Calculating Humidity

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SWBAT read a psychrometer and identify them as instruments that measure humidity | SWBAT use psychrometer readings to calculate relative humidity and dewpoint using the ESRT

Big Idea

In this lesson, students use their Earth Science Reference Tables to calculate both relative humidity and dew point, and they use a psychrometer to calculate the relative humidity and dew point in their own classrooms

Lesson Introduction

[Note: For embedded comments, checks for understanding (CFUs), and key additional information on transitions and key parts of the lesson not necessarily included in the below narrative, please go to the comments in the following document: 5.3 - Calculating Humidity & Dewpoint (Whole Lesson w/comments). Additionally, if you would like all of the resources together in a PDF document, that can be accessed as a complete resource here: 5.3 - Calculating Humidity & Dewpoint [Whole Lesson][PDF]. Finally, students may need their Earth Science Reference Tables [ESRT] for parts of the lesson (a document used widely in the New York State Earth Science Regents course) as well.]

This lesson is an extension of the previous day's lesson (found here) on humidity, in that students actually get to calculate humidity and dew point, including by making their own sling psychrometers (see below for information on where to buy)! Using information contained in their Earth Science Reference Tables [ESRT], students are able to determine the dry-bulb and wet-bulb temperatures, which allows them to determine the relative humidity and dew point. In addition to doing this for themselves, students also get the opportunity to tackle Regents-based problems which task them with solving for solutions of humidity and/or dew point. 

Materials Needed:

  • Pre-made sling psychrometers (Class Set) [I recommend this classroom set from Flinn Scientific]. If you don't have one, you can buy a single one from the same company here and do it as a teacher demonstration.

Do Now & Objective(s)

10 minutes

Students come in silently and complete the (attached) Do Now. In this case, the Do Now is a review of material from Unit 3 (Geologic History), in preparation for the upcoming interim assessment. After time expires (anywhere from 2-4 minutes depending on the type of Do Now and number of questions), we collectively go over the responses (usually involving a series of cold calls and/or volunteers), before I call on a student and ask them to read the objective out loud to start the lesson.

As a general note, the Do Now serves a few purposes:

  1. It serves as a general review of the previous day's material; (again, this is a bit different, as they are reviewing for the quarterly Interim Assessment)
  2. It is a re-activation of student knowledge to get them back into "student mode" and get them thinking about science after transitioning from another content area or alternate class;
  3. as a strategy for reviewing material students have struggled with (for example, using this as a focused review for material that they have struggled with on unit assessments or recent quizzes); and,
  4. It is an efficient and established routine for entering the classroom that is repeated each day with fidelity (I never let students enter the classroom talking. While it may seem potentially severe to have students enter silently each day, this is both a school wide expectation and a key component of my classroom. In many respects, I find that students readily enjoy the focus that starting with a quiet classrooms brings each day).

Calculating Dewpoint & Humidity

20 minutes

The lesson begins with one of my favorite things - Coke Zero (specifically, the Cherry variety)! My students know that I drink this (too) frequently, so when I produce a can with localized condensation on it and ask my students to imagine them sipping on a can of soda on a hot summer day, they can quickly visualize the thin film of condensation that often appears on the outside surface. While they know it's my favorite drink, they don't yet know why condensation forms, but we begin to tackle that problem.

The following is excerpted from the embedded comments in the attached Word document (see the Introduction above), but I think it can be helpful to put here for context on how to "roll out" the explanation to students and relate it to previously learned material:

  •  Statement 1: "Cold air holds less water than warm air"
  •  Statement 2: "A cold can of soda, or anything colder than the air, can often cool the surrounding air"
  •  Statement 3: "Cooler air can hold less water, so that often means when the temperature drops, it cools to what we call the dew point, the temperature at which the air is saturated"
  •  Statement 4: "When that happens, the air is saturated, and some of the water in the air actually needs to condense out, so it condenses on nearby objects like the can of Coca-Cola."

 In effect, the cold can of Coke acts as a mini-fridge, cooling the nearby air to the dew point, producing localized condensation. While these facts are there for me to guide the students if they get stuck, I try to lean on them as much as possible to do this thinking aloud. Once we have that information down and the condensation mystery solved, I indicate that we're going to be using out Earth Science Reference Tables [ESRT] in addition to our Calculating Dewpoint & Humidity resource to solve some problems, and calculate relative humidity and dew point ourselves. 

Before we jump into utilizing the Earth Science Reference Tables [ESRT], however, I want them to calculate relative humidity and dew point for themselves, so I indicate that we're going to utilize an instrument called a sling psychrometer to calculate both. We first read the brief paragraph on the Calculating Dewpoint & Humidity resource about the structure of the psychrometer and how it works, and I briefly explain the parts using a teacher model that I already have.

As posted in the image above, the psychrometers are relatively simple, and all they really require is that the paper towel "wick" that's on one of the thermometers (the wet-bulb thermometer) has been dipped or soaked in water. I then have a student helper pass out a psychrometer to each student (Note: If you purchase the kit from Flinn, please note that they come unassembled. While they don't take long to assemble, I would take the time to do this yourself and have them ready to go so as to maximize your class time). Once students receive them, I have them record the dry-bulb temperature (the air temperature), attach the wet wick (again, I just use a piece of paper towel soaked in water), and then have them swing it in the air for a few minutes. After a 1-2 minutes, the students record the wet-bulb temperature, and then I show them how to utilize the information on page 12 of their Earth Science Reference Tables [ESRT] to calculate the dew point and humidity. We then compare it to the outside RH/Dewpoint (pulled from a local weather report) to see how close we are. Two quick things here to summarize:

  1. I don't like this to take a lot of time - I would say that overall, this entire process, from passing out to actually recording the information, takes about 6-7 minutes flat (it will also depend on the size of your class); and
  2. To actually calculate the dew point or relative humidity, find the appropriate table on page 12 of the Earth Science Reference Tables [ESRT]. Then, since the dry bulb temperature is equal to the air temperature, use the thermometer (the one without the paper towel "wick") to find the dry bulb temperature in degrees Celsius. Swing the psychrometer in the air for 60-90 seconds (to allow air to evaporate from the wet wick, lowering the temperature) and then subtract the wet wick from the dry wick to get the difference, which is the number you use on the top x-axis. Then, like a multiplication table, see where they "meet," and that value is going to be your dew point (or relative humidity). 

After that, I have the student volunteer re-collect the psychrometers as we turn to some practice together. I'll usually model one or two more problems for students until they feel comfortable, and then release them in groups to try out more problems together. 


25 minutes

The Practice section in this lesson is, like the vast majority of questions found in all of my classwork and homework, is 100% Regents-based. All of the questions come from prior Regents examinations. Likewise, as I try to generally do with all of my lessons, the questions are mostly organized to get increasingly more difficult and increase in complexity, which is why the harder questions tend to come toward the end. For whatever reason, I've found that many students struggle with this content - they mix up the shadow lengths, the inverse relationships, all that stuff. The best way I've found to combat this is to give them the chance to identify and correct their mistakes in questions of this type, which is why this section is a little extended as per my usual lessons. 

In terms of student work habits, I tend to sometimes make this decision in the moment, and as a response of what I know about the students and how they're processing the material on, but I'll either ask them to work independently, in partners, or (sometimes) give them the option. Usually, before starting practice, we tend to go over some steps for self-help ("What should you do if you're stuck?"), and I might reference a previously used multiple-choice or free response strategy in order to build their skills while simultaneously learning content (as an example - one popular one we always use - "If you aren't sure what the right answer is, see if you can eliminate some wrong answer choices"). I tend to circulate for compliance and then hone in on specific students while they're doing this. 

After about 10 minutes, we go over their responses. Students who finish early are encouraged to work on the exit ticket (resource below) and double-check their responses. We use a combination of strategies (active voting, cold calling, popsicle sticks, volunteers) to go over the responses, where students correct their work and ask any clarifying questions. 

Exit Ticket & Closing

5 minutes

In the last few minutes of class, I have students complete the daily Exit Ticket. For the sake of time, I have students grade them communally, with a key emphasis on particular questions and items that hit on the key ideas of the lesson (Note: This usually manifests as students self-grading, or having students do a "trade and grade" with their table partners). After students grade their exit tickets, they usually pass them in (so that I can analyze them) and track their exit ticket scores on a unit Exit Ticket Tracker. 

After students take a few seconds to track their scores, we usually wrap up in a similar way. I give students time to pack up their belongings, and I end the class at the objective, which is posted on the whiteboard, and ask students two questions:

  1. Do you feel that you mastered the objective for the day?
  2. Can you reiterate one thing you learned about (in this case, information on where the vertical ray falls on the respective Equinoxes/Solstices, etc.)?

Once I take 2-3 individual responses (sometimes I'll ask for a binary "thumbs up/thumbs down" or something similar), I have students leave once the bell rings.