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Lesson plan Grade 6 Science

Why Halbeck Stays Dry: Modeling Wind Over Dalerift Ridge

A grade 6 science lesson in which students build and use a cross-section model to explain why a fictional coast town is wet and a town past the ridge is dry.

Objectives

  1. Students will build a model of moist air crossing a ridge and label where rain falls.
  2. Students will use rainfall records from three stations to explain the pattern their model predicts.
  3. Students will explain why air that has crossed a ridge arrives dry on the far side.

Vocabulary

Water vapor
Water vapor is water in the form of an invisible gas mixed into the air. There is always some water vapor around you, even when the sky is completely clear.
Condensation
Condensation is when water vapor in the air cools down and turns back into tiny liquid water droplets. It is how clouds form and why a cold glass gets wet on the outside on a warm day.

Also: prevailing wind, windward, leeward, rain shadow

Teacher overview

Two fictional towns sit 40 miles apart at the same latitude, with a ridge between them, and one receives several times the rain the other does. Students build a paper cross-section model of moist ocean air crossing that ridge, then test the model against a rainfall record from three stations. The lesson matters because the same pattern decides where a region's reservoirs are built and which communities have to import water. One misconception to address during the hook is the idea that the ridge blocks the clouds, as though a mountain were a wall that stops weather. Air does cross the ridge. What changes is its temperature, and warm air can hold more water vapor than cool air, so rising air sheds its water on the windward side and the same air arrives dry on the far side. Students who hold the wall picture will misplace the rain on their models and will not be able to explain why the leeward side is dry rather than simply cloudy.

Procedure

Hook (5 min). Project or read aloud two short descriptions of the same fictional afternoon: fog and steady drizzle in Stourmere on the coast, and bright dry wind in Halbeck, 40 miles inland. Both towns sit at the same latitude. Ask the class for two possible reasons and record them on the board without judging them. Expect "the mountain blocks the clouds" to appear; leave it posted and return to it at the end.

Read the record (10 min). Distribute the Student Handout and give pairs time to study Table 1, which lists average rainfall at three fictional stations: Stourmere on the coast, Dalerift Gap at the ridge crest, and Halbeck on the far side. Ask pairs to state the pattern in one sentence before they touch the model. Circulate and listen for pairs who describe the pattern as a steady decrease with distance; they will need the crest reading pointed out, because rainfall peaks at the crest rather than at the coast.

Build the model (15 min). Give each pair the ridge cross-section sheet and the set of four air parcel cards. The sheet shows the ocean on the left, Stourmere at the shore, Dalerift Ridge in the middle, and Halbeck on the right. The cards read: warm moist air leaving the ocean, air rising and cooling on the windward slope, cloud and rain at the crest, and air sinking and warming on the leeward slope. Pairs tape the cards in order along the arrow, then color the windward slope green and the leeward slope brown, and draw raindrops only where their model says rain falls. Require a one-sentence caption under each card saying what happens to the temperature and to the water vapor at that step.

Use the model (10 min). Pairs answer questions 1 through 6 on the Student Handout, using both the model and Table 1. Push pairs who finish early to explain why the crest station, rather than the coast station, records the most rain.

Formative check (7 min). Collect an exit ticket: each student writes one sentence answering "Why does the air arrive dry in Halbeck if it started over the ocean?" Sort the tickets into three piles as students leave: sentences that name cooling and condensation on the way up, sentences that name only the ridge as a blocker, and sentences that name neither. If more than a third of the class lands in the second or third pile, reteach with the demonstration listed under Support before moving to the next lesson.

Close (3 min). Return to the reasons posted during the hook. Ask one student to say what the class now knows that the "blocking" idea cannot explain, and point out that both towns draw drinking water from the same weather pattern in very different ways.

Why Halbeck Stays Dry: Modeling Wind Over Dalerift Ridge — Grade 6 Science
Name: ____________________________ Date: ______________

Student Handout

Table 1. Average yearly rainfall at three fictional stations

Station Location Elevation Average yearly rainfall
Stourmere Coast, at sea level 20 feet 46 inches
Dalerift Gap Ridge crest 3,400 feet 71 inches
Halbeck Inland, past the ridge 900 feet 13 inches

The prevailing wind at all three stations blows from the ocean toward the ridge. All three stations sit at the same latitude and are within 40 miles of one another. These stations are fictional and were written for this lesson.

Answer each question in complete sentences.

  1. Which station records the most rain, and which records the least?
  2. Describe the pattern in Table 1 in one sentence. Say what happens to rainfall from the coast to the crest, and then from the crest to Halbeck.
  3. On your model, at which step do cloud droplets first form? Explain why they form at that step and not at the step before it.
  4. Air reaching Halbeck came from the ocean, yet Halbeck is dry. Explain why, using what your model shows about the leeward slope.
  5. The Halbeck council asks whether planting trees along the ridge crest would bring more rain to town. Use your model to answer, and give your reason.
  6. Name one measurement you would take at Dalerift Gap to test whether your model is right, and say what result would support it.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key
  1. Answer: Dalerift Gap records the most and Halbeck records the least. The crest station reads 71 inches, the coast reads 46 inches, and Halbeck reads 13 inches.
  2. Answer: Rainfall rises from the coast to the crest, then drops sharply past the crest.
    Accept any wording that captures both the rise and the sharp fall.
  3. Answer: At the step where air rises and cools on the windward slope. Cool air holds less water vapor than warm air, so as the rising air cools, some of its vapor condenses into cloud droplets. At the step before it, the air is still warm enough to hold that vapor.
  4. Answer: The air lost most of its water as rain on the windward side, and on the leeward slope it sinks and warms. Warmer air can hold more water vapor, so the little vapor left stays in the air instead of condensing, and Halbeck gets dry wind rather than rain.
  5. Answer: No, because the rain at the crest is caused by air rising and cooling, not by the trees. Accept answers that add a reasonable secondary point, such as trees changing runoff or shade, as long as the student says trees would not change the rising and cooling of the air.
  6. Answer: Accept any measurement that could test the model, with a matching result. Common strong answers include air temperature at the base and at the crest on the same day, expecting the crest to be cooler; humidity on both slopes, expecting the leeward air to be drier; or cloud base height on the windward slope.

Differentiation

Support

Run a quick demonstration before the build: breathe onto a cold window or a chilled metal spoon and have students name what appeared and where the water came from. Give these pairs a cross-section sheet with the four card positions already outlined and the arrow drawn, so the work is placing and explaining rather than laying out. Provide the caption frames "The air here is getting ___" and "The water vapor here is ___." Allow them to answer questions 1 through 4 only, and to draw rather than write the answer to question 3.

Extension

Ask a pair to redraw the model for a year when the wind reverses and blows from Halbeck toward the coast, predicting where rain would fall and what Stourmere's rainfall record would look like over a year of reversed winds. A second option is to sketch where they would site a reservoir in this fictional landscape and to defend the site using Table 1.

ELL supports

Pre-teach "windward," "leeward," "vapor," "condense," and "prevailing" with the labeled cross section and hand motions for rising and sinking air. Provide a bilingual glossary card with the five terms. Let students state each caption aloud to a partner before writing it, and accept a labeled drawing with single-word labels for question 4. Pair each student with a partner who can talk through Table 1 in the student's stronger language.

Extensions

Have students look up the yearly rainfall for two real towns on opposite sides of a mountain range in their own state or region, record both numbers with the source, and write two sentences saying whether the pattern matches the model they built. A second option is to find out where their own community's drinking water falls as rain or snow, and to mark that place and their school on a simple map.