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The Difference Between Dew Point and Humidity

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Posted at 8:09 PM, Dec 25, 2020
and last updated 2020-12-25 22:09:40-05

TUCSON, Ariz. (KGUN) — The First Warning weather team often refers to dew point and humidity. Most people know these terms have something to do with moisture, but a lot of people don’t really understand the terms. Fear not, we are here to help explain!

Explaining the details of dew point and humidity can get pretty confusing, so we’ll try to provide you with an explanation that is easy to understand. We’ll start with humidity because that’s a term most of us have heard since we were children.

Humidity can be discussed in two ways… Absolute Humidity and Relative Humidity. The National Weather Service defines absolute humidity as the actual amount of water vapor in the air, regardless of the air temperature. The higher the amount of water vapor, the higher the absolute humidity. This reading is expressed in grams of water vapor per cubic meter volume of air.

Relative humidity is expressed as a percent and measures water vapor relative to the air temperature. Hence the term relative humidity. It’s a measure of the actual amount of water vapor in the air compared to the amount of water vapor that can exist in the air at its current temperature.

Warm air can hold more moisture than cold air. So, air with the same amount of absolute humidity will have a higher relative humidity if the air is cooler and lower relative humidity if the air is warmer. To put it into numbers, a relative humidity of 50% means the air is holding half of the water vapor it’s capable of holding.

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The First Warning weather team often refers to dew point and humidity. Most people know these terms have something to do with moisture, but a lot of people don’t really understand the terms. Fear not, we are here to help explain!

Dew point defines the point at which air will become saturated and condense. In other words, the point at which water droplets form. If suspended in air, you’ll see clouds form. If the air temperature and dew point meet on a surface such as a glass or on a leaf, dew will form and you’ll see drops of moisture on the surface. If the air temperature is below freezing, the condensation will appear as frost or ice.

I like to refer to dew point because it really helps define our “comfort” level. The closer the dew point and temperature, the more humid or “muggy” it will feel. This is especially noticeable in the summer during our monsoon. Unfortunately, in 2020, we didn’t feel much muggy or sticky air because our dew points rarely got very high.

Across southeastern Arizona, in the summer months, we commonly see dew points in the upper 60s and lower 70s as sub-tropical moisture gets transported over the region. When combined with air temperatures of 100° or higher, this combination can feel quite oppressive. These are the days when we have sweat rolling off our foreheads without even exerting ourselves. These are also the days that can produce heavy rain if we can get the heavy, wet air to lift.

In the winter months, we routinely see dew points in the single digits and teens. These are the times when our skin dries out quickly and it seems like no amount of hand lotion will help keep our hands from drying out. These conditions also play havoc with people’s sinuses and dehydration will take over much quicker than it would in humid conditions.

When it’s cold, a high dew point can make it feel even colder. If temperatures are in the 30s or 40s and we get a surge of moisture that brings the dew points up, it can feel much colder because of the increase in humidity. These are the nights when it feels like the air is chilling us to the bone and only a bowl of hot soup or a cup of hot chocolate will make us feel warmer!

Dew point also plays a key role in the formation of fog. If the air temperature and dew point get within 2 or 3° F of each other, the air will condense and a cloud will form. Remember, fog is just a cloud close to the ground. If the air temperature is below freezing, we will see freezing fog that will cause icy streets and coat everything with a layer of frost. It can make for quite the sight to see ice crystals on everything in sight! Here is a photo I took in Arches National Monument after a foggy start to the morning when temperatures were below freezing.

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The First Warning weather team often refers to dew point and humidity. Most people know these terms have something to do with moisture, but a lot of people don’t really understand the terms. Fear not, we are here to help explain!

Figuring out the difference between dew point and humidity can be tricky. How dew point and humidity relate to one another can be confusing. Hopefully, we have helped clear up what can be a pretty cloudy subject! Just remember, dew point directly relates to how the air feels to us. If we step outside and it feels really humid (or muggy as some folks like to say), it means the dew point is really high compared to the air temperature. If the dew point and air temperature get within about 3° F of each other, we’ll likely see some fog developing.

I hope we can soon be talking about higher dew points because that means we’re looking at more moisture and more moisture equals a better chance of rain! For now, it looks like we’re in for more low dew points which equals more dry skin. Keep drinking that water!