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Almond Orchards in a Dry Winter: Following the Storm Belt

Use rain records and a model of wind belts to explain a farm town's climate.

Answer Key

Oakbend's rain and the wind belt model

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

Rain Gauge Records at Oakbend

1Oakbend is a farm town in the Central Valley of California, at about 38°N. The town and its numbers are made up, but they are based on real records from the region. Kaveh Morrison's aunt, Mrs. Beatriz Lindqvist, grows almonds there and has kept a rain gauge for 30 years. Last winter was one of the driest she has seen, so she had to pump much more water from her well. Kaveh, a sixth grader, wanted to know why the rain at Oakbend comes when it does, and why some winters bring so little.

2His science teacher shared a model of how air moves around Earth. Sunlight strikes Earth most directly near the equator, so the air there is heated the most. The warm air rises, cools and drops heavy rain. High above the ground, this air spreads out toward the poles. Around 30° north and south of the equator, much of it sinks. Sinking air warms and dries, so few clouds form there, and many of the world's large deserts lie under these belts.

3Near the ground, some of the sinking air flows back toward the equator and some flows toward the poles. Because Earth rotates, air that moves a long way across its surface curves. In the Northern Hemisphere, it curves to the right; in the Southern Hemisphere, it curves to the left. Air flowing from about 30°N toward the North Pole curves toward the east, so between about 30°N and 60°N, winds near the ground usually blow from the west. These westerlies carry storms from the Pacific Ocean onto the west coast of North America.

4The belts are not fixed. During the year they shift partway north and south, following the latitude where the Sun's noon rays are most direct. That latitude is north of the equator from late March to late September and south of it for the rest of the year. In some dry winters, a large area of sinking air and high pressure stays over the ocean west of California for weeks, and the storms carried by the westerlies pass farther north.

Table 1. Monthly Rain at Oakbend (mm)
Month30-year averageLast year (dry)
October255
November5010
December7520
January8515
February8030
March6025
April3015
May125
June30
July00
August10
September50
1.

In an average year, Oakbend gets 350 mm of rain from November through March, and only 4 mm from June through August.

2.
Use paragraphs 2 and 4. Which explanation of Oakbend's dry summers fits the model best?
  • A. In summer, strong heating makes air rise over the valley and push the clouds away.
  • B. In summer, the westerlies turn to blow from the east and carry dry air from inland deserts.
  • C. Earth is closest to the Sun in summer, so the extra heat dries up the clouds.
  • D. In summer, the belt of sinking, drying air near 30°N shifts north over California.
3.
Use paragraphs 2 and 3 to complete the model. Use each word once. Two words in the bank are left over.
Word bank: equator · rotation · sinks · west · east · poles

Sunlight is most direct near the equator, so the air there is heated and rises. High up, the air spreads out, and much of it sinks near 30°N. Near the ground, air flowing from there toward the North Pole is turned by Earth's rotation, so at Oakbend's latitude the winds usually blow from the west.

The dry winter along the coast

Use Table 1, Table 2 and paragraph 4.

Table 2. January Rain Along the West Coast (mm)
PlaceLatitude30-year averageLast year (dry)
Southern California town33°N505
Oakbend (Central Valley)38°N8515
Northern California coast41°N180110
Oregon coast45°N250310
Washington coast48°N280370
4.
Use Table 1. First add up the yearly totals. Round the percent to the nearest whole number.

Last year, Oakbend got 125 mm of rain in all. That is about 29% of its 30-year average yearly total.

5.
Which statement best describes Table 2?
  • A. The two northern places got less January rain than usual, while the three southern places got more.
  • B. The Northern California coast had the largest drop in January rain, from 180 mm down to 50 mm.
  • C. Places at 41°N and south got less January rain than usual, while the two places farther north got more.
  • D. Oakbend got less January rain than usual, but the other four places stayed within 10 mm of average.
6.
Which explanation of the pattern in Table 2 fits paragraph 4 best?
  • A. The westerlies turned around and blew from the east, carrying dry inland air to the whole coast.
  • B. Sinking air and high pressure stayed off California, so the westerlies carried storms farther north.
  • C. Storms formed over the land in Oregon and Washington instead of over the ocean, so they stayed north of 41°N.
  • D. The Sun heated the equator less that winter, so fewer storms formed over the entire Pacific Ocean.
7.
Use paragraphs 2–4. Sort each part of the model by what it helps explain at Oakbend.
The belt of sinking air sits farther north in July and August.Westerlies carry Pacific storms onto the west coast.In December, the Sun's most direct noon rays are south of the equator.Sinking air warms and dries, so few clouds form under it.The storm track reaches 38°N when the belts shift south.In July, the Sun's most direct noon rays are north of the equator.
Wet wintersDry summers
Westerlies carry Pacific storms onto the west coast.
In December, the Sun's most direct noon rays are south of the equator.
The storm track reaches 38°N when the belts shift south.
The belt of sinking air sits farther north in July and August.
Sinking air warms and dries, so few clouds form under it.
In July, the Sun's most direct noon rays are north of the equator.

Use the model somewhere else

Apply the model from paragraphs 2–4 to a farm region in the Southern Hemisphere.

8.
Central Chile is on the west coast of South America at about 35°S, nearly as far south of the equator as Oakbend is north of it. Its farms also depend on rain from storms carried by westerlies. In which months does central Chile most likely get most of its rain?
  • A. June to August, when the belts shift north and the storm track reaches 35°S
  • B. October and November, when the Sun's noon rays first move south of the equator
  • C. March and September, when the Sun's noon rays are most direct at the equator
  • D. December to February, the same months as Oakbend's wettest season
9.
Use paragraph 3. From which direction do central Chile's winter storms most likely arrive?
  • A. From the west, because air moving toward the South Pole curves left, toward the east
  • B. From the east, because air in the Southern Hemisphere curves the opposite way
  • C. From the south, because cold air near the South Pole pushes storms toward the equator
  • D. From the north, because the storms start near the equator and move straight south
10.
A neighbor says, "Last winter was dry because the Pacific Ocean made fewer storms. The whole West Coast must have had a drought." Do you agree? Write a claim, give at least two pieces of evidence from Table 2, and explain your reasoning using the model.

I disagree. Claim: The storms did not disappear; the westerlies carried them farther north than usual. Evidence: Last January, Oakbend got 15 mm of rain instead of its 85 mm average, and the Southern California town got 5 mm instead of 50 mm. But the Oregon coast got 310 mm instead of 250 mm, and the Washington coast got 370 mm instead of 280 mm. Reasoning: Paragraph 4 says that in some dry winters, sinking air and high pressure stay off California for weeks. The westerlies, which blow from the west because Earth's rotation turns air moving toward the pole to the east, then carry Pacific storms around the north side of that area. So the northern coast was wetter than usual while California was dry, which would not happen if the ocean simply made fewer storms.