Local News

Weather Quiz: Which Place Has More Moisture In The Air?

February 12, 2016 / 1:13 PM MST / CBS Colorado

By Chris Spears

DENVER (CBS4) - Take a look at the following and decide which scenario has more moisture in the air.

The desert with the following conditions...

Temperature: 95°F
Dewpoint Temperature: 50°F
Humidity: 22%

Or the arctic?

Temperature: 28°F
Dewpoint Temperature: 28°F
Humidity: 100%

Which did you choose?

Most people will choose the second picture because there is an ocean, low clouds or fog, and the relative humidity is 100%.

But the answer is there's more moisture present in the hot, sunny desert than in the cold, foggy arctic.

THE WEATHER REPORT

As consumers of weather information we look to various sources each day to find the current weather outside.

Most of the time people are interested in the temperature and relative humidity, which is why meteorologists cater to providing this information.

The general public uses the temperature to know how to dress and the relative humidity to understand human comfort.

Some people look at the relative humidity to know what kind of hair day to expect.

But the relative humidity can be very misleading if you don't understand it.

RELATIVE HUMIDITY

Relative humidity is a calculation made by using air temperature and dewpoint temperature. But it's not as simple as solving a quick math problem using those two numbers.

You actually need to know the vapor pressure of each to determine the relative humidity.

Luckily there is a great online tool that will do the work for you.

Relative humidity tells us how close the air is to saturation at that moment in time. It doesn't tell us anything about how much moisture is actually in it.

If the relative humidity is 50%, then the air is holding half of the moisture it can at the current air temperature.

DEWPOINT

The dewpoint is a measure of the actual amount of moisture, or water vapor, currently in the air.

The lower the number, the drier the air, regardless of what the relative humidity suggests.

In other words, the dewpoint is the temperature that air would need to cool down to for it to be saturated with a relative humidity of 100%.

WHY IS THE RELATIVE HUMIDITY MISLEADING?

Relative humidity is a number that can change even if the actual moisture content stays the same.

In fact, it can change by the hour and even the minute.

You may wake up in the morning and see a humidity of 95% on the weather report, but it will have dropped to less than 50% by the time you go to lunch.

All the while the dewpoint, or actual water content of the air, stayed the same.

The relative humidity will go down as the air temperature goes up.

SUMMARY

if your air temperature and dewpoint are equal, then your relative humidity will be 100%, because the air is holding all the moisture it can for that temperature, and is said to be "saturated"

saturated air can still be "dry" ... meaning it doesn't have a lot of water vapor in it

the closer your air temperature is to the dewpoint, the higher your relative humidity will be (this is why relative humidity is typically highest in the morning around sunrise)

colder temperatures have a low capacity to hold moisture while warmer temperatures have a much higher capacity (which is why in the picture example at the top of this story, the desert has more water vapor in it despite the fact that it "looks" the driest)

associating terms like wet and dry with dewpoint can be subjective depending on the part of the country and the climate (i.e. air with a dewpoint of 55°F in Colorado would be considered muggy or humid while in Florida it would be considered dry air)

the air temperature can cool down to the dewpoint temperature, but not below it

EXAMPLE OF VERY DRY, BUT SATURATED AIR

Take a look at the following maps for a good example of why the relative humidity can be misleading when trying to determine human comfort, especially during the winter.

At 9 a.m. on Friday, Feb. 12, the following conditions were being reported in Gunnison, Colorado...

Temperature: -2°F
Dewpoint Temperature: -2°F
Humidity: 100%

While the air in Gunnison was the most saturated of all the major reporting stations in Colorado with a relative humidity of 100%, it was also the driest air in Colorado with a dewpoint of -2°F.

Meteorologist Chris Spears writes about stories related to weather and climate in Colorado. Check out his bio, connect with him on Facebook or follow him on Twitter @ChrisCBS4.