the mouth. Wind induced under catch is the dominant error in amateur rainfall records, which is why siting instructions insist on open ground and a mount clear of obstructions by several times their height. Our roundup notes in rain gauges cover what the different mechanisms publish.
Indoor Damp Rarely Tracks the Rainfall Total
People reach for both instruments when a wall goes dark after a storm, expecting the rainfall total to confirm the diagnosis. It usually will not, because indoor humidity responds to occupancy, heating and ventilation far more strongly than to the weather outside.
Cooking, washing and breathing add several litres of water a day to a household. Heating raises the air temperature, which drops relative humidity without removing any of that water. Opening a window in cold weather almost always dries a room, because cold outdoor air at a high percentage holds very little vapour in absolute terms and warms up to a low percentage indoors. None of those mechanisms care what the gauge recorded.
What does correlate with rainfall is wind driven rain on a specific elevation, and that is a function of direction and intensity rather than depth. A modest total arriving horizontally on an exposed gable can soak a wall that a larger total falling vertically leaves untouched. So the honest pairing is a rainfall record with a wind direction record for the exposure question, and a continuous indoor humidity record for the interior question. The Govee H5179 states a stable WiFi and Bluetooth connection with 20 days of online data and app alerts, which is enough to show whether the indoor trace moved at all on the night in question.
Running Both Without Mixing Up the Numbers
Keep the two records side by side and label them properly. The gauge total belongs to a period and to a location outdoors. The humidity trace belongs to a room and to a moment, and it is the shape of the trace over a day that carries the information rather than any single value.
Before trusting the indoor half, check it. The saturated salt routine in how to calibrate a hygrometer is the practical method, and the JEDEW hygrometer and allsun thermometer hygrometer notes set out what the basic displays publish. Note that none of the consumer sensors here publish a long term drift figure, so annual drift is Not stated in the listing bullets and a periodic check is the only way to know. With a checked sensor indoors and a level gauge outdoors, you can finally answer the two questions separately, which is the only way either answer is worth anything.
Frequently Asked Questions
Why does rainfall get reported in millimetres rather than litres?
Because depth is independent of the size of the collector, while volume is not. A given rain event deposits the same depth everywhere in its footprint, so quoting depth lets a small garden gauge and a large reference gauge be compared directly. One millimetre of rain is one litre spread over a square metre, which is the conversion that turns the depth back into a volume once you know the area you care about.
Does high humidity mean it is about to rain?
Only weakly, and not reliably at ground level. Rain forms when air aloft is lifted and cooled past saturation, which is a process happening well above your garden. Surface relative humidity rises most evenings simply because the air cools, with no rain following. A falling barometric pressure trend tells you far more about what is coming than the humidity percentage does.
What resolution should a rain gauge have?
Tipping bucket designs commonly resolve about 0.2 millimetres or 0.01 inches per tip, and the resolution is set by the bucket volume against the funnel area rather than by electronics. Finer resolution means more tips per event and a better picture of intensity, but it also means a bucket that is easier to upset by wind, by a bird or by a small level error, so a stable mount matters as much as the published figure.
Do rain gauges without moving parts work differently?
Yes. Instead of counting tips, they sense the impact of individual drops on a plate and infer rainfall from the pattern and energy of those impacts. The advantage is nothing to clog or wear out. The trade is that the inference depends on drop size distribution and the firmware behind it, so very light drizzle and very heavy downpours are the conditions where the two technologies most often disagree.
Can a rain gauge reading explain damp inside a wall?
It can establish exposure, not cause. Knowing that a large depth fell in a short period tells you whether a gutter, a parapet or a sill was being asked to handle an unusual load. Whether water got into the fabric is a question for a moisture meter on the wall and a hygrometer in the room, because driving rain depends on wind direction and detailing rather than on depth alone.