When you want to check the upcoming weather, you probably look to your smartphone’s five-day forecast. However, there is one aspect of the forecast that’s not there — and it will help you better prepare for the days ahead.

This aspect, called the dew point, will help you understand why air ends up feeling so humid at times.

First, we need to review a few properties of air: Air primarily consists of nitrogen, oxygen, carbon dioxide and water vapor. All of those air parts make up what is floating around us all the time. The piece that goes up and down the most dramatically from day to day is the water vapor. Water vapor fits into the spaces between the other gases.

The warmer the air, the more space there is between the other gases, and the more water vapor can fit in there.

Colder air is more dense and hold less water vapor

Colder air is more dense and holds less water vapor.

RELATIVE HUMIDITY

The term “relative humidity” tells you little about how much actual moisture is in the air; it only tells you how close the air is to being full of water vapor.

A relative humidity of 100 percent means the air is full of the potential moisture. A relative humidity of 50 percent means the air is half full of potential moisture.

However, if the air is cold, the potential for total moisture is small. So when you have 100 percent relative humidity in cold air, the air is full of moisture, but the air molecules are packed tighter and there’s not much space in there for water vapor.

Hot air with a relative humidity of 50 percent has half the potential moisture, but because the air can hold so much water vapor, there’s a lot present.

Think about the difference between an 8-ounce glass of water that’s full and a swimming pool that’s half full. There’s a lot more water in the 50 percent full pool. This is why relative humidity is rather meaningless when it comes to how you feel.

Relative humidity does tell us how close the air is to being saturated. You can assume that if it’s over 80 degrees and the relative humidity is over 50 percent, there must be a lot of moisture in the air.

Relative humidity tells you how close the air is to being full of water, but doesn't tell you how much is in there.

Relative humidity tells you how close the air is to being full of water, but doesn’t tell you how much is in there.

DEW POINT

Because relative humidity isn’t a good descriptor for how you will feel, years ago, meteorologists starting using the dew point to better explain how much water is actually in the air. The chart below shows how much water vapor is in the air at various dew points.

Warmer air can potentially contain more water vapor, but it doesn't mean it's always present.

Warmer air can potentially contain more water vapor, but it doesn’t mean it’s always present.

Notice how much more water vapor is present as the dew point rises. If the dew point is 68 degrees Fahrenheit, you have 15 grams of water vapor in a kilogram of air. That’s enough water vapor for us to feel it — we would say it’s a humid day.

It might actually be better if meteorologists started using the grams of water vapor as an actual variable on TV. I think people would quickly get the concept of more water versus less.

If you like numbers, use the current temperature and dew point along with the chart above and the equation below to see for yourself how relative humidity, dew point and temperature are all related.

Calculating relative humidity uses a simple formula

Calculating relative humidity uses a simple formula.

HOW HIGH CAN THE DEW POINT GET?

I consider a high dew point in the upper 60s or above. In Portland it’s rare for dew points to reach the upper 70s, but it does happen. This month we have seen several days with dew points in the 70s, considered oppressive by most.

The next week or two will bring some high dew point days with lots of moisture in the air.

The chart below along with the forecast dew point will help you know when the sticky air is arriving and when the drier air will replace it. Humidity levels will be fluctuating a bit over the next 10 days with more sticky, uncomfortable ones on the horizon.

The higher the dew point the more moisture in the air and the worse it feels

The higher the dew point, the more moisture in the air and the more uncomfortable it feels.