Understanding Dew Point
Have you ever noticed droplets forming on a cold glass of water on a warm day? Or perhaps you've seen dew coating the grass in the early morning hours? These phenomena are closely related to a concept known as dew point.
A higher dew point means the air has more water in it than air with a lower dew point. But what exactly is dew point, and why is it important?
What is Dew Point?
Air can carry moisture in the form of water vapor, meaning water that has evaporated, or turned into a gas. But did you know that air's ability to carry water vapor changes with the air temperature?
Warmer air can carry more water. Cooler air can carry less.
Visualize a volume of air as a bucket, able to hold water.
Relative humidity, then, is a measure of how full the bucket is. 50% relative humidity means that the air is carrying half of the water it could possibly hold. Our imaginary bucket is half (50%) full.
Now, without adding or removing any moisture, what happens if the temperature of that air changes?
Remember, we said that warm air can carry more water than cooler air. So as the temperature of the air changes, the size of our imaginary bucket changes. But the amount of water does not.
Let's work through an example:
We’ll begin with air at 72°F (22°C) and 50% humidity, like the inside of a nice comfortable living space.
Here's what our bucket looks like:
Now, let's say the air is cooled down to 60°F (16°C). This could be air that finds its way into a cooler basement or attic, or perhaps behind the insulation in a wall cavity. Or maybe it's near a window on a cold day.
The cooler air has less capacity to carry moisture, by about one-third in this case. The imaginary bucket gets smaller, shrinking to two-thirds of its original size. But the moisture didn't go anywhere. It's still there.
Here's what it looks like now.
The amount of water is the same, but the capacity is smaller. As a result, now the bucket is 75% full. In real terms, this means the relative humidity has increased to 75%. Remember, there's no more water, but because of the temperature change, the vapor capacity of the air – the size of the bucket – has changed.
OK, let's cool the imaginary air once more, this time down to 52°F (11°C). If that seems like an arbitrary choice of temperature, hang in there, it'll all make sense in a moment. After this temperature change, the air loses more of its capacity. Now the imaginary bucket is even smaller:
As a result, the bucket, and the air, is full. This is 100% relative humidity. That's why we picked this temperature. For air with this amount of moisture, 52°F (11°C) is the dew point. This just means the temperature at which the air's water vapor capacity is exactly full – the temperature where relative humidity is exactly 100%.
Why is this significant? Let's cool the air down just a little more and find out. Remember, when we cool the air, our bucket still shrinks.
Once air is cooled to below its dew point the air can no longer hold all of the moisture as water vapor. As a result, the excess turns back into liquid water.
This how fog is formed. Or the drops of condensation on your cold drink on a hot humid summer day. Or the frost on the inside of your windows on a cold winter day (same condensation, but the window is cold enough to turn it to ice). Or, as a less pleasant example, the dampness and risk of mold that can result when warm moist air finds its way to the cold side of a home's insulation. This might be due to a missing or improperly installed vapor barrier or because of excess ambient humidity inside the home.
Very different situations, but they're all a result of the same effect: warm, relatively moist air is cooled, its capacity to hold moisture shrinks, and as a result the air becomes oversaturated and some of its water vapor begins turning to liquid water, simply because it has nowhere else to go.
Incidentally, this is also how rain and other precipitation occurs in the atmosphere.
Another example (why indoor air is dry in winter)
Imagine a cold, snowy day, with perhaps a bit of foggy haze in the air. The outdoor temperature is 20°F (-6°C) and the relative humidity is 100%. That's a damp, soggy day, so we wouldn’t have any problem with dry air inside our homes, right? Not exactly.
Let's go back to our imaginary bucket. Here it is, starting at 20°F (-6°C) and 100% relative humidity.
The air is saturated at 100% relative humidity, so the bucket is full of water. But remember, cold air can't carry as much moisture, so the bucket is small.
Outside air is always infiltrating homes. Some homes allow more outside air in than others, but there's always outside air coming in replacing the air inside. Since we don't like our homes at 20°F (-6°C), that air gets heated.
Let's say we heat that air up to 72°F (22°C), the same temperature we started with in our first example. We now know that as a result of heating the air, it can carry more moisture, right? Its bucket gets bigger, back to its original size from the first example. That's more than *seven times* bigger!
When the outside air is heated, even though it was completely full of moisture and that moisture didn’t go anywhere, the moisture capacity of the air increases so much that its relative humidity drops to less than 14%.
That's dry! For comparison, according to the National Park Service, a typical summer day in Death Valley has a relative humidity around 10%.
Dew Point Measurements and Applications
The entire line of SensorPush temperature/humidity smart sensors is able to display dew point in conjunction with the SensorPush app.
When deciding whether it's better to open a window or turn on a ventilation fan, or whether you need to be concerned about humidity in a space, don't just look at temperature and humidity – remember the dew point.
For instance, in the summer outdoor air is generally hotter than the indoor air, and indoor humidity is often an issue. Without an understanding of dew point and a way to easily measure it, you might decide to bring in some outside “dry” air if your indoor humidity is 65% and the outdoor humidity is 50%.
But what if it’s 85F (29C) outside and 75F (24C) inside?
That would make the dew point outside is 64F (18C) while inside it is 62F (17C).
Remember a higher dew point means more water vapor. That means the outdoor air actually has more moisture, even though the relative humidity is lower. If you let that outside air in, the net effect is more dampness, not less.
With a SensorPush sensor inside and another outside, and what you now know about dew point, you could see this at a glance, increasing your comfort and efficiency.
Here are a few quick things to take away:
Warmer air holds more moisture than cooler air
Air with a higher dew point contains more water than air with a lower dew point.
If you're trying to lower (or raise) interior humidity through ventilation, compare the exterior and interior *dew points*, not relative humidity. Relative humidity will change when the air changes temperature. Dew point does not.
If air comes into contact with a surface that has a temperature at or below the air's dew point, condensation will begin to form. So if your basement walls tend to be 55°F (13°C), condensation will form on them when the air's dew point is at or below 55°F (13°C).
Conclusion
Dew point is a fundamental concept in meteorology that helps us understand moisture levels in the air and its implications for weather and comfort. By grasping this concept, we can better interpret weather forecasts, make informed decisions about outdoor activities, and manage indoor humidity levels effectively. So, the next time you see dew forming on a cool morning or feel the stickiness of a humid day, remember, it all comes down to the dew point.