Fronts Without Rain: A Dry Summer at the County Fair

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Fronts Without Rain: A Dry Summer at the County Fair

Use a summer of front records to see when colliding air masses bring rain.

The summer front log

Read the passage and Table 1. Then answer questions 1–5.

A Dry Summer in Willow Creek County

1Willow Creek County is farm country in Nebraska, where families grow corn and soybeans and raise cattle. The county and its numbers are made up, but they are based on real summers on the Great Plains. This year, only 64 mm of rain fell from June through August, while an average summer brings about 250 mm. Sixth grader Anneliese Kowalczyk joined the youth weather team at the county fairgrounds, which keeps a weather station next to the cattle barns.

2Team leader Mr. Desmond Achterberg, who farms 300 hectares of corn, explained air masses. An air mass is a huge body of air with about the same temperature and humidity throughout. Air that sits over the Gulf of Mexico becomes warm and humid. Air over the high plains and mountains to the west is usually dry, and air from Canada is usually cooler and drier. The dew point is the temperature the air would have to cool to before its water vapor starts to condense, so a higher dew point means more water vapor in the air.

3A cold front is the edge of a cooler, denser air mass that is pushing in. Air rising over or ahead of a front cools as it rises. Six cold fronts crossed the fairgrounds this summer. For each one, the team recorded the noon dew point on the day before the front arrived, the noon dew point on the day after it passed, the wind direction in the two days before, and the rain. These six days brought all 64 mm of the summer's rain.

4Anneliese remembered the July 8 front well. The sky stayed clear all day. At about 3 p.m., the wind turned from the west to the northwest, and the noon temperature, 35 °C on July 8, was only 27 °C on July 9.

Table 1. Cold Fronts at the Fairgrounds
Front	Wind, 2 days before	Dew point before (°C)	Dew point after (°C)	Rain (mm)
June 3	South	20	9	34
June 22	West	10	5	1
July 8	West	8	3	0
July 27	West	11	6	2
August 14	South	19	10	27
August 30	West	9	4	0

Dew point before = noon on the day before the front. Dew point after = noon on the day after it passed.

1.

The two fronts that came after south winds brought mm of rain together. The other four fronts brought mm of rain together.

2.

Which statement best describes Table 1?

A. The rain from each front grew larger from June to August.B. The fronts with the most humid air ahead of them brought the most rain.C. Each front brought about 10 mm of rain to the fairgrounds.D. The fronts with the driest air behind them brought the most rain.

3.

Use paragraph 3. What best explains the heavy rain when the June 3 front arrived?

A. Strong sunlight heated the humid air until its water vapor changed into raindrops.B. The denser cool air pushed the humid air upward, and as it cooled, its vapor condensed.C. The cool air carried rain clouds south from Canada and dropped them on the county.D. The humid air sank under the cool air and squeezed water out of the ground.

4.

Use paragraph 4 and Table 1. Underline each change on July 8 and 9 that shows a new air mass had arrived, even though no rain fell.

5.

Why did the July 8 front bring no rain?

A. The air pushed upward held little water vapor, so little of it could condense.B. The sunlight on the clear day dried up the raindrops before they reached the ground.C. The front stopped moving before it reached the fairgrounds that afternoon.D. The cool air behind the front held too much water vapor to let rain fall.

Where was the humid air?

Table 2 shows noon readings on July 7, the day before the July 8 front. The stations are listed from south to north, plus one station to the west. Use it to answer questions 6–8.

Table 2. Noon on July 7
Station	Where	Wind from	Dew point (°C)
Gulf coast	Southern Texas	South	24
Central Oklahoma	700 km south	South	21
Central Kansas	350 km south	South	17
Fairgrounds	Willow Creek County	West	8
High plains	300 km west	West	6

6.

At noon on July 7, the dew point in central Oklahoma was °C higher than at the fairgrounds.

7.

Where was the warm, humid air mass from the Gulf most likely located at noon on July 7?

A. South of the fairgrounds, with south winds still carrying it north through KansasB. West of the fairgrounds, moving east toward the county from the high plainsC. Over the whole region, from the Gulf coast north to the high plains stationD. Over the fairgrounds and the high plains, but not yet in Oklahoma or Kansas

8.

Sort each reading by the air mass that was most likely over that place.

Plan for fair week

Use both tables and the passage to answer questions 9–11.

9.

The forecast for fair week says: south winds on Monday and Tuesday, then a cold front arrives on Wednesday afternoon. On which day is heavy rain most likely at the fairgrounds?

A. Tuesday, because the wind will have blown from the south for longestB. Monday, because that is when the south winds begin to blowC. Wednesday, because the front will push up humid air brought north by the south windsD. Thursday, because the cooler air behind the front will have arrived

10.

Mr. Achterberg wants to judge whether the next front is likely to bring rain to his corn. Which readings should he check a day before it arrives?

A. How far the temperature dropped after the last front passedB. How much rain fell from the last front that crossed the countyC. The dew point at the high plains station after the front has passedD. The dew point at the fairgrounds and which way the wind has been blowing

11.

Mr. Achterberg says, "We hardly got any rain in July because no fronts came through all month." Do you agree? Write a claim, give at least two pieces of evidence from Tables 1 and 2 and the passage, and explain your reasoning using air masses.