What does it mean when it says there's a 15% chance of rain?
If a meteorologist said that there was a 15% chance of rain, and it rained, then there would probably be a lot of angry people. But 15% is not that small of a percentage—it should rain 3 out of every 20 such days.
The rain percentage on your weather app refers to the chance you will experience a measurable amount of precipitation (0.01”) in the given period. Meteorologists determine the Probability of Precipitation by multiplying their forecasting confidence in rain occurring with the expected areal coverage of rain.
The Abstract: What does it mean when you say there's a 10% (or it could be 20 or 70 — we're using 10 as an example) chance of precipitation? Lackmann: It means that at any given fixed location within the forecast area, there is a 10% chance of receiving 1/100th of an inch or more of precipitation.
The "Probability of Precipitation" (PoP) simply describes the probability that the forecast grid/point in question will receive at least 0.01" of rain. So, in this example, there is a 40 percent probability for at least 0.01" of rain at the specific forecast point of interest!
If the chance of rain is less than 50%, there's a greater likelihood of it staying dry than not. For example, a 20% chance of precipitation means there's an 80% chance of dry weather. On the other hand, an 80% chance of precipitation means there's only a 20% chance of dry weather.
And what does this mean? The "PoP" represents the confidence - "C" - that a meteorologist has that rain will occur somewhere in the area for which the forecast is being prepared, and "A" represents the percentage of the area where a forecast is expected to receive a measurable amount of rain.
If there's a 10 percent chance of rain, it means the current conditions yield rainfall one out of every 10 times observed. If it's 20 percent, then you'd see rain two out of every 10 times, and so on.
A forecast of a 20 percent chance of rain means the forecaster believes there is a one in five chance (20 percent) that measurable rain will occur in the time period covered by the forecast.
That's because weather forecasts give their “percentage chance” in probabilities. The answer? An “80% chance of rain” means that there is an 80% chance that rain will fall somewhere within the forecasted area. Rain refers to 0.01 inch or more.
When we give a percent chance of rain, we as meteorologists know that it will rain at some point that day. The percent is the amount of area coverage or people that will see rain for sure.
How do you calculate chances of rain?
Confidence multiplied by the percentage of the area forecasted equals the "percentage of precipitation." So if there's a 100% confidence that 30% of the area will see rain, then it's a 30% chance [(1 x 0.3)100 = PoPs].
A 100% chance of rain means that meteorologists are very confident that precipitation will occur; the odds of it not raining are extremely low.
When a meteorologist says that there is a 20 percent chance of rain, that is not a ''cover your rear'' percentage. It usually means that the atmosphere is generally stable but there's just enough of a particular ingredient (i.e. moisture, heat, lift) to squeeze out a shower of a very limited area.
However, if the forecaster has only a 60 percent confidence that it will rain, but is sure that if it does rain the whole region will receive that rain, then what you read in the forecast is the identical “60 percent chance of rain.”
This term is usually used when the forecast indicates the possibility of an isolated thunderstorm covering only a small portion of the forecast area or the expected occurrence of very light precipitation (snow flurries, for example).
Using a 40% probability of rain as an example, it does not mean (1) that 40% of the area will be covered by precipitation at given time in the given forecast area or (2) that you will be seeing precipitation 40% of the time in the given forecast area for the given forecast time period.
The same as it means on any other weather forecast; the probability that the rain or other precipitation will occur.
Fifty percent of the area will get rain.
You're probably used to seeing forecasts that claim a "30% chance of rain" or a "70% chance of snow." But what do these probabilities actually mean? The basic definition is the likelihood of measurable precipitation (0.01 inches or 0.254 mm) occurring at a location during a specific timeframe.
This definition means that at any given point in the forecast area, there is a PoP percent chance of measurable precipitation during the time period of the forecast.
How accurate is chance of rain?
An Easy Rule of Thumb
The good news is that the average person doesn't need to get out a pencil and paper every time they read the forecast. According to the National Weather Service, if you see a 40 percent chance of rain, "there is a 40 percent chance that rain will occur at any given point in the area."
40%-50% - SCATTERED - Roughly half of the area will encounter a shower or storm. 60%-70% - NUMEROUS - Much of the area is covered so it's likely you will get wet.
According to the National Weather Service (NWS), it actually means that, in 5 out of 10 cases, there will be a measurable amount of precipitation somewhere in the zone over 12 hours. Remember, probabilities are always given for a point in space over a standard period.
IT DOES NOT MEAN...
A 50 percent chance of rain means there is a 50 percent chance for any one spot in the forecast area to get wet during the forecast period. So what's the forecast area and forecast period?
Chance of doesn't tell you how much rain will occur. Putting it all together — a 40% chance of rain suggests that you more likely than not can have your happy hour outdoors. But it may just sprinkle on you. Or you might get soaked.
But excessive rainfall — especially at fast rates — can do more harm than good. When rain falls too quickly for soil to absorb, the ground can't soak it all up. Instead, stormwater runoff collects and flows through yards and roadways, increasing the risk of floods and soil erosion.
Bottom line... the correct way to interpret the forecast is as follows: if there is a 50 percent chance of rain in the forecast, then we are calling for half of our viewing area to see some kind of wet weather at any given point throughout the day/night we are forecasting for.
Instead, it means a certain percentage of the forecasted area will definitely see rain—so if you see a 40% chance, it means 40% of the forecasted area will see rainfall.
Light rain gives up to 2–4 mm (0.07–0.15 in) of precipitation; Moderate rain gives 5–6 mm (0.19–0.23 in);
Cold spots create condensation and we can get a shower. It is not unusual this can become a “stray thunderstorm”. It is sudden and short lived and freaky as it can seem to happen out of a clear sunny sky. Related questions (More answers below) Why do we usually get feared during thunderstorms?
What does 100 chance of snow mean?
A scattered chance, 40-50%, will impact several towns while some still stay dry. Numerous rain or snow showers, 60-70%, will impact most towns in the viewing area. A widespread chance, 80-100%, would likely be a line of thunderstorms or a snowstorm impacting the entire viewing area.
Probabilities of precipitation enable forecasters to convey certainty (or uncertainty) of the likelihood of precipitation. A 40 percent chance of snow means the forecaster believes you will experience snow 4 out of every 10 times you hear such a forecast (assuming the forecasts verify perfectly).
When we give a percent chance of rain, we as meteorologists know that it will rain at some point that day. The percent is the amount of area coverage or people that will see rain for sure.
That's because weather forecasts give their “percentage chance” in probabilities. The answer? An “80% chance of rain” means that there is an 80% chance that rain will fall somewhere within the forecasted area. Rain refers to 0.01 inch or more.
Rain is classified as light, meaning rain falling at a rate between a trace and 0.10 inch per hour; moderate, 0.11 to 0.30 inch per hour; heavy, more than 0.30 inch per hour.
A seven-day forecast can accurately predict the weather about 80 percent of the time and a five-day forecast can accurately predict the weather approximately 90 percent of the time.
A 50 percent chance of rain means there is a 50 percent chance for any one spot in the forecast area to get wet during the forecast period.
40%-50% - SCATTERED - Roughly half of the area will encounter a shower or storm. 60%-70% - NUMEROUS - Much of the area is covered so it's likely you will get wet. 80%-100% - WIDESPREAD - The entire area is covered with showers and storms so everyone gets rain!
Drizzle is a term used for a very light, fine rain. The drops tend to be very small and not very dense. Mist is similar to drizzle, only the number of drops in any given area is greater.
Chance of doesn't tell you how much rain will occur. Putting it all together — a 40% chance of rain suggests that you more likely than not can have your happy hour outdoors. But it may just sprinkle on you. Or you might get soaked.
How bad is 60% chance of rain?
If they say 60% chance of rain, 60% of their area will experience rain at some point during the forecast period. The forecaster will often mention which interpretation they adhere to. Keep in mind, PoP only refers to the chance of precipitation, NOT how much rain you will get.
Rainfall rate is generally described as light, moderate or heavy. Light rainfall is considered less than 0.10 inches of rain per hour. Moderate rainfall measures 0.10 to 0.30 inches of rain per hour. Heavy rainfall is more than 0.30 inches of rain per hour.
3/4 (0.75) of an inch of rain – A light moderate rain never reaches this amount, heavy rain lasting for 2-4 hours. There would be deep standing water for long periods of time. One (1.00) inch of rain – A light moderate rain never reaches this amount, heavy rain for several hours (2-5 hours).
One inch of water should penetrate the soil somewhere between 6 and 15 inches deep. But your soil type can dramatically affect this. Clay soils, which are denser, won't be as deeply penetrated by a 1-inch rain event as loamy and sandy soils.
