When see it, I’m sure you’ve noticed it

When you think of all of the things you measure, water vapor probably doesn’t come to mind. This lesson will explore different ways to measure water vapor in the air!

Water Vapor

Think of all of the things you’ve measured in the past week: 4 cups of flour for your bread, a quarter of a gallon of gas for your chainsaw, a 5-mile trail for your run and a 32-inch belt for your waist. So, I know you’re a bread baking, chainsaw-wielding runner with a nice belt, but I don’t know if you can measure water vapor.

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Well, don’t worry; I’m here to help. Water vapor is water that is a gas. In fact, it’s floating all around you right now. Even though you can’t see it, I’m sure you’ve noticed it before. For example, when it’s humid out, there’s a lot of water vapor in the air, so you’ve probably felt it.

Or, when you see dew, fog or clouds, water vapor has condensed, or become a liquid. Measuring water vapor can help scientists make predictions about weather.So, how in the world do you measure water vapor? A ruler? A measuring cup?

Vapor Pressure

Let’s start with vapor pressure, which is the atmospheric pressure due to water vapor molecules. Now, depending on where you read, this definition can get a little more complicated, but for the scope of this lesson, let’s keep it simple. You might not realize these water vapor molecules are exerting pressure, but they are constantly bumping into you!Since you can’t see these water vapor molecules, how do you measure vapor pressure? Excellent question. You can measure vapor pressure by using a manometer. No, that isn’t a man-thermometer hybrid.

Instead it’s a U-shaped tube usually filled with mercury. As the pressure on one side of the tube increases, it pushes the mercury to the other side. So, now you can be a bread baking, chainsaw wielding runner with a nice belt and a manometer.Before we move on, how do you think vapor pressure is related to the number of water vapor molecules in the atmosphere? Well, if you have a lot of water vapor in the air, they would be exerting more pressure, so you would have a high vapor pressure! Okay, onto humidity!

Humidity

When you think of humidity, what comes to mind? Being sweaty? Hot? Your hair going crazy? You probably already know that humidity has something to do with how much water vapor is in the air, but there are several ways to measure humidity. There is relative humidity, absolute humidity and specific humidity. Let’s look at each.

Relative humidity measures how close the air is to saturation. Saturated air is air that’s full of water vapor and can’t hold anymore. So, relative humidity is water vapor in the air divided by how much water vapor the air could hold. Relative humidity is measured as a percentage, so if the relative humidity is 100%, it means the air is saturated.

Sometimes it’s hard to wrap your mind around definitions, so let me give you an example. Let’s use cups to represent air and water to represent water vapor. Our first cup is five ounces and is holding five ounces of water. To make that a percentage (like relative humidity), I’d take 5 / 5 * 100.

We’d say this cup has a relative humidity of 100%.Our second cup is 50 ounces and is holding 5 ounces of water. To make that a percentage, I’d take 5 / 50 * 100. We’d say this cup has a relative humidity of 10%.In our analogy, the small cup represents cold air, whereas the big cup represents warm air.

Warm air can hold more water vapor without ‘filling up,’ or becoming saturated, hence the bigger cup. Now, technically the warm air isn’t ‘holding’ more water vapor, but realize that warmer air has more water vapor molecules (without becoming saturated).Another measurement of humidity is called absolute humidity, which measures the mass of water vapor in a certain volume, or the density of water vapor. So, here, you can see one of these has more water vapor molecules within a certain area, so you could say it has a greater absolute humidity.

Absolute humidity measures the mass of water vapor within a certain volume.
Type of Humidity How It’s Calculated
Relative Water vapor in the air / how much water vapor the air can hold
Absolute The density of water vapor, which is the mass of water vapor / volume
Specific Mass of the water vapor / total mass of the air

Mixing Ratio

Next, let’s see how you can measure the mixing ratio, or the mass of water vapor divided by the mass of dry air.

Although this definition sounds similar to specific humidity, you’ll notice the mixing ratio looks at the mass of dry air, whereas specific humidity looked at all air. Like I mentioned for humidity, there are formulas you can use to help you calculate the mixing ratio. We won’t get into the specifics, but you know which tool you’ll need to do all of that math!

Dew Point

The last measurement we’re going to discuss is the dew point. This is the temperature air needs to be cooled to in order for saturation to occur at a specific temperature and pressure.

In other words, the temperature where the air cannot hold anymore water vapor, which results in condensation. Dew is condensation on objects, hence the name dew point.You can measure dew point by using another type of hygrometer made out of a mirror, a light beam and a light detector. The light beam shines into the mirror and then bounces off and is read by the detector.

When the mirror is cooled, dew forms on its surface, which makes the beam less bright, which is noted by the light detector, and voila the dew point is recorded. Now you have another hygrometer to add to your collection.

Lesson Summary

So, you’re a bread baking, chainsaw wielding runner with a nice belt, a manometer, a couple of hygrometers (including one psychrometer), some math formulas, a table and a calculator. But let’s take a moment to review all of the ways we can measure water in the air!You probably remember that water vapor is water that’s a gas in the air, but did you remember what vapor pressure measured? Vapor pressure is the pressure exerted by water vapor molecules that are in the atmosphere. Humidity measures how much water is in the air, and you can measure it several different ways. The mixing ratio sounded similar to specific humidity, but it was actually a little different.

Let’s take a look at a table that summarizes humidity and mixing ratio.

Type of Humidity How It’s Calculated
Relative Water vapor in the air / how much water vapor the air can hold
Absolute The density of water vapor, which is the mass of water vapor / volume
Specific Mass of the water vapor / total mass of the air
Mixing Ratio Mass of water vapor / total mass of dry air

Last but not least, there was the dew point, which is the temperature the air must cool in order for dew to form. And remember all of the tools you can use: formulas, tables, calculators, manometers and hygrometers (including one called a psychrometer).So, now that you’re armed with a whole new set of tools, you can spend all of your free time measuring the water vapor in the atmosphere. And think of all the things you’ll be able to tell your friends! When dew will form, how saturated the air is or what’s the humidity today!

Learning Outcomes

Once you are done with this lesson, you should be able to:

  • Describe how to measure vapor pressure
  • Explain and compare relative, absolute, and specific humidity
  • Discuss how to measure the mixing ratio and the dew point
  • Name some of the tools used in measuring humidity and dew point
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