Many people interpret a UK forecast with a chance of rain and assume that the percentage refers to the percentage of the area that will experience rain. In fact the chance of rain figure is a specific piece of data that is interpreted in a number of different ways, and, when understood, can be very useful.
What The Percentage Actually Measures
It is often misinterpreted in terms of the amount of rain that may fall, how long it will last and how widespread it will be. The figure of 30% for example simply means that given the current atmospheric state there is a 30% chance of rain at that specific point within the stated time period.
Myth One: It Covers 30% Of The Map
A third of the map to see 30% of it rain is a common assumption not covered by the chance of rain percentage at a single point. The percentage does not describe how widespread the rain will be and does cover cases where it’s patchy and mostly misses you off. Conversely it also covers cases where it’s widespread and pours in sheets.
Myth Two: The Chance of Rain is the amount of time for which rain will fall.
This is another very common misunderstanding of the chance of rain percentage – that it refers to the length of time for which it will rain. This is known as the duration of the rain and is often very variable. It is quite common for short, intense showers to occur, lasting for only a few minutes. Conversely, it is also quite common for prolonged periods of rain to occur, lasting for many hours. The chance of rain percentage does not give any indication of the duration of the rain.
Myth Three: It’s Just A Confidence Score
The common perception of the percentage of chance of rain seems to be that the figure is a rough estimate of the meteorologist’s confidence in their own forecast. In other words, a 30% chance of rain might suggest that the meteorologist is not very confident in their own forecast and a 90% chance of rain might suggest that the meteorologist is confident that rain is going to occur. However, this is not really the case as the percentage of chance of rain is calculated in a strict manner and is based upon past observations and the frequency with which similar situations have led to rain.
How The Figure Is Calculated
Using ensemble forecasting models (which run the same weather situation hundreds of times with slight variations in the initial conditions), the same forecast for a given time and place can be generated many times. For any given run of the model, if it does rain, then the amount, time and location can all vary. The model is run hundreds of times for any given situation, and for each run a forecast is generated. The forecast that is issued is a statistical average of all the individual forecasts from the hundreds of runs. This is why the probability of precipitation can change from one update to the next as more data becomes available.
Why Low Percentages Can Still Catch You Out
This forecast is saying that the chance of rain at a given point is small, but it can be heavy and of short duration. Even though the percentage is low, there is still a possibility of a sharp shower and so you would still take an umbrella with you.
Why High Percentages Don’t Guarantee A Soaking
But a reading of 80% or 90% for your location does not necessarily mean that you will get a soaking. The percentage only states the likelihood of rain at that location and time. How long the rain lasts and how intense it is, can vary. The Met Office’s public guidance explains the difference between the likelihood of rain and the intensity and duration of it in more detail should you require it.
When To Take The Umbrella
Your forecast summary.
Sun symbol, 10%: no need for an umbrella, rain is unlikely.
Light shower 40% – Take a light coat or fold up umbrella. It may pour down after a while if you are out for a long time.
Rain cloud symbol, 60-70%: take a proper umbrella, as it is likely to rain.
Heavy rain symbol, 80-90%: it’s going to rain a lot, so try to avoid going out if possible.
Common Mistakes
You need to read the percentage as well as the symbol (eg Sun symbol with percentage shows likelihood of rain).
You would also be mistaken to think 30% equals 30% of the hour (or day). What it actually means is that the chances of any rain falling at all equals 30%.
Mistaking short but intense showers with low percentages for longer periods of less intense rain with higher percentages.
Each hour’s probability is also calculated for the hour before and the hour after it. Hence, between updates of a forecast, the figure for a given hour may have changed slightly as more information becomes available about the hours surrounding that time.
Knowing what your percent really means won’t alter the weather but it can save you from a lot of wasted umbrellas and wet clothes.
