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When the Cold Front Hit the School

Use weather station data to explain how moving air masses change the weather.

Answer Key

The station log

Read the passage and study the station log. Then answer the questions.

A Tuesday in October

1The weather club at Ridgeview Middle School, in the central United States, runs a small weather station on the roof. It records temperature, humidity, air pressure in hectopascals (hPa), and the direction the wind blows from. On Monday night, club members saw an alert on their forecast app: a cold front was moving east across the region and would reach the school on Tuesday.

2Large bodies of air called air masses take on the temperature and moisture of the places where they form. Air that sits over the warm Gulf of Mexico usually becomes warm and humid. Air that forms over central Canada in fall is usually cold and dry. Cold air is denser than warm air. When air is pushed upward, it cools, and when humid air cools, some of its water vapor can condense into clouds and rain. A front is the boundary where two air masses meet.

3On Tuesday, club member Keisha Morgan copied the station's readings every three hours. Around 4 p.m., a line of thunderstorms swept over the school with gusty wind and heavy rain for about 30 minutes. At lunch the air had felt warm and sticky, with a steady breeze from the south. By the time Keisha walked home that evening, the air felt crisp and she was glad she had brought a jacket.

4The club also checked two other weather stations on a straight east–west line through the school. Westfield is 90 km west of the school and Eastbrook is 60 km east. At each station, the club found the time when the wind suddenly turned to blow from the northwest.

Ridgeview Roof Station Log
TimeTemp (°C)Humidity (%)Pressure (hPa)Wind fromSky
Tue 9 a.m.24781008SouthHazy sun
Tue noon28701005SouthSome clouds
Tue 3 p.m.29721002SouthwestDark clouds to west
Tue 6 p.m.18801006NorthwestLight rain ending
Tue 9 p.m.14551012NorthwestClearing
Wed 9 a.m.10401020NorthClear
1.
Between which two readings did the cold front most likely pass the school?
  • A. Tue 3 p.m. and Tue 6 p.m.
  • B. Tue noon and Tue 3 p.m.
  • C. Tue 9 a.m. and Tue noon
  • D. Tue 9 p.m. and Wed 9 a.m.
2.

By how many degrees did the temperature drop from Tue 3 p.m. to Wed 9 a.m.? 19 °C

3.
Sort each set of readings by the air mass over the school at that time.
24 °C, 78% humidity, wind from the south10 °C, 40% humidity, wind from the north29 °C, 72% humidity, wind from the southwest14 °C, 55% humidity, wind from the northwest
Warm, humid air massCold, dry air mass
24 °C, 78% humidity, wind from the south
29 °C, 72% humidity, wind from the southwest
10 °C, 40% humidity, wind from the north
14 °C, 55% humidity, wind from the northwest
4.
Which statement best describes the air pressure data?
  • A. It rose through the afternoon, then fell steadily after the front passed.
  • B. It went up and down at random, with no link to the change in wind.
  • C. It fell through the afternoon, then rose steadily after the front passed.
  • D. It stayed near 1008 hPa all day, then dropped sharply on Wednesday.
5.
What best explains the thunderstorms around 4 p.m.?
  • A. Warm air sank toward the ground and squeezed water out of the soil.
  • B. The afternoon sun heated the cold air until its water vapor turned to rain.
  • C. The cold air mass carried rain clouds all the way from central Canada.
  • D. Dense cold air pushed under the humid air, which rose, cooled and formed rain.

Tracking the front

Use the front timing table and paragraph 4 to answer the questions.

Wind Turns to the Northwest (Tuesday)
StationLocationTime of wind shift
Westfield90 km west of school1 p.m.
Ridgeview School0 km4 p.m.
Eastbrook60 km east of school?
6.

How fast did the front move from Westfield to the school? 30 km/h

7.

If the front keeps the same speed, how many hours after it passes the school will it reach Eastbrook? 2 hours

8.
Based on the school's data, which weather would you predict in Eastbrook at 9 p.m. on Tuesday?
  • A. Cooler and drier, with wind from the northwest and pressure rising
  • B. About the same as at noon, since fronts lose strength as they move
  • C. Warmer and more humid, with wind from the south and pressure falling
  • D. Warmer and drier, with wind from the south and pressure rising
9.
A student says the cold air formed right over the school during the storm. Which evidence best shows that the cold air moved in from somewhere else?
  • A. The humidity at the school was highest at 6 p.m., just after the rain.
  • B. The wind shift reached Westfield first and the school three hours later.
  • C. The temperature at the school rose from 9 a.m. to 3 p.m. on Tuesday.
  • D. The sky at the school was hazy in the morning and clear on Wednesday.
10.
Explain how moving air masses caused the weather to change at the school on Tuesday. Write a claim, give at least two pieces of evidence from the station log, and explain your reasoning.

Claim: A cold, dry air mass from the northwest moved in and replaced a warm, humid air mass, which caused storms and then cooler, drier weather. Evidence: At 3 p.m. it was 29 °C with 72% humidity and wind from the southwest, but by 6 p.m. it was 18 °C with wind from the northwest, and by Wednesday at 9 a.m. it was 10 °C with 40% humidity. Pressure fell to 1002 hPa at 3 p.m. and then rose to 1020 hPa. Reasoning: The warm, humid air likely came from the Gulf of Mexico, and the cold, dry air from central Canada. When the denser cold air arrived, it pushed the humid air upward, where it cooled and formed thunderstorms. After the front passed, the cold, dry air mass stayed over the school, so the weather turned cooler, drier and clear.