How do they calculate chance of rain?
Meteorologists calculate the Probability of Precipitation (PoP) using a simple formula: PoP = C x A. Where “C” stands for the confidence a forecaster has that rain will occur somewhere in the forecast area, and “A” stands for the percentage of the area where they expect a measurable amount of rain to occur.
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.
If a forecast for a given county says that there is a 40% chance of rain this afternoon, then there is a 40% chance of rain at any point in the county from noon to 6 p.m. local time.
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 a 100 percent chance of rain mean? 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 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.
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.
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!
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.
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.
10 inches of rainfall is 0.833 ft of water. 18,000 ft x 0.833 ft is 15,000 cubic ft or 112,200 gallons of water. That's 467.9 tons of water, which is a lot of water that was suspended in the sky above your house.
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."
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.
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).
It also doesn't mean it will rain 30% of the day. The percent chance of rain (or snow or thunderstorms) is called the “Probability of Precipitation,” or PoP. The figure refers to the chance that the forecast area will see at least 0.01 inches of rain, according to the National Weather Service.
Heavy rainfall can lead to numerous hazards, for example: flooding, including risk to human life, damage to buildings and infrastructure, and loss of crops and livestock. landslides, which can threaten human life, disrupt transport and communications, and cause damage to buildings and infrastructure.
When it rains, the humidity is at 100%, which is why the clouds are unable to hold any more water. When it rains, it will increase the relative humidity because of the evaporation. The air where the rain is falling may not be completely saturated with water vapor.
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.
Can it rain at 100 degrees?
Rain is rare at temperatures above 100°F
It is rare to get rain when the temperature rises above 100°F (38°C), since heat of that intensity is usually accompanied by a high pressure system with sinking air, which discourages clouds and rainfall.
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.
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.
| Nr | Description |
|---|---|
| 19 | Mostly cloudy |
| 20 | Mostly cloudy and few cirrus |
| 21 | Mostly cloudy and cirrus |
| 22 | Overcast |
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.
For example, a 70% chance of rain represents a 7 in 10 chance that precipitation will fall at some point during that period. Chance of precipitation represents how likely it is that rain (or other precipitation: sleet, snow, hail, drizzle etc.) will fall from the sky at a certain time.
1/10 (0.10) of an inch of rain – A light rain for 30-45 minutes, moderate rain for 10 minutes or heavy rain for 5 minutes. Small puddles would form but usually disappear after a short while. 1/4 (0.25) of an inch of rain – A light rain for 2-3 hours, moderate rain for 30-60 minutes or heavy rain for 15 minutes.
structure, approximately 90% of its composition is air. This air becomes trapped in between ice crystals and slows the rate of heat transfer. With less exchange of heat between the ground and the air, snow acts like a blanket insulating the ground below.
The U.S. 2022-2023 Winter Forecast
The Farmer's Almanac is predicting an earlier, heavier, and colder snow season in many areas of the country, including possible record-breaking temperatures as low as -40°F may be seen in several regions.
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).
How high is 50% chance of rain?
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.
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.
Greatest rainfall in 24 hours
The biggest rainfall in a day occurred with the passage of Cyclone Denise in Foc-Foc, La Réunion, an island in the southern Indian Ocean. Some 1.825 meters (71.8 inches) of rain fell over 24 hours, from January 7 to 8, 1966.
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.
The main thing to realize is that 1 inch of rainfall is enough to create a 1 inch deep layer of water on your roof. So the total volume of water on your roof is the square footage of your roof—let's say it's 1,000 square feet—times 1 inch = 1/12 of a foot.
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.
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. However, a 10-day—or longer—forecast is only right about half the time.
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.
Why is rain so hard to predict?
Why is predicting the weather so difficult for meteorologists? Well, meteorologists need to track multiple conditions and variables at the same time across vast areas, and many of these variables interact and affect one another. Weather prediction is more than just a moving target.
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.
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.
Accuracy. Certain rain gauges more accurately measure rainfall than others. For homeowners, tipping bucket systems are often the most accurate, and some display rain totals down to 0.01 of an inch. Standard gauges are less accurate and may only offer 0.25 or 0.1-inch increments.
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.
The alerts are a reliable short-term forecast based on near real-time data. This app is more precise and reliable than a traditional forecast. This weather app alerts you when rain is approaching. The alerts are a reliable short-term forecast based on near real-time data.
