Hot Blacktop: Monitoring and Cooling a Schoolyard

Use sensor data to measure how paving heats a schoolyard, then judge ways to cool it.

Answer Key

Monitoring the heat

Read the passage and Table 1, then answer the questions.

The Schoolyard Heat Study

1Palo Seco Middle School is in a desert city in the southwestern United States. The school and its numbers are made up, but they are based on real measurements in hot, sunny cities. The school's weather club keeps a weather station on the roof and checks a forecast app each morning. Last September, the app showed a heat warning on 9 school days, and the nurse saw several students with red, sore hands after they fell on the blacktop play area.

2When people replace soil and plants with pavement and roofs, they change the land's surface. Dark pavement absorbs most of the sunlight that hits it, gets very hot and warms the air above it. Plants and moist soil stay cooler, because water evaporating from leaves and soil carries energy away, and trees also shade the ground. Hot pavement keeps giving off heat after sunset, so the air near it can stay warm at night. A city that is warmer than the land around it is called a heat island.

3Club member Zofia Benally and her teammates set up four sensor stands around the schoolyard, at sites with different surfaces. At each stand, they measured the surface temperature with an infrared thermometer and the air temperature 1 m above the ground inside a white, shaded shield. They took readings at 3 p.m. and at 5 a.m. on five sunny days in September and found the average of each set.

4The principal, Dr. Marguerite Osei, asked the club to help choose a way to cool the 2,000 m² play area. The school set three requirements. By next September, the play area's 3 p.m. surface reading on sunny days must be 45 °C or less. The project must cost $65,000 or less. It must use no extra city water. The club tested each idea on a small patch of blacktop and estimated the cost for the whole area. Young trees grow slowly, so their patch reading is what the club expects next September; full shade would take about 10 years.

Table 1. September Averages, Five Sunny Days
Site	Surface	Surface, 3 p.m. (°C)	Air, 3 p.m. (°C)	Air, 5 a.m. (°C)
A. Play area	Black asphalt, full sun	62	40	29
B. Gravel garden	Gravel and shrubs, sun	51	38	26
C. Sports field	Watered grass, full sun	37	37	24
D. Tree grove	Bare soil, tree shade	34	36	25

At 3 p.m., the play area's surface was 25 °C hotter than the sports field's surface, but the air above it was only 3 °C warmer.

Which statement best describes Table 1?

A. Asphalt and gravel got much hotter than grass and shaded soil, and the play area's air was warmest.
B. At each of the four sites, the air 1 m up was within 2 °C of the surface temperature at 3 p.m.
C. The grass surface was hotter than the gravel surface, because watered grass soaks up more sunlight.
D. The tree grove had the hottest surface at 3 p.m. and also the coolest air of the four sites at 5 a.m.

Use paragraph 2. Why was the air at the play area still the warmest at 5 a.m., long after sunset?

A. The asphalt kept giving off heat that it absorbed during the day.
B. The asphalt makes heat of its own at night, without any sunlight.
C. Water evaporating from the asphalt at night warmed the air above it.
D. The play area got the most sunlight at 5 a.m., since it has no trees.

Why did the club use the same kind of sensor stand, the same times of day and only sunny days at all four sites?

A. So the tree shade would reach the play area stand
B. So the sensors would warm up and give higher readings
C. So differences would come mainly from the surfaces
D. So they could skip the 5 a.m. readings at some sites

Underline each piece of evidence that paving the play area changed the schoolyard's local environment.

The asphalt surface averaged 25 °C hotter than the grass surface at 3 p.m.The forecast app showed a heat warning on 9 school days for the whole city.The air at the play area stayed 4 to 5 °C warmer than at the grass and trees at 5 a.m.The club took its readings at 3 p.m. and 5 a.m. on five sunny days in September.The air above the play area was the warmest of the four sites at 3 p.m.

Choosing a design

Use Table 2 and the three requirements in paragraph 4.

Table 2. Test Patches and Cost Estimates
Idea	Surface, 3 p.m. (°C)	Cost, whole area ($)	Extra city water?
Plain blacktop (no change)	62	0	No
1. Light-colored cool coating	50	30,000	No
2. Shade sails over the area	38	60,000	No
3. Shade sails plus cool coating	35	90,000	No
4. Replace with watered grass	36	50,000	Yes
5. Young trees in planters	55	20,000	No (rain tank)

Use paragraph 4 and Table 2. Sort each idea. Each idea that misses fails only one requirement.

Idea 1: cool coatingIdea 2: shade sailsIdea 3: sails plus coatingIdea 4: watered grassIdea 5: young trees

Meets every requirement	Too hot next September	Costs too much	Needs extra city water
Idea 2: shade sails	Idea 1: cool coating Idea 5: young trees	Idea 3: sails plus coating	Idea 4: watered grass

Compared with plain blacktop, the shade sails cooled the patch's surface by 24 °C. Adding the cool coating under the sails cooled it 3 °C more, for an extra cost of $30000.

A parent group gives the school $25,000, so the budget grows to $90,000. The other two requirements stay the same. Which idea now meets every requirement and gives the coolest surface?

A. Idea 4, because its 36 °C surface is cooler than the sails alone
B. Idea 5, because grown trees will shade the area for decades
C. Idea 2, the sails alone, because they meet the 45 °C goal
D. Idea 3, the sails plus coating, with a 35 °C surface

What is the main drawback of Idea 5, the young trees, for this year's goal?

A. Their shade will be small next September, so the surface would reach about 55 °C.
B. They would leave the surface even hotter than the plain blacktop with no change.
C. They would use more city water each week than the watered grass in Idea 4.
D. They would cost $20,000 more than the shade sails in Idea 2.

Did the sails work?

The school chose Idea 2. The next September, the club repeated its 3 p.m. surface readings on five sunny days.

Table 3. 3 p.m. Surface Readings, Sunny-Day Averages (°C)
Site	September before	September after
A. Play area (sails added)	62	37
B. Gravel garden (no change)	51	48
C. Sports field (no change)	37	34

10.

Dr. Osei says, "The sails cooled the blacktop surface by 25 °C." Which response do the data in Table 3 support best?

A. She is right, since the play area dropped from 62 °C to 37 °C after the sails went up.
B. The sails cooled it by about 22 °C, since the unchanged sites also cooled by 3 °C.
C. The sails cooled it by about 3 °C, the same amount as the sites with no change.
D. The sails cooled it by about 28 °C, since this September was 3 °C warmer than last.

11.

A parent says, "Blacktop only matters for the few seconds when a kid touches it. It does not change the schoolyard's environment." Do you agree? Write a claim, use at least two pieces of data from Table 1 or Table 3 as evidence, and explain your reasoning.

I disagree. Claim: Paving the play area made that part of the schoolyard hotter, day and night, and the shade sails showed that this impact can be measured and reduced. Evidence: At 3 p.m., the asphalt surface averaged 62 °C, 25 °C hotter than the grass, and the air above it was 40 °C, the warmest of the four sites. At 5 a.m., the air at the play area was still 29 °C, 4 to 5 °C warmer than at the grass and the trees. After the sails went up, the play area's surface dropped to 37 °C, while the unchanged sites cooled by only 3 °C. Reasoning: Dark asphalt absorbs sunlight, heats the air above it and keeps giving off heat after sunset, so the area stays warmer for many hours, not just for a few seconds. That warming is a change to the local environment caused by replacing soil and plants with pavement.