When you see a forecast for rain or snowstorm or check local indoor or outdoor humidity levels, you’ll often see a reference to rain percentages. But what do rain percentages mean? Rain percentages tell you how much of the moisture in the air is falling as rain, and how much is coming from other sources.
Understanding what these numbers mean and how they can impact your personal comfort, safety, and visibility when driving can help you make smart decisions about possible activities on your next rainy or snowy day.
How Rainfall is Measured?
The first thing we should understand about rain percentages is how we measure rain itself. While a rain gauge can be used to measure rainfall in a specific location at a specific time, it doesn’t take into account the way the rain is impacting whatever surface it lands on.
If a rain gauge in your backyard measures 1/2 inch of rain, you can’t assume that 1/2 inch of rain is also falling on your lawn. The two surfaces might react very differently, with your lawn absorbing most of the water and your grass growing, while your driveway is quickly covered in a puddle.
Rainfall is measured in inches, but if it’s falling on a surface, it’s only part of the story. More importantly, it’s only part of the story for the rain percentage forecast. There are other percentages that may be just as important.
What Does 20% Rain Mean?
When you see a rain forecast of 20%, it means that 20% of all the water in that rain cloud is falling to the ground as rain. If a rain cloud has 10 inches of water in it, 2 inches of rain will fall to the ground. If a cloud has 1 inch of water in it, 2/3 of an inch will fall as rain.
The rest of that water will evaporate before it reaches the ground, or it could be blown sideways by the wind. If there is no ground for the water to fall on, it will accumulate in lakes, rivers, and low spots like rooftops until it can be absorbed.
What Does 50% Rain Mean?
If a rain forecast says there will be 50% rain, that means 50% of the water in the rain cloud is falling. If you have a 10-inch rain cloud and 5 inches is being dropped to the ground, that’s a lot of water that has to be absorbed by something. It will also cause a lot of flooding in low-lying areas if it doesn’t have a chance to evaporate.
Think about trying to pick up a 5-gallon bucket of water with a 5-foot-long shovel. Then think about trying to pick up 5 of those buckets at once. That’s a lot of water that is going to have to be moved and it isn’t going to happen quickly.
What Does 80% Rain Mean?
An 80% rain forecast is a lot of rain. In fact, an 80% rain forecast is an extreme rainfall level. An 80% rain level means that 80% of the water in the rain cloud is falling.
While it sounds like it would be the same amount of water that is falling at a 50% rain level, it actually results in much more water being dropped to the ground. Why? Because when that water is falling, it isn’t evaporating. Some of the water will be blown sideways, but most of it will be falling to the ground.
When Is 100% Rain?
100% rain is possible, but it is rare. In fact, a 100% rainfall level isn’t just rare, it’s practically unheard of. A 100% rain forecast indicates that every drop of water in the cloud is falling to the ground. A 100% rain level will cause catastrophic flooding and possibly damage to infrastructure. It is not a forecast you want to see.
Understanding the Language of Rainfall Percentages
When you see a rainfall forecast, it will be accompanied by a number. That number represents the percentage of all the water in the rain cloud that is falling to the ground. This is important for several reasons. It tells you how much water might be flowing in your local river or stream. It tells you how much water is falling on your property. It also tells you how much moisture is in the air, which can have an impact on visibility.
If you see a forecast with a 90% rain percentage, that means that 9 out of every 10 water molecules in that rain cloud are falling to the ground. That also means that the air around you is very moist, which can result in degraded visibility, especially at night.
Why Knowing the Difference Matters.
Knowing the difference between a rainfall percentage and a rainfall amount can really come in handy. For example, if you are visiting someone and they tell you that it rained in their area, you can look up the rainfall percentage. This will tell you how much water is in the air and how much moisture might be impacting your personal comfort indoors.
Likewise, if you are visiting someone and they tell you that it rained in their area, but you know how much it rained and what the rainfall percentage was, you will know whether they have enough water flowing in their river or stream.
You can also see if there is a flood risk. If you are camping or hiking outdoors, you can use the same information to know when conditions are best for being outside and when you might want to seek shelter.