Rain Gauge Readings Across a Schoolyard
On a rainy day, students place four identical rain gauges around the schoolyard at 8:00 AM: Gauge 1 in the open field, Gauge 2 under a large oak tree, Gauge 3 against the wall of the building on the windy side, and Gauge 4 on the roof. At 3:00 PM they read: Gauge 1 = 22 mm, Gauge 2 = 6 mm, Gauge 3 = 31 mm, Gauge 4 = 24 mm. A student claims, "It rained more near the building than in the field, so the building must attract rain." Another student wants to report a single number for the whole schoolyard.
1. Explain why the four gauges recorded different amounts even though they sat in the same storm. Which reading is least trustworthy as a measure of how much rain fell, and why?
Read the worked answer
The gauges sat in the same storm, so the differences come from where they were placed, not from different rainfall.
- Gauge 2 (under the oak, 6 mm): the tree's leaves and branches blocked rain from reaching the gauge. This is the least trustworthy reading - it measures rain that got past the tree, not the rain that fell.
- Gauge 3 (windy wall, 31 mm): wind blowing against the wall pushed extra slanted rain into the gauge, inflating the catch.
- Gauge 4 (roof, 24 mm): higher and more exposed, catching slightly more than the open field.
- Gauge 1 (open field, 22 mm): the most representative, since nothing blocked or funneled rain into it.
A strong answer ranks Gauge 1 as the best single estimate and explains each odd reading by its surroundings.
2. The second student wants one number to represent the schoolyard. Which value should they report, and how would you defend that choice to the class?
Read the worked answer
Report Gauge 1's 22 mm as the best estimate of actual rainfall.
Defense: Gauge 1 was in the open with no tree overhead and no wall to funnel wind-driven rain, so it caught rain the way the ground does. The other readings are distorted by their locations: the tree removed rain (6 mm), the wall added wind-blown rain (31 mm), and the roof was more exposed (24 mm).
A strong answer explains that averaging all four would be misleading, because three of the values are biased by obstacles. If the class wanted a true average, they would first move all gauges to open, unobstructed spots and repeat the measurement.
3. The class wants to know whether the roof really catches more rain than the field. Describe a fair test, naming what to change and what to keep the same.
Read the worked answer
Change: the location - one gauge on the roof, one in the open field.
Keep the same: identical gauge design and size, same start and end times, same storm, and both spots open to the sky with nothing overhead.
Measure: the depth of rain collected in each gauge.
Reasoning: With identical gauges and the same time window, any difference in catch must come from the location (height and exposure), not from the gauge or the timing. Repeating across several storms makes the result more reliable.
A strong answer notes that the field gauge should be placed well away from the building so the wall does not funnel rain into either gauge.