What Does A Chance Of Rain Really Mean?

You check the forecast before heading outside and see a 30% chance of rain. Does that mean it will rain for 30% of the day? That 30% of your town will get wet? Or that the meteorologist is only 30% sure it's going to rain?

None of those is quite right.

A chance of rain—officially called the probability of precipitation, or PoP—is the probability that a particular location in the forecast area will receive a measurable amount of precipitation during the specified time period.

So if your forecast says there is a 30% chance of rain this afternoon, the simplest way to read it is: there is a 3-in-10 chance that your location will receive measurable rain during that forecast period.

What it does not tell you is almost as important.

What Does a 20% Chance of Rain Mean?

A 20% chance of rain means there is a relatively low probability that your location will receive measurable precipitation during the forecast period. It does not mean it will rain for 20% of the day or that exactly 20% of the forecast area will get rain.

Likewise, a 50% chance means your location has a 1-in-2 chance of measurable precipitation during that period. At 100%, measurable precipitation is expected at your location.

The key word is measurable. The National Weather Service generally defines measurable precipitation as at least 0.01 inch.

That means the percentage is answering a fairly narrow question: Will measurable precipitation occur at this location during this period?

It isn't answering: How much rain will fall?

A 100% Chance of Rain Doesn't Mean It Will Rain All Day

This is one of the easiest forecast numbers to misinterpret.

If the forecast says there is a 100% chance of rain tomorrow, you might picture a steady downpour from breakfast until bedtime. But the percentage says nothing about duration.

Rain could last most of the day, or a fast-moving band of showers could pass through in an hour. Both could justify a very high probability of precipitation if forecasters are confident your location will receive measurable rain.

The same is true at lower probabilities. A 30% chance doesn't mean a brief shower, and an 80% chance doesn't necessarily mean hours of heavy rain.

Probability and duration are two different things.

It Doesn't Tell You How Hard It Will Rain Either

A high rain probability also doesn't necessarily mean heavy rain.

Imagine two days. On the first, there is a 90% chance of a light shower that might produce 0.05 inch. On the second, there's a 40% chance of a slow-moving thunderstorm that could drop more than an inch over your neighborhood.

The first day has a much greater chance of rain. The second could produce much more rain if the storm happens to pass over you. That's because rainfall intensity and total rainfall are different measurements, and neither is described by the probability percentage.

That's an important distinction for gardeners. A high percentage doesn't automatically mean your plants received a useful soaking, while a lower-probability thunderstorm could provide more than enough water for several days.

Why Can It Pour at My House When the Forecast Said 20%?

Because 20% isn't the same as zero.

Scattered showers and thunderstorms are especially good at demonstrating this. One neighborhood may get drenched while another a few miles away stays completely dry. In fact, rainfall can vary substantially over surprisingly short distances.

If you happen to be underneath one of those showers, a 20% forecast can feel spectacularly wrong. But a low probability doesn't mean rain can't occur. It means the odds of measurable precipitation at a particular location were relatively low.

This is one reason rainfall can be so frustrating to judge from a weather app alone. The forecast tells you the probability before the event. Radar can show where rain is occurring during it. Neither necessarily tells you exactly how much landed in your yard.

Where Did the “Confidence Times Area” Explanation Come From?

You may have seen an explanation online that says meteorologists calculate rain probability by multiplying how confident they are that rain will occur by the percentage of the forecast area expected to receive it.

For example, the explanation might say that if a forecaster is 50% confident rain will occur and expects it to cover 80% of the area, the forecast becomes a 40% chance of rain.

This has become a popular way to explain PoP, but it can create the wrong impression about what the number on your forecast actually means.

For the person reading the forecast, the important definition is much simpler: the percentage represents the probability of measurable precipitation at a point in the forecast area during the stated period.

You don't need to calculate how much of your county will get wet to understand your own forecast.

Why Does the Percentage Keep Changing?

You may see a 30% chance of rain several days ahead, 50% the next morning and 80% by the time the storm arrives. That doesn't mean meteorologists keep changing their minds randomly.

Forecasts improve as new information becomes available. Satellites, radar, weather balloons, surface observations and computer models continually provide updated information about the atmosphere. As a weather system gets closer, its timing and path usually become clearer. That's also why weather forecasts become less certain the farther ahead we try to predict.

Different weather apps can also incorporate models and new information differently, which helps explain why weather apps sometimes disagree and why their forecasts keep changing.

Thunderstorms can remain particularly difficult to pinpoint because they may develop over relatively small areas. Forecasters may be confident that thunderstorms will occur somewhere in a region while being much less certain about which exact communities will get one. Summer thunderstorms are especially good examples of how localized rainfall can be.

What Should You Look at Besides the Chance of Rain?

The percentage is useful, but it works best when you read it along with the rest of the forecast.

Look at the expected timing and description. “Scattered afternoon thunderstorms” tells you something different from “steady rain developing overnight,” even if both happen to carry the same probability. Expected rainfall totals can also tell you much more about whether the rain is likely to be meaningful.

Radar becomes useful once precipitation is actually developing, especially for localized showers and thunderstorms. But radar rainfall totals are still estimates rather than measurements taken in your yard. Here's how weather radar estimates how much rain is falling.

And after the storm is over, an actual rainfall measurement tells you something none of those forecasts can: how much rain your particular spot received.

Forecast Probability vs. Actual Rainfall

For anyone deciding whether a lawn or garden needs water, this distinction matters.

Suppose yesterday's forecast showed an 80% chance of rain. You might assume watering isn't necessary. But if the storm passed just north of you, your garden may have received nothing. On another day, a 30% chance might turn into a thunderstorm that drops an inch directly over your house.

The forecast percentage describes the chance of an event before it happens. It isn't a measurement of what happened afterward.

That's why a rain gauge and a weather forecast answer two different questions. The forecast helps you decide what might happen. Your rain gauge tells you what did happen. It's also why your rain gauge may not match the rainfall reported by your weather app.

So the next time your phone shows a 40% chance of rain, don't read it as 40% of the day, 40% of the area or 40% of some expected amount. Read it for what it is: the probability that your location will receive measurable precipitation during that forecast period.

Whether that turns out to be a few drops, a ten-minute shower or a soaking thunderstorm is another question entirely.