Atmosphere & Weather — Problem set, Grade 9

Trace water and energy through the atmosphere and interpret changing weather evidence.

Review the key ideas

The Water Cycle

The sun's energy lifts water into the air as vapor (evaporation and transpiration), and gravity pulls it back down as precipitation and runoff — the same water endlessly recycles through air, land, and ocean.

Atmospheric Energy & Circulation

Unequal heating, water’s phase changes, and Earth’s rotation help organize atmospheric circulation.

Air Masses, Fronts & Pressure

Air masses take on the temperature and moisture of where they form; cold fronts bring fast, intense weather while warm fronts bring slow, gentle weather; high pressure means sinking air and clear skies, and low pressure means rising air and storms.

Reading Weather Patterns

Weather maps use H, L, fronts, and isobars to show pressure patterns; because mid-latitude weather generally moves west to east, watching pressure trends and what's happening upwind predicts what's coming next.

Problem 1

Which process describes water vapor turning into liquid droplets that form clouds?
Answer and explanation

Condensation

As water vapor rises and cools, it condenses into tiny liquid droplets, which cluster together to form clouds.

Problem 2

Name two drivers of the water cycle, and state the role of each.
Answer and explanation

Solar energy supports evaporation; gravity drives precipitation falling and downhill flow.

Sunlight is an energy source, not itself a force. Gravity moves water downward while solar energy supports phase changes and circulation.

Problem 3

A hiker notices a mountain snowpack has been shrinking all spring even though it hasn't rained. Aside from ordinary melting, what water cycle process could be turning some of that snow and ice directly into the air without ever forming liquid water?
Answer and explanation

Sublimation

Sublimation is the process where ice or snow turns directly into water vapor without first becoming liquid water. Along with melting, it can shrink a snowpack over time.

Problem 4

An air mass forms over the ocean near the equator. Give both its moisture and temperature classification labels.
Answer and explanation

Maritime tropical (mT)

It forms over water, so it's maritime (moist); it forms near the equator, a warm region, so it's tropical (warm). Together: maritime tropical (mT).

Problem 5

A front is approaching where warm air is gradually gliding up and over a retreating cold air mass on a gentle slope. What type of front is this, and what weather does it typically bring?
Answer and explanation

A warm front; it typically brings gradually thickening clouds and a longer period of light, steady precipitation.

The gentle slope and slow-gliding motion of warm air over cold air describes a warm front, which moves slowly and spreads lighter precipitation over a longer stretch of time, unlike the fast, steep, intense weather of a cold front.

Problem 6

Which instrument would you use to measure wind speed?
Answer and explanation

An anemometer

An anemometer is the instrument specifically designed to measure wind speed.

Problem 7

Isobars on today's weather map are spaced much closer together than they were yesterday over the same region. What does this suggest about the wind?
Answer and explanation

Winds are likely getting stronger.

Isobars packed closely together indicate a bigger pressure difference over a short distance, and bigger pressure differences drive stronger winds.

Problem 8

A barometer reading is well below the standard sea-level average, and a cold front is approaching from the west. Predict the sequence of weather changes you'd expect over the next several hours, and explain your reasoning.
Answer and explanation

Increasing clouds and a chance of showers ahead of the front, then a brief but intense burst of wind and rain (or thunderstorms) as the front passes, followed by clearing skies and noticeably colder temperatures behind it.

The below-average pressure reading signals rising air and an increased chance of clouds and precipitation even before the front fully arrives. As the fast-moving, steep cold front itself passes through, expect a short, intense period of gusty wind and rain or storms. Once it moves past, the region comes under the influence of the fresh, cold air mass behind it, so skies clear and temperatures drop.

Problem 9

Explain how a meteorologist in Westchester County, NY could use both a falling barometric pressure trend AND the typical west-to-east movement of weather systems to improve a forecast for tomorrow.
Answer and explanation

By combining the falling-pressure trend, which signals rising air and a likely storm, with knowledge of what's currently happening in states to the west, since weather generally travels west to east in the mid-latitudes.

A falling barometer at the local station already hints that stormier weather is developing overhead. Because mid-latitude weather, including New York's, is steered generally from west to east by the prevailing westerlies, the meteorologist can also look at current conditions a state or two to the west and expect Westchester to experience something similar within a day or so — combining both clues sharpens the forecast.

Problem 10

A catchment receives 800 mm of rain a year and loses 500 mm to evaporation and transpiration. How many millimetres remain for runoff and recharge?
Answer and explanation

300 mm

800 − 500 = 300 mm.

Problem 11

Explain why a water budget must specify a time period and an area.
Answer and explanation

Inputs and outputs are rates, so the figures only mean something over a stated interval — 800 mm in a year is very different from 800 mm in a month. The area matters because a depth of rainfall converts to a volume only when multiplied by the catchment area. Without both, the numbers cannot be compared between places or used to size a reservoir.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

Problem 12

Chapter review: A lake starts at 250 m³. Daily input is 40 m³ and output 28 m³ for five days. Find final storage and explain why the height change cannot be determined from these data alone.
Answer and explanation

Net input is 12 m³/day, so storage gains 60 m³ and reaches 310 m³. Lake area/shape or a volume-level relationship is needed for height change.

Subtract 40−28 = 12 m³/day.

Multiply by five days to obtain 60 m³ and add to 250.

Distinguish volume from height and request geometry.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the physical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 13

A dark and a pale surface receive equal sunlight. What must you measure before concluding that color alone determines temperature?
Answer and explanation

Measure reflected/absorbed energy and temperature while controlling material, moisture, mass, airflow, and exposure. Several properties affect temperature.

Problem 14

Chapter review: A Northern Hemisphere air parcel initially moves north, while another moves east. Give the Coriolis deflection direction for each and explain why the effect does not begin a wind from rest.
Answer and explanation

Right of northward motion is east, and right of eastward motion is south. Coriolis modifies existing motion in the rotating Earth frame; pressure-gradient or other forces initiate the wind. The complete wind also depends on friction and other forces.

Right of northward motion is east, and right of eastward motion is south. Coriolis modifies existing motion in the rotating Earth frame; pressure-gradient or other forces initiate the wind. The complete wind also depends on friction and other forces.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the physical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 15

Air rising in a low-pressure system cools because it
  1. loses energy to space
  2. expands as pressure drops with height
  3. touches cold ground
  4. gains moisture
Answer and explanation

expands as pressure drops with height

Expanding air does work and its temperature falls, which is why cloud forms aloft.

Problem 16

A barometer falls steadily through the afternoon. Predict the weather and explain your reasoning.
Answer and explanation

Unsettled weather is likely — increasing cloud, strengthening wind and probably rain — because a falling barometer means a low-pressure system is approaching. Rising air in the low cools and condenses, producing cloud and precipitation, and the pressure gradient between the low and surrounding higher pressure drives wind. The speed of the fall indicates how vigorous the system is.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

Problem 17

Chapter review: Two maps have the same ground scale. Map A shows a 4 hPa change over 100 km; map B a 2 hPa change over 25 km. Compare pressure-gradient magnitudes and explain why neither gives a full wind direction alone.
Answer and explanation

A is 0.04 hPa/km; B is 0.08 hPa/km, twice as large. Actual wind also responds to rotation and friction, so pressure-gradient magnitude alone is not a complete directional forecast.

Calculate 4/100 and 2/25 in matched units.

Compare 0.08 with 0.04: factor two.

Include rotation/friction and distinguish pressure-gradient force from wind.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the physical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 18

A storm 180 km away approaches at 30 km/h. How many hours until it arrives?
Answer and explanation

6 hours

180 / 30 = 6 hours.

Problem 19

Two stations report the same temperature but very different dew points. Describe the difference in how the air feels and why.
Answer and explanation

The station with the higher dew point has far more water vapour in the air, so it feels humid and sticky, and sweat evaporates poorly. The one with a low dew point feels dry despite the identical thermometer reading. Dew point measures the actual moisture content, which is why it describes comfort better than relative humidity alone.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

Problem 20

Chapter review: A front is 90 km from a town. Model A gives steady motion 30 km/h and model B 45 km/h. Calculate the two arrival times and define a fair observation-based timing check.
Answer and explanation

A gives 3 h and B 2 h. Record the forecast issue time, defined passage indicator/location and predicted intervals, then compare with later observations using the original criteria. Changing criteria after the event undermines verification.

A gives 3 h and B 2 h. Record the forecast issue time, defined passage indicator/location and predicted intervals, then compare with later observations using the original criteria. Changing criteria after the event undermines verification.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the physical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.
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