Evaporative Cooling: Tree Sweat, Bottle Sweat
Introduction: Evaporative Cooling: Tree Sweat, Bottle Sweat
Using infrared thermometers students investigate the cooling effects of evaporating water. To find examples, we don’t have to look further than, well, sweat. Sweat is critical to keeping our bodies cool when we exercise or spend any time in the heat. When the water in sweat evaporates from our skin it carries off heat. The same principle -- called evaporative cooling -- can be used to cool other things: water, food, even buildings. Some evaporative cooling technologies date back thousands of years, long before electric refrigerators or air conditioners were an option. Evaporative cooling also explains one of the ways trees and other plants can help cool the area around them.
Evaporative Cooling: Tree Sweat, Bottle Sweat was developed as part of the Science of Sweat Workshop, including investigations of how sweat, and more generally evaporating water, can cool things down, and the impacts that extreme heat can have on our bodies. Designed for out-of-school-time educators, the workshop is easily adaptable to other educational settings. Topics include evaporative cooling, the water cycle (including the role of energy), urban heat islands, and staying cool while outdoors. These topics are important to spending time outdoors in the heat, they bring up environmental justice and equity issues, and they provide a hands-on introduction to widely taught science concepts.
Supplies
* 1 or 2 zip-close plastic bag (quart-size, or any size available)
* 2 metal water bottles
* Unglazed clay flower pot (1 or 2 inches larger than the diameter of the water bottle)
* Cork or stopper (to fit the drainage hole of the flower pot, or tape over the hole with duct tape)
* Clean sand (approximately 3 to 4 cups)
* Optional: unglazed clay water bottle or pitcher (see resources section)
* Student Thermometers, preferably IR Thermometers
Note: IR (infrared) thermometers -- sometimes listed as non-contact thermometers, or “temperature guns” -- allow students to digitally measure the temperature of objects without touching the object. They are wonderful tools for science investigation and can be purchased for $10-$12 and up. Having several will allow students to work in small groups. Choose an IR thermometer that is designed for uses other than (or in addition to) checking body temperature. For example, an IR thermometer that is listed as useful for cooking, refrigeration, or swimming pools. 440
Many models have a built-in laser pointer -- this is helpful for aiming the thermometer, and most models allow you to turn this feature off. You can find models without the laser feature if you are not confident that your students can use a laser pointer safely.
Step 1: Tree Sweat?
You sweat. Trees sweat… sort of. For plants, it is called transpiring, at least in polite company.
Every leaf has tiny holes on the bottom that can open and close (called stomata). The holes let air in, so the plant can absorb gasses it needs from the air, and also lets the leaf release excess gases. Our lungs do the same thing for us.
When the stomata are open, water also evaporates from inside the leaf. Evaporating water carries heat away from the plant — cooling the plant. In fact, if there are enough plants, they can cool the area around them. What are other ways trees can keep you cool?
You can catch some tree sweat by putting a zip lock bag around a few leaves on a tree. It may take an hour or two (less on a sunny day) but you should start to see some condensing water on the inside of the bag. Lucky for you, tree sweat doesn't smell!
Use the IR thermometers to check the temperature of leaves, and compare them to other surfaces and materials.
Be Safe: Many IR thermometers have a built-in laser pointer to help you aim the thermometer. Never point the laser into your eye, or into the eye of another person or animal.
Step 2: Bottle Sweat
In many parts of the world, clay water jars are used to keep drinking water cool without a refrigerator. Unglazed clay lets a very small amount of water seep through (it is slightly porous). The water on the outside of the jar then absorbs heat from the jar as it evaporates, removing heat from the jar and cooling the water inside.
This phenomenon was used starting thousands of years ago in areas including India, parts of Africa, and the Near East. There are even ancient Egyptian drawings of people using fans to cool clay water jars.
You can buy a clay water jar or pitcher (see Resources), or follow the directions below to build a “Zeer”. 440