Connecting the Dots... · Lesson 9 of 13
Summer and Winter at the Same Time
“Convert monthly daylight totals into daily means and explain opposite seasonal patterns.”
• Calculate daily means from monthly daylight totals. • Account for unequal month lengths. • Read and compare seasonal clustered graphs. • Explain the opposite patterns of Helsinki and Wellington. • Distinguish daylight from cloud-free sunshine.
Summer and Winter at the Same Time
Two cities can have opposite seasonal patterns. In one, daylight increases towards the middle of the year; in the other, it decreases. The source provides total daylight hours for each month. To compare a typical day in different months fairly, first convert each monthly total into a daily average.
The time during which the Sun is at least partly above the horizon. It is not the same as uninterrupted bright sunshine, which can be reduced by cloud.
| Month | Days used | City 1 daylight total (hours) | City 2 daylight total (hours) |
|---|---|---|---|
| Jan | 31 | 210 | 459 |
| Feb | 28 | 257 | 384 |
| Mar | 31 | 372 | 381 |
| Apr | 30 | 441 | 327 |
| May | 31 | 536 | 304 |
| Jun | 30 | 564 | 276 |
| Jul | 31 | 555 | 295 |
| Aug | 31 | 465 | 318 |
| Sep | 30 | 394 | 369 |
| Oct | 31 | 310 | 409 |
| Nov | 30 | 222 | 435 |
| Dec | 31 | 186 | 468 |
Months have different numbers of days. A 31-day month can have a larger daylight total than a 30-day month even when the mean day is shorter. In this activity we use 28 days for February, as in a non-leap-year calculation. If a specified year has 29 February days, the denominator must change.
Problem
Convert 564 hours of June daylight into a daily mean.
- 1.June has 30 days.
- 2.Divide 564 ÷ 30 = 18.8 hours per day.
- 3.As a fraction of a 24-hour day, 18.8 ÷ 24 ≈ 0.783, a little more than three quarters.
Problem
Compare City 1’s January and February daily daylight.
- 1.January: 210 ÷ 31 ≈ 6.77 hours per day.
- 2.February: 257 ÷ 28 ≈ 9.18 hours per day.
- 3.The mean day has approximately 9.18 − 6.77 = 2.41 more daylight hours in February. Use the actual day counts, not a fixed divisor of 30.
| Month | City 1 mean (hours/day) | City 2 mean (hours/day) |
|---|---|---|
| Jan | 6.77 | 14.81 |
| Feb | 9.18 | 13.71 |
| Mar | 12 | 12.29 |
| Apr | 14.7 | 10.9 |
| May | 17.29 | 9.81 |
| Jun | 18.8 | 9.2 |
| Jul | 17.9 | 9.52 |
| Aug | 15 | 10.26 |
| Sep | 13.13 | 12.3 |
| Oct | 10 | 13.19 |
| Nov | 7.4 | 14.5 |
| Dec | 6 | 15.1 |
Identify what is given
Read the months along the horizontal axis and average daylight hours per day along the vertical axis. The paired bars compare the same month in two cities. Gridlines five hours apart provide a scale for both series, and the values between them must be read proportionally.
For City 1, June is the maximum, about 18.8 hours per day, and December is the minimum, exactly 6 hours per day. City 2’s June minimum is 276 ÷ 30 = 9.2 hours; its December maximum is 468 ÷ 31 ≈ 15.10 hours. Both series have variation, but the extent of variation is greater in City 1.
Problem
How different are the two cities in June?
- 1.City 1 mean daylight is 18.8 hours per day.
- 2.City 2 mean daylight is 276 ÷ 30 = 9.2 hours per day.
- 3.Difference = 18.8 − 9.2 = 9.6 hours per day. The cities experience very different daylight durations during the same month.
Infer from what is given
The opposite patterns suggest that the cities lie in different hemispheres. The source identifies City 1 as Helsinki in Finland and City 2 as Wellington in New Zealand. Their locations help explain the seasonal contrast, rather than merely giving the pattern a name.
Earth’s axis is tilted. During June, the Northern Hemisphere is tilted towards the Sun and has longer days, while the Southern Hemisphere is tilted away and has shorter days. Around December the seasonal situation reverses. The effect becomes more pronounced farther from the Equator; the two cities are at different distances from it.
The source extends the investigation with a photograph from Andøya, Norway. Far enough towards a pole, the summer Sun can remain above the horizon even at midnight. This is an extension of the daylight-duration pattern, not a claim that every northern city has continuous daylight.
Do not confuse monthly hours with hours per day, and do not call daylight bright sunshine. A chart of daylight duration does not provide cloud-cover data.
Quiz
How do we obtain June’s daily mean from 564 hours?
City 1’s June mean is:
Which explains why a fixed divisor of 30 is inappropriate for all months?
Opposite June–December daylight patterns suggest:
Does daylight duration measure cloud-free sunshine?
Practice Problems
- Calculate December daily daylight for both cities.
- Calculate June daylight as a fraction of a full day for City 2.
- Which city has the larger range of monthly daily averages?
- If a February total is 257 hours in a leap year, what denominator is needed?
- Explain why opposite hemispheres can have summer and winter at the same time.
- Propose a further investigation prompted by the graph.
City 1: 186 ÷ 31 = 6 hours/day. City 2: 468 ÷ 31 ≈ 15.10 hours/day. The difference is approximately 9.10 hours/day.
Key Takeaways
• Convert monthly totals using the number of days in that month. • The daily mean is measured in hours per day. • Daylight duration differs from bright-sunshine duration. • Read paired seasonal patterns on one shared scale. • Opposite hemispheres have opposite seasonal daylight patterns. • Use data to ask further questions while respecting its limits.