What the Relative Humidity Gizmo Actually Tests
The Relative Humidity Gizmo is an ExploreLearning simulation that runs in a browser and asks students to manipulate variables like temperature and water vapor to see how relative humidity changes. The answer key exists because teachers assign it as homework and students end up stuck on specific questions. I have graded labs that use this gizmo for about eight years, and the main issue is not that the questions are hard. It is that the simulation behaves slightly differently depending on which browser you run it in and what resolution your screen is set to. I do not host answer files directly. ExploreLearning controls the simulation, and the official answer key is locked behind the teacher dashboard. What I can do is walk you through the standard activity questions and the logic you need to solve them, which is more useful than copying answers anyway. If you have a class code, log into ExploreLearning, open the Relative Humidity Gizmo, and click the teacher resources tab. You will find the printable activity sheet and the answer key together. That is the only version that matches the current build of the simulation exactly. Relative humidity is the ratio of actual water vapor pressure to the saturation vapor pressure at a given temperature, expressed as a percentage. The gizmo visualizes this by showing you a container of air where you can add or remove molecules and change the temperature slider. When you lower the temperature, saturation vapor pressure drops, so relative humidity rises even if the absolute amount of water vapor stays the same. That is the core mechanic, and most wrong answers come from confusing those two concepts.
Here is the formula you actually need to remember: RH equals the actual vapor density divided by the saturation vapor density at that same temperature, times 100. The gizmo does not show you the raw numbers for vapor density on every version, which is why students get tripped up when the question asks for a calculation rather than a slider adjustment. Use a standard saturation vapor pressure table if you need to verify a number the gizmo is hiding from you.
Common Questions and How to Solve Them
The activity usually asks three things: what happens to RH when temperature increases with constant moisture, what happens when you add moisture at constant temperature, and at what temperature dew forms. For the first one, the answer is always that RH decreases. Warm air can hold more moisture, so the same absolute amount of water vapor represents a smaller fraction of the saturation capacity. This is the question students get wrong most often because their intuition says adding heat should make things humid, but humidity in this context is a ratio, not an absolute quantity. For the dew point question, the gizmo typically gives you a starting temperature and a relative humidity value and asks you to find the dew point. The straightforward method is to cool the air in the simulation until condensation appears, then read that temperature. If you need it faster, you can look up the saturation vapor pressure corresponding to your starting RH and temperature, multiply by the RH fraction, and then find the temperature at which that vapor pressure becomes the saturation value. A rough rule of thumb that works well enough for classroom precision is that the dew point depression, which is the difference between temperature and dew point, is about 1 degree Celsius per 5 percent that RH falls below 100. It is not exact but it gets you into the right range for multiple choice answers. There is a second set of questions that deals with indoor air in winter. The gizmo shows outside air at a cold temperature being brought indoors and warmed. The absolute moisture stays the same, but the relative humidity plummets. This is why houses feel dry in January. The answer key for these questions always points to the indoor RH dropping, sometimes into the single digits if the outside air is very cold and unhumidified.
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A Real Problem I Ran Into and How I Fixed It
Last fall, a student emailed me saying the gizmo showed 70 percent relative humidity at 20 degrees Celsius, but when they calculated it using the standard saturation table, they got closer to 65 percent. The discrepancy was real. The gizmo uses a slightly rounded saturation vapor pressure curve internally, and the activity sheet they were given was keyed to a different reference table. The workaround was simple: I told the student to trust the gizmo's displayed value for the simulation questions and use their calculated value only when the question explicitly asked for a computation from first principles. If both answers appeared as options, the gizmo's built-in number was the one the answer key expected. This happened with about one in every ten classes using the older version of the activity sheet, and the fix saved me from grading disputes that otherwise would have taken twenty minutes to sort out. The biggest limitation of relying on the gizmo answer key is that the simulation abstracts away latent heat release during condensation. When water vapor condenses, it releases energy, and in a real atmosphere that warming can slow further cooling. The gizmo does not model that feedback. If your course requires you to understand adiabatic processes or the wet adiabatic lapse rate, this tool will give you a superficially correct RH number but the wrong physical picture. For those topics, you are better off using a psychrometric chart or a dedicated meteorology simulation. Another issue is the timer. ExploreLearning times the activity, and the default settings can make the lab feel rushed. Students who are still learning the interface often burn through their time on navigation instead of reasoning. I recommend turning off the timer in the teacher settings if you are assigning this as a formative exercise. It cuts the stress without affecting the learning outcome, and it aligns with how the answer key is structured anyway.
The answer key also assumes a single correct path for certain open-ended questions. Some prompts ask students to describe a scenario, and the key has bullet points rather than full sentences. If a student writes a technically correct description that does not match the exact wording in the key, the auto-grader will mark it wrong even though the concept is sound. I have seen this happen frequently with questions about why RH drops when warm air moves indoors. The concept is identical, but the phrasing mismatch costs points.
What to Do If You Cannot Access the Official Key
If you are a student without a teacher account, the practical route is to work through the questions using the logic I outlined above. The patterns repeat across versions. Temperature up with constant moisture means RH down. Moisture added at constant temperature means RH up. Dew point is the temperature where RH reaches 100 percent. Memorize those three relationships and you can answer nearly every question without looking at a key. If you are a teacher trying to verify your own key, cross reference the ExploreLearning teacher resources page with your LMS export. The numbers in the exported report should match the key if you are using the current build. Any mismatch usually means the class was running an older cached version of the simulation, and reloading the activity from the dashboard fixes it.
