“Today’s weather will be wet, so don’t forget your umbrella… It’s a dry one today… Moisture from the ocean will make today a pleasant one… Those hot, dry, Santa Ana winds blowing from inland will raise today’s fire danger level…” So the TV or radio weather report might go.

But why wait for the report? You don’t need a fancy weather station to track the weather; you can do it yourself!

Today’s lesson: building and using a HYGROMETER to measure the humidity.

What you’ll need:

• A glass jar—one with straight sides (tube shaped)
• A ruler—preferably with a millimeter scale
• Water
• Patience—as scientific observation often needs!

What you’ll do:

1. Fill the jar about halfway with water.

2. With a permanent marker, carefully draw a horizontal line on the jar at exactly the top level of the water.

3. On a notepad, write down the date and time that you filled the jar.

4. Set the jar somewhere that it won’t be disturbed (and where your pet won’t drink it).

5. The next day, at exactly the time that you filled the jar the first day, look to see where the water level is. If it is below the line you drew, use the ruler to measure how far the water went down.

6. On your note pad, write down the date, the time, and how far you measured the water to drop (for example, if you measure 4 millimeters between the water level and the marked line, write down 4 mm; if it didn’t seem to drop at all, write down 0 mm).

7. Also on your note pad, write down a few words describing the weather. For example, “sunny,” “rainy,” “cloudy,” “windy and warm,” and so on.

8. Now, carefully pour water into the jar, filling it back to the line you drew the first time you filled it.

9. Now, make the same measurement everyday for a week (or longer if you like); measure how much the water level fell, write it down with a few words describing the weather, and then carefully fill the jar back to the original line.

Did the water level drop by the same amount everyday, or was it sometimes different? Were there days it didn’t seem to drop at all? Do you see a connection between the numbers you wrote down and your descriptions of the daily weather? If so, what connections do you see?

What’s happening?

You have built a “hygrometer.” Hygrometers measure the humidity of the air—that is, how much water is held by the air, as a vapor (or gas). On days of high humidity, when there is a lot of water vapor in the air, conditions are moist; wet objects don’t dry as quickly; clouds and fog are more likely to form; some peoples’ hair gets frizzy! On days with less humidity, things dry out quickly, our lips can get chapped, and we can get thirsty more easily.

With your hygrometer, on humid days the water doesn’t evaporate easily, and so less water is lost from the jar; on dry days, the water evaporates easily and the water level will drop more quickly. So, measuring how much water has evaporated in a day is a good way of measuring humidity.

What OTHER things about the weather could you measure and record on your note pad? Hmmmm….