Reading passage Grade 8 Health & Consumer Science
Why Coach Ibarra Moved Practice: Sweat, Humidity, and Heat
A passage about a fictional cross country team explains how sweat cools only when it evaporates, why humidity slows that, and the warning signs of heat illness.
Objectives
- Students will explain, citing the passage, why sweat cools the body only when it evaporates and why humidity slows that cooling.
- Students will distinguish the warning signs of heat exhaustion from those of heat stroke and state the correct response to each.
- Students will connect evaporation from skin to evaporation in the water cycle.
Vocabulary
- Evaporation
- Evaporation happens when liquid water, often warmed by the sun, turns into an invisible gas called water vapor that rises into the air. It is how water leaves oceans, lakes, and puddles.
- Humidity
- Humidity is the amount of water vapor floating in the air. Air that feels sticky and damp has high humidity, while dry air has low humidity.
- Water vapor
- Water vapor is water in the form of an invisible gas mixed into the air. There is always some water vapor around you, even when the sky is completely clear.
Also: heat exhaustion, heat stroke, dehydration
Teacher overview
This passage follows the fictional Orlan Bay Middle School cross country team on a humid August afternoon when their coach moves practice to the next morning. Through the coach's explanation, students learn that sweat cools the body only when it evaporates, why humid air slows that evaporation, and how to recognize and respond to heat exhaustion and heat stroke. The last paragraph connects evaporation from skin to evaporation in the water cycle. Teaching note: a common misconception is that heavy sweating means the body is cooling itself well. On a humid day heavy sweat can signal the opposite, because sweat that drips off without evaporating removes little heat, and students who believe the misconception may keep exercising when they should stop.
Passage
The Orlan Bay Middle School cross country team was supposed to run hill repeats at four o'clock on a Tuesday in late August. At lunch, Coach Ibarra checked the school's weather station. The air temperature was only a little higher than it had been all month. The humidity was much higher. By the time the team met in the gym after school, Coach had changed the plan. Practice would move to seven the next morning, and that afternoon the runners would stretch indoors.
Some of the runners grumbled. "It's not even that hot," said Mateo. Coach Ibarra wrote one word on the whiteboard: evaporation. Then she explained why the temperature alone did not tell the whole story.
When you run, your muscles make a lot of heat. Your body has to get rid of that heat, or the temperature deep inside you climbs. Your main tool is sweat. Sweat glands push water and salt out onto your skin. Sweat by itself does not cool you, though. The cooling happens when the sweat evaporates, which means it changes from liquid water into water vapor, a gas.
Evaporation takes energy. The water molecules in a drop of sweat are always moving, and some move faster than others. The fastest ones break away from the drop and escape into the air as vapor. They carry energy with them, and they take that energy from your skin. That is why you feel a chill when you step out of a pool into a breeze. The same thing happens, more slowly, every time sweat dries on your arms.
Humidity is a measure of how much water vapor is already in the air. On a dry day, the air can take in much more vapor, so sweat evaporates quickly and carries heat away. On a humid day, the air near your skin already holds a lot of vapor. Molecules still escape from your sweat, but many others move from the air back into the sweat, so the drop shrinks slowly. Sweat that does not evaporate cannot cool you. It drips off your forehead and soaks your shirt, and your body keeps making more.
"That is why a runner can be dripping with sweat and still be overheating," Coach said. "On a humid day, heavy sweat can be a sign that your cooling system is struggling."
Heavy sweating also drains water and salt from the body. Blood carries heat from the muscles to the skin, where it can escape. A runner who loses too much water has less blood to do that job, so overheating becomes even more likely. Thirst can lag behind the body's needs during hard exercise. Coach Ibarra asked the team to drink water before practice, take sips at every break, and drink again afterward.
Then she described two kinds of heat illness. Heat exhaustion often comes first. Its signs include heavy sweating, a headache, dizziness, an upset stomach, muscle cramps, and weakness. A runner with these signs should stop, tell the coach, move to shade or air conditioning, drink water, and cool off with wet towels or a fan. If the signs get worse or last more than an hour, the runner needs medical help.
Heat stroke is a medical emergency. The body's temperature rises so high that the brain begins to be affected. The clearest warning sign is a change in behavior, such as confusion, stumbling, slurred speech, or acting strangely. Some people with heat stroke stop sweating, but many athletes are still sweating when it begins, so sweat is not a safe sign. If a teammate shows these signs, call 911 and start cooling the person right away with cold water while help is on the way.
The next morning, the runners met at seven. The air was cooler, and the humidity had dropped. Coach still set a cooler of water at the bottom of the hill and scheduled a break after every two repeats. Mateo noticed that his shirt was damp but not soaked, and that the sweat on his arms dried in the breeze.
On the bus home from the next meet, Mateo looked out at the low clouds over the bay. The water that had evaporated from the team's skin that week was out there somewhere as vapor in the air. Some of it might already be part of those clouds. The same process that cooled his body lifts water from the bay into the sky, where the vapor cools, condenses into droplets, and falls again as rain.
Questions
According to the passage, what has to happen to sweat for it to cool the body?
Using details from the passage, explain why sweat evaporates more slowly on a humid day than on a dry day.
Name two changes Coach Ibarra made to the team's plans because of the humidity.
A runner says, "I'm dripping with sweat, so my body must be cooling me well." Use evidence from the passage to explain what is wrong with this thinking.
What warning sign does the passage say most clearly separates heat stroke from heat exhaustion?
The passage says that losing too much water makes overheating more likely. Explain the connection, using what the passage says about blood.
Why might a fan or a breeze help a runner cool down on a humid day? Use what the passage says about the air near the skin.
How does the last paragraph connect sweating to the water cycle?
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Answer Key
Answer: The sweat has to evaporate, changing from liquid water into water vapor.
The passage states that sweat by itself does not cool you and that the cooling happens when the sweat evaporates.Answer: On a humid day the air near the skin already holds a lot of water vapor, so many molecules move back into the sweat and the drop shrinks slowly.
The passage explains that dry air can take in much more vapor, while humid air cannot, so evaporation slows.Answer: She moved practice to seven the next morning and replaced the afternoon run with indoor stretching.
Accept also the cooler of water at the bottom of the hill and the break after every two repeats, which she arranged for the morning practice even though the air was cooler by then.Answer: Heavy sweat does not prove the body is cooling, because sweat cools only when it evaporates.
The passage says sweat that does not evaporate drips off and soaks the shirt, and the coach explains that a runner can be dripping with sweat and still be overheating. On a humid day, heavy sweat can mean the cooling system is struggling.Answer: A change in behavior, such as confusion, stumbling, slurred speech, or acting strangely.
The passage calls this the clearest warning sign of heat stroke and notes that sweating is not a safe sign either way.Answer: Blood carries heat from the muscles to the skin, and a runner who has lost too much water has less blood to do that job.
With less heat reaching the skin to escape, the body's temperature rises more easily, as the paragraph about water and salt explains.Answer: Moving air replaces the damp air next to the skin with air that can take in more vapor, so sweat can evaporate faster.
This is an inference. The passage says humid air near the skin already holds a lot of vapor and that a breeze makes you feel a chill after swimming. Accept answers that link moving air to faster evaporation.Answer: It shows that sweat that evaporates becomes water vapor in the air, the same process that lifts water from the bay into clouds.
The paragraph says the vapor from the team's skin may join the clouds, then cool, condense into droplets, and fall as rain.
Differentiation
Support
Provide a two-column chart with "What happens to sweat on a dry day" and "What happens to sweat on a humid day" for students to fill in while reading paragraphs four and five. Read the heat exhaustion and heat stroke paragraphs aloud and have students list the signs of each in a T-chart before answering questions 5 and 6.
Extension
Ask students to design a one-page heat plan for a school practice that uses both temperature and humidity to decide when to move, shorten, or cancel a session. The plan should state who checks conditions, what breaks and water are required, and what each coach does if a runner shows signs of heat illness.
ELL supports
Pre-teach "evaporate," "humidity," "vapor," "exhaustion," and "confusion" with a simple diagram of a sweat drop losing its fastest molecules to the air. Allow literal questions 1, 3, and 5 to be answered with phrases, and provide a sentence frame for question 4: "This thinking is wrong because the passage says ___."
Extensions
Students can record the temperature and humidity at their own school for a week and note which afternoons would call for the kind of change Coach Ibarra made. An athletic trainer or school nurse can be invited to describe the school's own plan for hot, humid practices and how staff respond to a suspected case of heat stroke.