Dew Point and How it Affects You
Full disclosure: I chose that title because it kind of almost rhymes. But also, it’s something a lot of people either don’t know or they overlook. We’ve already talked about heat index and how dangerous that can be, but dew point is just as important and also plays a huge part in heat index. In fact, some heat index charts plot Dew Point vs. Temperature instead of Humidity vs. Temperature. So, let’s take a look at the technical details of dew point without boring you to death.
Dew Point and the Science Behind It
From Wikipedia: “Dew point is the temperature to which air must be cooled to become saturated with water vapor. When further cooled, the airborne water vapor will condense to form liquid water (dew). When air cools to its dew point through contact with a surface that is colder than the air, water will condense on the surface. It is sometimes called the frost point when the temperature is below the freezing point of water, as frost is formed rather than dew. The measurement of the dew point is related to humidity. A higher dew point means there will be more moisture in the air.”
From that paragraph, we see that it’s always talked about in degrees. For the sake of this article, we’ll talk about it in degrees Fahrenheit (apologies to anyone outside of the U.S. that happens to be reading this).
Dew Point vs. Humidity
Both measurements are a function of moisture so it’s easy to get the two confused or think that they’re interchangeable. But they’re very different. Dew point is the temperature at which the vapor in the air would condense into liquid. Relative humidity is the ratio between the current amount of water vapor in the air at a given temperature, to the maximum amount of water vapor possible in the air at that same temperature.
Dew point and Atmospheric Pressure
To make it slightly more complex, atmospheric pressure can also affect it, so it’s not just humidity and temperature. We’ll discuss pressure in greater detail in a separate post later, but just know that it increases and decreases as air pressure increases and decreases.
For this, let’s think of a clear plastic box filled with water vapor that’s still in its vapor form. Now suppose the top side of this box can move up and down. As the top moves down, the volume inside the box decreases and the pressure inside the box increases. This makes the molecules start bouncing off the walls, and each other, faster and faster. And the faster that happens, the higher the temperature becomes. Thus, raising the dew point.
So that’s how, on two separate days with the exact same temperature and the exact same humidity, they can have different dew points…because of the atmospheric pressure.
Our comfort zone
It should now come to no surprise that humidity alone is not the only culprit when we feel hot and sticky in the summer. And in fact, the humidity number alone can be extremely misleading.
For example, if the air temperature is 50 and the dew point is 50, the relative humidity will be 100%. But a temperature of 90 and a dew point of 70 produces a relative humidity of 52%. Pretty comfortable, right? (Well, depending on what part of the country you’re from.)
However, counter to what seems obvious based on those numbers, it would actually “feel” much more humid on the 90-degree day with only 52% relative humidity compared to the day with 100% humidity. This is because there is more actual water vapor in the air on the 90-degree day. Remember, with a colder dew point, that water has already condensed into water. On the higher dew point day, that water is allowed to hang out in the air and collect on your skin. Regardless of the actual humidity reading.
Dew points between 60 – 63°F already start to “feel” more humid. Once they hit 70°F or higher, they become rather oppressive on a summer day.
Why it really matters
Other than causing some uncomfortable and sticky situations, the main reasons we’re concerned with humidity and dew point are how they affect heat index.
Dew Point and Heat Index
In the previous post about heat index, the chart graphed heat index as a function of temperature and humidity. The chart below also shows heat index but in terms of temperature and dew point. And just like the other chart, extreme caution needs to be taken starting at just 85 degrees if the dew point is high.