Worksheet

Rain on Snow: Landslide Watch in a Mountain Town

Grade 6 earth science: valley and ridge station data from a mountain town show how a mild, humid air mass brought rain on snow and raised landslide risk.

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1 NGSS standard
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11 questions
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Format: Interactive · Printable
Grade: Grade 6
Subject: Science
Question types: Fill in the blank, Multiple choice, Circle the answer, Sorting, Short answer

About this worksheet

What students practice

Students analyze temperature, dew point and wind data to tell when a different air mass arrived.
Students explain how a mild, humid air mass changed rain, snow and soil water on mountain slopes.

How to use

Read the passage together and have students mark where the wind direction changes in Table 1 before they answer. Students can then compare the two tables row by row.

Curriculum

NGSS standard covered

MS-ESS2-5

Use data on moving air masses to explain how they cause changes in weather.

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Name Date

Rain on Snow: Landslide Watch in a Mountain Town

Use station data to explain how a change in air masses raised the landslide risk.

The valley station log

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

Three Days in Cedar Gap

1Cedar Gap is a mountain town in the Pacific Northwest, about 150 km inland from the ocean. Steep, forested slopes rise on both sides of its valley. The town and its numbers are made up, but they are based on real winter storms in the region. The town's hazards office runs a set of earthquake sensors and also watches slopes that could slide. In January, sixth grader Mattias Ferreira spent three days at the office with geologist Dr. Aiyana Whitcomb for a school project.

2Dr. Whitcomb explained that small earthquakes are felt near Cedar Gap a few times a year, but most of the town's landslides happen during long, heavy winter rain. Water that soaks into the soil makes a slope heavier and weaker. In a cold storm, snow piles up on the slopes instead of soaking in. The danger is often greatest when heavy rain falls on deep snow, because melting snow adds its water to the rain.

3An air mass is a huge body of air with about the same temperature and humidity throughout. It takes on the conditions of the place where it forms. In winter, air that forms over the cold interior of Canada is usually cold and dry. Air that forms over the warm Pacific Ocean far to the southwest is usually mild and very humid. The dew point shows how much water vapor the air holds: the higher the dew point, the more water vapor. When humid air is pushed up the mountain slopes, it cools, and much of its water vapor condenses into clouds and falls as rain or snow.

4The office has a weather station on the valley floor, at 600 m, and another on Lookout Ridge, at 1,500 m. A sensor buried in the old slide slope above Fir Street measures how much of the soil is water. When it reads 40% or more, the office warns the families who live below the slope. Mattias copied the readings every 12 hours, from Monday morning to Thursday morning.

Table 1. Valley Station (600 m)
Time	Wind from	Temp (°C)	Dew point (°C)	Precip., last 12 h (mm)	Falling as
Mon 6 a.m.	North	−6	−14	0	Nothing
Mon 6 p.m.	Southwest	−2	−4	15	Snow
Tue 6 a.m.	Southwest	4	3	40	Rain
Tue 6 p.m.	Southwest	9	8	70	Heavy rain
Wed 6 a.m.	Southwest	10	9	55	Heavy rain
Wed 6 p.m.	Northwest	3	−2	10	Rain, then snow
Thu 6 a.m.	North	−5	−15	0	Nothing

From Monday at 6 a.m. to Tuesday at 6 p.m., the valley temperature rose by °C, and the dew point rose by °C.

Which change in Table 1 best shows that a different air mass had moved over Cedar Gap by Tuesday evening?

A. The wind kept blowing from the north, but the temperature rose to above 0 °C by evening.
B. The temperature fell below the dew point, and fog filled the valley.
C. The wind turned to the southwest, and the dew point climbed from −14 °C to 8 °C.
D. The precipitation stopped, and the dew point dropped to below −10 °C by evening.

Use paragraph 3 and Table 1. Where did the air over Cedar Gap on Tuesday and Wednesday most likely form?

A. Over the warm Pacific Ocean, far to the southwest
B. Over the cold interior of Canada, far to the north
C. Right above the valley, as the snow there melted
D. High above Lookout Ridge, where the air is colder

Circle each reading that is evidence that the air over the valley on Tuesday held much more water vapor than the air on Monday morning.

A dew point of 8 °C on Tuesday at 6 p.m.A wind from the north on Monday at 6 a.m.A ridge snow depth of 170 cm on Tuesday at 6 a.m.A valley temperature of 4 °C on Tuesday at 6 a.m.A dew point of 3 °C on Tuesday at 6 a.m.

Snow, rain and the slope

Use Table 2, Table 1 and paragraph 2.

Table 2. Lookout Ridge (1,500 m) and the Slope Sensor Above Fir Street
Time	Ridge temp (°C)	Falling on ridge	Ridge snow depth (cm)	Slope soil water (%)
Mon 6 a.m.	−11	Nothing	150	28
Mon 6 p.m.	−6	Snow	165	28
Tue 6 a.m.	−1	Snow	170	31
Tue 6 p.m.	5	Rain	140	41
Wed 6 a.m.	6	Rain	115	46
Wed 6 p.m.	−3	Snow	118	44
Thu 6 a.m.	−10	Nothing	117	37

From Tuesday at 6 a.m. to Wednesday at 6 a.m., the snow on Lookout Ridge got cm shallower.

Rain or snow fell on Lookout Ridge on Monday evening, Tuesday and Wednesday, yet its snow got shallower after Tuesday morning. Which explanation fits Tables 1 and 2 best?

A. Strong north winds blew the loose snow off the ridge and down into the valley below.
B. The ridge got colder each day, so the snow froze into a thinner layer of ice.
C. Mild air brought rain, not snow, to the ridge, and the rain and warm air melted snow.
D. Less precipitation fell on Tuesday than on Monday, so the old snow had time to settle.

Use paragraph 2 and both tables. Sort each event by what it did to the water reaching the slope soil.

Rain falling on the ridge snow on TuesdaySnow piling up on the ridge on Monday eveningRidge snow melting on Tuesday and WednesdaySnow falling on the ridge on Wednesday evening70 mm of rain falling in the valley by Tuesday at 6 p.m.

Sent more water into the soil	Kept water from soaking in for now

Use paragraph 4 and Table 2. When did the office first need to warn the families below the slope, and what was the weather like in the 12 hours before?

A. Tuesday at 6 a.m., after the first snow of the storm fell on the ridge
B. Wednesday at 6 a.m., after the ridge temperature first rose above 0 °C
C. Wednesday at 6 p.m., after rain changed to snow in the valley
D. Tuesday at 6 p.m., after heavy rain fell on the ridge snow in mild air

The cold air returns

Use both tables to explain the change and plan for the next storm.

What best explains the changes at Cedar Gap between Wednesday at 6 a.m. and Thursday at 6 a.m.?

A. Melting snow on the ridge cooled the valley air, and the cooler air pulled in a wind from the north.
B. Cold, dry air moved in on northwest and north winds, so rain turned to snow and then stopped.
C. The humid air ran out of water vapor, then cooled down on its own while it sat over the valley.
D. Night fell, and without sunlight the humid air over the valley froze and turned into snow.

10.

The next week, the forecast app says southwest winds will bring mild, humid Pacific air for two days, with rain up to the top of Lookout Ridge. The ridge has 180 cm of snow, and the slope sensor reads 33%. Which plan fits the data best?

A. Get ready to warn families early, because rain on deep snow raised the soil water quickly last time.
B. Get ready to warn families once north winds arrive, because the cold air brought the heaviest rain.
C. Wait to warn families until a quake is felt, because quakes caused the jump in soil water last time.
D. Wait, because the sensor is now below 40% and the deep ridge snow will soak up the rain.

11.

A neighbor on Fir Street says, "The slope got soaked because the snow on the ridge slowly melted on its own as the days got longer." Do you agree? Write a claim, give at least two pieces of evidence from Tables 1 and 2, and explain your reasoning using air masses.