Weather explained · Rain
What do rain chance and rainfall amount mean?
Read the quantity, the period and the provider before comparing two weather screens. Similar-looking numbers can describe different things.
Three measurements to keep separate
| You see | Read it as | Do not read it as |
|---|---|---|
| A percentage | Likelihood for the provider’s defined event and period | The fraction of the day that will be wet |
| Millimetres | Rainfall amount for the stated interval | A guarantee for your exact address |
| dBZ on radar | Radar reflectivity | Millimetres per hour or accumulated rain |
Why the BOM rainfall range may look different
The Bureau’s daily rainfall explanation describes the displayed range using amounts with a 75% and a 25% chance of being reached. Amounts outside that range remain possible. Those figures are not simply the minimum and maximum readings from a rain gauge.
Atmosphr’s forecasts are model estimates supplied through Open-Meteo. Do not assume an Atmosphr amount uses the Bureau’s range definition. Compare the source labels, units and forecast periods shown in each product.
How the Bureau’s rain forecast is built
The Bureau describes its rainfall forecasts as probabilistic: instead of one number, they give a range of amounts and the chance of receiving at least each amount. When rain is forecast, the website and BOM Weather app show two numbers. The first has a high (75%) chance of being reached and the second a low (25%) chance. The expanded view for the day adds the medium (50%) chance amount.
Its own example: a Port Augusta forecast of 7 to 25 mm means a 75% chance of at least 7 mm and a 25% chance of at least 25 mm. Amounts outside the range are still possible.
The Bureau treats 0.2 mm as the smallest measurable amount, because that is what its rain gauges record. Amounts below 0.2 mm are not displayed, so on some days only the 25% figure appears. When all three chances are below 0.2 mm, the forecast reads “No rain”, sometimes with a high percentage such as 95% to show confidence that no rain will be recorded.
Worked examples
| What you see | A plain reading |
|---|---|
| 70% chance of at least 1 mm | Rain adding up to 1 mm or more is judged more likely than not. If forecasts like this are well calibrated, about 3 days in 10 with this forecast will end up with less than 1 mm. It says nothing about when in the day the rain falls. |
| 7 to 25 mm (Bureau example, Port Augusta) | At least 7 mm is likely (75%); 25 mm or more is possible but less likely (25%). |
| 10 to 110 mm (Bureau example, Byron Bay) | A very wide range signals uncertainty, typical of showers and thunderstorms: some places may get a little and others a lot. |
| Only a 25% chance of 1 mm shown (Bureau example, Boonah) | The 75% and 50% amounts were below 0.2 mm, so light rain is possible but a dry day is more likely. |
| 40% in an hourly column on Atmosphr | Open-Meteo’s estimate of the probability of more than 0.1 mm falling in that hour, from ensemble models at about 27 km resolution. |
Showers versus steady rain
The Bureau explains that ranges are wide on days of showers and thunderstorms, for example 5 to 50 mm, because heavy bursts can hit one suburb and miss the next. They are narrower with widespread rain from layered cloud, for example 5 to 10 mm. Showers and storms are typical of the warmer months over northern and eastern Australia; steadier frontal rain is typical of the cooler months in the south.
The Bureau also notes that its point forecast comes from a 6 km by 6 km grid square, while observed rain usually comes from one automatic weather station that may be some distance away. Other providers use different models and grids, so two apps can reasonably disagree.
Radar dBZ and rain rate, roughly
Radar measures reflectivity, reported in dBZ, not rainfall. A widely used textbook link between the two is the Marshall-Palmer relation, Z = 200R1.6, where R is rain rate in mm/h. Converting with it gives these approximate rates:
| Reflectivity | Approximate rain rate (Marshall-Palmer) |
|---|---|
| 20 dBZ | about 0.6 mm/h |
| 30 dBZ | about 2.7 mm/h |
| 40 dBZ | about 12 mm/h |
| 50 dBZ | about 49 mm/h |
Treat these as a rough guide only. The Bureau notes that radar colours often do not match gauge measurements, because drop size matters more than the amount of rain: tropical maritime rain with many moderate-sized drops reflects much less than rain clouds over land at a similar rainfall rate. The US National Weather Service describes using different relationships for tropical rain, and hail can make rates look far higher than they are. An echo also has to fall to the ground, move and last before it becomes rain at your location.
Why forecasts change as the day approaches
The Bureau explains that several days out, computer models can disagree sharply: one may show a rain band while another shows clear skies. As newer observations arrive and the timeframe shortens, confidence grows and forecasters add local knowledge such as terrain effects and known model biases. Its advice is to check regularly for updates and warnings. For an important plan, note the forecast you relied on, check again the evening before and the morning of, and decide in advance what change would make you switch to a backup.
Will it rain during my plans?
Look at the hourly window that overlaps the activity, then check a recent radar sequence and its timestamps. For a three-hour event, a daily total leaves the timing unanswered. Showers can also miss the forecast point or arrive differently from the model estimate.
Use outdoor planning for events and travel, construction rain planning for work, or the radar guide for colour and playback controls.
Sources and further reading
South East Queensland rain event
Compare the Brisbane rain outlook with observations and reported impacts.