Buzz
When checking the weather on my iPhone, I often see forecasts showing a 40%, 85%, 90% chance of rain for the day. These numbers inform us about the likelihood of rain during the day but not simply that 40% of the day will be rainy throughout the entire area, or that 40% of the area will experience rain all day long... The reality is quite different.Data Usage
Before delving into analysis, it's crucial to understand which data is utilized and what it's based on to recognize it as a rainstorm, warranting notification. Apple previously relied on third-party data for the weather app on iPhone (now also on iPad, Mac), but currently, they use their own data to forecast the likelihood of rain (Apple Weather). In reality, Apple doesn't manually do this (collecting, analyzing, processing weather data); in 2020, they acquired Dark Sky, a weather app, and third-party data provider. Other weather apps like Carrot allow us to choose data sources for reference, to see weather forecasts, but Apple doesn't, they have only one source of their own.What defines a rainfall?
According to information I found on weather.gov of the United States, a rainfall forecast is formed when there is a precipitation of 0.254mm in a specific area over a certain period of time. Timeframes can be measured over a day, daytime, nighttime, or even hourly. Daytime spans from 6 AM to 6 PM, totaling 12 hours, similarly for nighttime.Understanding Rain Probability
So, how do we calculate that 40% figure and what does it really mean? It doesn't imply that 40% of a particular area (let's say, Hanoi) will experience rain within a certain timeframe. Nor does it mean you'll witness rain 40% of the time during the forecasted day in the area. In reality, if a weather station or forecasting app predicts a 40% chance of rain in Hanoi this afternoon, it indicates a 40% likelihood of rain occurring at any point in Hanoi up until 6 PM.The 40% figure is derived from the following calculation: The meteorological station, the entity providing weather data, estimates that rain will form or move towards the Hanoi area, multiplied by the likelihood of rain covering a certain percentage of the Hanoi area (PoP = C x A). For instance, if Apple Weather predicts an 80% chance of rain forming in the Hanoi area today and anticipates the rain to cover 50% of the Hanoi area, the final number displayed by the weather app will be 40% for the entire day.Forecasting Rainfall Volume?
Having known that 40% of the Hanoi area will experience rain throughout the day, do we discern heavy rain from this? The answer seems to be no. We all understand that rain clouds are unpredictable. On a hot noon, if we see a 30% chance of rain accompanied by nearby lightning, it might be a heavy downpour. However, if only light rain is forming and heading our way, even with an 80% chance of rain, it remains just a light shower.However, data is becoming increasingly accurate and detailed. Alongside the 40% rainfall figure, you may notice additional descriptions like localized, scattered, widespread... to provide clearer insights into the nature of the rainfall.What does it mean for us?
You might have read this far and still wonder what significance it holds and how it differs from the old perception, no worries, I thought the same. In reality, if you see a 20% forecast for the day, chances of rain are minimal, and you can go about your day without much preparation. However, if that number rises to 80% or like today, 100%, then rain is definitely on the horizon. The purpose here is to help everyone understand better how the calculation works and how weather apps operate. Besides tracking rain, Weather apps also provide us with various other information such as wind, UV index, sunset, temperature, storms... which are also very useful.
