A First Course in Physics · Ch. V, §§ 133–150 · Chapter 5: Molecular Motions
15. Dew, fog, humidity, and cooling by evaporation
Goal: Your student can explain dew, fog, clouds and the dew-point, find relative humidity from the table, and explain why evaporation cools.
You'll need
- The book, open to these pages
- A shiny metal can or cup, ice cubes and water
- A kitchen or outdoor thermometer (alcohol type, not mercury)
- A small piece of cloth or a cotton ball and a rubber band
- A fan or a piece of cardboard to fan with
- A notebook and pencil
- A calculator (optional, for the problems)
What's in the book
Condensation of water vapor; dew, fog, clouds, rain, hail and snow; finding the dew-point with ice water in a polished cup; relative humidity worked out from the vapor table; cooling by evaporation; freezing by evaporation; the wet-and-dry-bulb hygrometer; evaporation of solids; the three states of matter. Ten questions and problems follow.
The lesson, step by step
Hook question
SayOn a hot day, why does a glass of iced lemonade get wet on the outside? Did it leak?
Read together
Read §§ 133–150. The dew-point:
The temperature to which the atmosphere must be cooled in order that condensation may begin is called the dew-point.
Try it: find the dew-point
Fill the shiny can halfway with water and the thermometer. Add ice a piece at a time, stirring gently with the thermometer. Watch the outside: the moment it fogs, read the thermometer. That is the dew-point (§ 137). Write down the room temperature too.
Work out the humidity
Use the table on page 95 (P column) like the example in § 138. If the dew-point was 15° C and the room 25° C: 12.7 ÷ 23.5 = 0.54, so the relative humidity is 54%. Do the same with your own numbers.
Try it: a wet-bulb thermometer
Read the thermometer. Wrap its bulb in a wet cloth and fan it for a minute: the reading drops. The fastest molecules escape, so the ones left behind are slower, and slower means colder (§ 141). On a very humid day the drop is small. Fan a dry thermometer: no change at all (§ 143).
Watch only: ether and iodine
Watch only: don't try this at home. §§ 140–145 use ether (very flammable) and an air pump, and § 148 heats iodine crystals, whose vapor is harmful. Describe the results: ether cools so fast it can freeze water, and iodine turns straight from solid to purple vapor (sublimation).
Say it back
Your student explains how dew, fog, clouds and snow are all the same process happening in different places.
Problems
Work questions 1–10 on book page 106. Question 7 uses the table on page 95.
Afterwards
Check the dew-point and humidity in a weather app each morning this week, and look for dew on the grass when the night was clear and still.
Tip
An alcohol (red- or blue-liquid) thermometer or a digital one is perfect. Avoid old silver mercury thermometers.
Answers to the book's problems
1. Evaporation of the water takes heat from the air and street. 2. The air is nearly saturated, so sweat cannot evaporate to cool us. 3. No. 4. No; in saturated air nothing evaporates. 5. The porous vessel is cooler, because water seeps through and evaporates from its outside; the difference is greater in dry air. 6. The cold ice chills the moist air around it below its dew-point. 7. About 774 g (75 cu. m × 0.0000172 g per cc × 0.60). 8. The vapor collects in the narrow neck, so the air above is nearly saturated and evaporation is slow; the shallow dish has a large open surface. 9. The sun warms the air above its dew-point and the droplets evaporate. 10. Winter air indoors is dry, so its dew-point is lower than the temperature of the pitcher.
Worked out for this site from the scan. If a number in your copy differs, trust the book.