Is 20% chance of rain bad?
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.
If we're just expecting a small storm or two, we'd say 20% of the area will see rain. On the other hand, if we're expecting more widespread rain, the area that will see rain would be more like 70% or 80%.
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 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.
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).
If there are 100 days in which the forecast for rain is 10%, then it should rain on 10 of those days and not rain on the other 90. So if you hear a forecast of 10% chance of rain, and it rains, it doesn't mean that the forecast is wrong, it just happens to be one of those rainy days.
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.
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?
A 100% chance of rain means it will certainly rain in a particular area during a particular time, but it may or may not include your forecast zone or area.
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.
Is 20% of rain a lot of rain?
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.
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!
"I just found out that on the weather app, when it says 30 per cent rain, I thought it meant 30 per cent chance of rain," Moffatt says. "It doesn't mean that. It means 30 per cent of your area will be covered in rain."
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.”
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.
We often glance at the percentage of rainfall for our location, but what does this number actually mean? Some people think that if you see 60% next to expected precipitation in the forecast then you have a 60% chance of rainfall. This is actually not the case. Nor is it the case that rain will fall in 60% of your area.
One inch of rain means that the rain that fell in a single geographic location would sit one-inch deep on top of the ground, if it were not absorbed into the earth or if it did not run downhill.
An 80% chance of rain means that 80% of the time the same combination of conditions occurred it rained. It does not mean that it will rain all day, just that rain will likely occur 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!
Fifty percent of the area will get rain.
Is half an inch of rain a lot?
1/2 (0.5) of an inch of rain – A light rain never reaches this amount, moderate rain for 1-2 hours or heavy rain for 30-45 minutes. There would be deep standing water and they would last for long periods of time.
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."
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.
Heavy rain — when the precipitation rate is > 7.6 mm (0.30 in) per hour, or between 10 mm (0.39 in) and 50 mm (2.0 in) per hour. Violent rain — when the precipitation rate is > 50 mm (2.0 in) per hour.
Very heavy rain: Greater than 8 mm per hour. Slight shower: Less than 2 mm per hour. Moderate shower: Greater than 2 mm, but less than 10 mm per hour. Heavy shower: Greater than 10 mm per hour, but less than 50 mm per hour.
A 0% chance indicates there are not adequate mechanisms in order to generate precipitation. The troposphere is too stable to generate precipitation. A 100% chance guarantees precipitation will occur. A 100% chance will be issued when it is already precipitating or mechanisms are in place to guarantee precipitation.
It means it will rain in Houston, but not at your exact location. iOS uses precise location to show you the best accurate report, so, it will still rain, just not where you are. If you go a few miles from your location, you might see a different %.
The term "100-year flood" is used in an attempt to simplify the definition of a flood that statistically has a 1-percent chance of occurring in any given year. Likewise, the term "100-year storm" is used to define a rainfall event that statistically has this same 1-percent chance of occurring.
In general, 2 inches of rain in a 24-hour period is considered a significant amount of rainfall, and can cause flooding or flash flooding in some areas. However, some places may receive 2 inches of rain in a day frequently, and it may not be considered a lot.
Keep in mind, PoP only refers to the chance of precipitation, NOT how much rain you will get. A 100% chance of rain could mean buckets of rain all day or just a light sprinkle in the afternoon. All you can tell is you are likely to see some sort of precipitation.
What does 13 percent rain mean?
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.
An 80 percent chance of rain (or of any other kind of precipitation) means the weather forecaster believes there will be an eight in ten chance (or 80 chances out of 100) of measurable precipitation (0.01 inch or more) in the area under consideration during the time interval that is specified in the weather forecast ( ...
The same as it means on any other weather forecast; the probability that the rain or other precipitation will occur.
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].
An Easy Rule of Thumb
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."
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.”
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.
A 100% chance of rain means that meteorologists are very confident that precipitation will occur; the odds of it not raining are extremely low.
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.
At no time does a percentage given in a forecast tell you how long it will rain, how much rain is expected, what time the rain will arrive or what the potential impacts are to you.
How bad is 30 percent rain?
A 30% chance of rain also means a 70% chance it won't rain. Which odds seem better to you? A '30% chance of rain' means every point in the forecast area has an 30% chance of observing measurable precipitation (at least 0.01") during the 12 hour forecast period. It also means rain expected over 30% of the forecast area.
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.
What does a 50% chance of rain actually mean? “C” is the percentage confidence that rain will form across the area, and “A” is the percentage area that is expected to get rainfall. PoP is expressed in a 12-hour window (6 a.m. to 6 p.m. and 6 p.m. to 6 a.m.).
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].
