D. Earth and Space Science: Climate Change · Climate ChangeHeat Transfer in the Atmosphere and Hydrosphere
D3.2 D2.5 A1.2 A1.4 A1.5 A1.13About 75 minutes
Learning goals- I can describe radiation, conduction and convection and give an example of each in the atmosphere and hydrosphere.
- I can use specific heat capacity and Q = mcΔT to explain why water heats and cools slowly.
- I can explain sea breezes, land breezes and why coastal climates are milder.
- I can describe how temperature changes with height in the troposphere.
- I can plan and safely carry out an investigation of heat transfer, controlling variables.
Words to know
heat
Energy that moves from a warmer object to a cooler one.
radiation
Transfer of energy by electromagnetic waves, such as sunlight or infrared; it needs no matter and can cross empty space.
conduction
Transfer of heat by direct contact, particle to particle.
convection
Transfer of heat by the movement of a fluid (liquid or gas): warm, less dense fluid rises and cooler, denser fluid sinks.
convection current
A loop of rising warm fluid and sinking cool fluid.
specific heat capacity (c)
The energy needed to raise the temperature of 1 g of a substance by 1 °C. For water, about 4.18 J/(g·°C).
density
Mass per unit volume. Warm air and warm water are less dense than cold air and cold water.
troposphere
The lowest layer of the atmosphere (up to about 8 to 18 km), where weather happens and temperature usually falls with height.
Let's start at the beach
It's a hot July afternoon at Wasaga Beach on Georgian Bay. The sand is so hot you hop across it, but the water is still cool enough to make you gasp. In the evening the sand has cooled, but the water feels warm.
Same Sun, same day, opposite behaviour. The sand and the water are absorbing similar amounts of sunlight. The difference is in how they store and move heat. That difference, multiplied across oceans and continents, shapes Earth's climate.
Remember from earlier grades
In Grade 7 Heat in the Environment you met conduction, convection and radiation and the particle theory. In Grade 9 you saw energy flowing through the four spheres. This lesson adds numbers (specific heat capacity and the equation Q = mcΔT) and applies heat transfer to the atmosphere and oceans.
Three ways heat moves
Heat is energy moving from a warmer place to a cooler place. It always flows from hot to cold, never the other way on its own.
- Radiation: energy carried by electromagnetic waves. It needs no matter, so it can cross the vacuum of space. Sunlight reaches Earth by radiation, and Earth gives off infrared radiation to space.
- Conduction: heat passing by direct contact as fast-moving particles bump into slower ones. The ground conducts heat to the thin layer of air touching it. Air is a poor conductor, so conduction only affects the first few millimetres or centimetres.
- Convection: heat carried by a moving fluid. When air or water is warmed, its particles spread out, so it becomes less dense and rises. Cooler, denser fluid sinks to take its place. The result is a loop called a convection current.
In the climate system, radiation brings energy in and sends it out, while convection (winds and ocean currents) does most of the work of moving heat around the planet.
Specific heat capacity: why water is slow to change
Different materials need different amounts of energy to warm up. The specific heat capacity (c) is the energy needed to warm 1 g of a substance by 1 °C.
Q = mcΔT
- Q = heat energy (J, joules)
- m = mass (g)
- c = specific heat capacity (J/(g·°C))
- ΔT = change in temperature (°C)
| Substance | Specific heat capacity (J/(g·°C)), approximate |
|---|
| Liquid water | 4.18 |
| Ice | 2.1 |
| Air (dry) | 1.0 |
| Dry sand | 0.8 |
| Granite (rock) | 0.8 |
Water's specific heat capacity is unusually high. It takes about five times as much energy to warm a gram of water as a gram of sand by the same amount. Water also mixes (waves and currents carry surface heat down into deeper water), and sunlight penetrates several metres into water instead of heating only a thin surface layer as on land. Together these mean oceans and large lakes heat up slowly in summer and cool down slowly in winter.
Worked exampleExample 1: Same energy, different results
A beach receives 20 900 J of solar energy on a 1 kg (1000 g) patch of dry sand and the same energy on 1 kg of water. Find the temperature change of each.
- Rearrange Q = mcΔT to get ΔT = Q ÷ (mc).
- Water: ΔT = 20 900 ÷ (1000 × 4.18) = 20 900 ÷ 4180 = 5.0 °C.
- Sand: ΔT = 20 900 ÷ (1000 × 0.80) = 20 900 ÷ 800 = 26.1 °C (rounded to 3 significant digits).
- The sand warms about 26.1 ÷ 5.0 ≈ 5 times as much. That's the hot sand and cool water at Wasaga Beach.
Water warms 5.0 °C; sand warms about 26.1 °C.
Practice data from a home investigation: 200 g of sand and 200 g of water under the same lamp. The sand warmed 14 °C in 20 minutes; the water warmed only about 4 °C, because water has a much higher specific heat capacity.Sea breezes, land breezes and coastal climates
Because land heats faster than water, the air above the land gets warmer during the day. It expands, becomes less dense and rises, leaving slightly lower air pressure near the ground. Cooler, denser air over the water flows in to replace it: a sea breeze (or lake breeze on the Great Lakes). At night the land cools faster than the water, and the circulation reverses: a land breeze.
The same idea works on a larger scale and over longer times. Places near large bodies of water have a maritime climate: cooler summers, milder winters, a smaller yearly temperature range. Places far from water have a continental climate with hot summers and cold winters.
Heat transfer in the atmosphere: up and down
The atmosphere is heated mainly from below. Sunlight passes through the air and warms the ground; the ground then warms the air above by conduction, convection and infrared radiation. That's why, in the troposphere (the lowest layer, where weather happens), the temperature usually drops with height, on average by about 6.5 °C for every kilometre you go up.
When a patch of ground heats strongly, a bubble of warm air (a thermal) rises by convection. As it rises, it expands and cools. If it cools enough, its water vapour condenses into a cumulus cloud (Lesson 4). On hot summer afternoons, strong convection can build towering thunderstorm clouds.
Sometimes the pattern flips and a layer of warm air sits above cooler air near the ground. This is a temperature inversion. Because the cool air below is denser, it can't rise through the warm lid, so convection stops and pollution gets trapped near the ground (Lesson 12).
Worked exampleExample 2: How cold is it up there?
On a summer day it is 24 °C at the shore of Lake Ontario (about 75 m above sea level). Use the average rate of 6.5 °C/km to estimate the temperature at the top of a cloud at 3.0 km and at the cruising height of a jet, 11 km.
- The shore is close to sea level, so treat the heights as 3.0 km and 11 km above the surface.
- At 3.0 km: drop = 6.5 × 3.0 = 19.5 °C, so T ≈ 24 − 19.5 = 4.5 °C.
- At 11 km: drop = 6.5 × 11 = 71.5 °C, so T ≈ 24 − 71.5 = −47.5 °C.
- This is an average; real values vary with weather. But it explains why mountaintops are cold and jet windows frost up.
About 4.5 °C at 3.0 km and about −48 °C at 11 km.
Heat transfer in the oceans and lakes
The sea surface is warmed by sunlight. Wind and waves mix this heat down through the top tens of metres (the mixed layer). Below that, water gets colder with depth. Cold water is denser than warm water, and salty water is denser than fresh water, so differences in temperature and salt drive slow, deep convection currents in the ocean (Lesson 5).
Fresh water has a surprising property: it is densest at about 4 °C. In an Ontario lake in winter, the 4 °C water sinks to the bottom, while colder water (0 to 4 °C) floats on top and freezes into ice. That's why fish survive the winter in the deeper water under the ice.
The ocean has absorbed more than 90% of the extra heat trapped by human-caused greenhouse gases in recent decades. That makes the ocean a huge buffer, but also means ocean warming will continue for centuries.
Just for funRiddle: I'm full of energy all summer, I hold onto it all fall, and I keep the fish cozy all winter. What am I?
A great big lake (and its high specific heat capacity).
Hands-on activityInvestigation: sand versus water, and a convection current you can see (A1.2, A1.4, A1.5)
Part 1: Which heats faster? You need: 2 identical cups (paper or plastic), dry sand or soil, water, a kitchen scale, 2 digital thermometers (or one used in turn), a desk lamp or a sunny window, a timer.
- Question: Does sand or water warm faster when it receives the same energy?
- Hypothesis: write one with a reason (for example, "If sand and water get the same light, then the sand will warm faster, because its specific heat capacity is lower").
- Variables: independent = material; dependent = temperature change; controlled = mass (use the scale: 200 g of each), cup type, distance from the lamp (about 30 cm), starting temperature, time.
- Record the starting temperature, then every 4 minutes for 20 minutes. Put the thermometer tip just under the surface each time.
- Remove the lamp and record for 20 more minutes as they cool.
- Graph both materials on one set of axes. Calculate each temperature change, and express your answers to a sensible number of decimal places (A1.13).
Part 2: See convection. You need: a clear glass baking dish or large jar of lukewarm water, food colouring, an ice cube tray.
- The day before, freeze water with several drops of food colouring in it.
- Gently place one coloured ice cube in the lukewarm water. Don't stir. Watch from the side for 5 minutes.
- Sketch what you see. Explain: the coloured meltwater is cold and dense, so it sinks, and warmer water moves up to replace it.
What to look for: sand should warm and cool faster than water. In Part 2, ribbons of colour sink to the bottom. Good conclusions name the controlled variables and connect results to specific heat capacity and density.
Safety: desk lamps get hot; don't touch the bulb and keep water away from the cord. Use glass carefully. Wash hands after handling sand or soil.
Your turn
1Name the three methods of heat transfer and give an example of each in the climate system.
Radiation: sunlight reaching Earth, or Earth giving off infrared to space. Conduction: warm ground heating the air touching it. Convection: warm air rising in a thermal, or a sea breeze; ocean currents.
2Which method of heat transfer can cross empty space? Why can't the others?
Radiation. Conduction and convection need particles (matter) to carry the energy, and space has almost none.
3Calculate the energy needed to warm 500 g of water from 15 °C to 25 °C.
Q = mcΔT = 500 × 4.18 × 10 = 20 900 J (20.9 kJ).
4Explain why warm air rises.
When air is warmed, its particles move faster and spread out, so the air becomes less dense than the cooler air around it. The denser cool air sinks and pushes the warm air up.
5Draw or describe the air movement at a beach on a sunny afternoon.
Air over the hot sand warms and rises; air higher up flows out over the water and sinks over the cooler water; near the ground, cool air flows from the water to the land. The wind you feel blows from the water toward the land (a sea or lake breeze).
6A town is at 1200 m elevation. A valley town nearby at 200 m has a temperature of 18 °C. Estimate the temperature in the higher town.
Height difference = 1.0 km. Drop ≈ 6.5 °C. Temperature ≈ 18 − 6.5 = 11.5 °C.
7Why do coastal places have a smaller yearly temperature range than inland places?
The nearby water heats slowly in summer and cools slowly in winter, because of its high specific heat capacity and mixing. Air moving over the water takes on its moderate temperature, keeping summers cooler and winters milder.
More practice: warm-up
1What is the specific heat capacity of water?
2Which direction does heat always flow on its own?
3In which layer of the atmosphere does weather happen?
4What is the name for a loop of rising warm fluid and sinking cool fluid?
5Does land or water heat up faster in the same sunshine?
More practice: on your own
12000 J of energy is added to 100 g of dry sand (c = 0.80 J/(g·°C)). By how much does its temperature rise?
ΔT = Q ÷ (mc) = 2000 ÷ (100 × 0.80) = 2000 ÷ 80 = 25 °C.
2How much energy is released when 1.0 kg of lake water cools from 20 °C to 4 °C in the fall?
Q = 1000 × 4.18 × 16 = 66 880 J (about 67 kJ). This heat warms the air above the lake, which is one reason the shore stays milder in fall.
3On a calm, clear night, a valley fills with cold air and the morning has fog and smoke trapped near the ground. Explain using the idea of a temperature inversion.
The ground cools quickly at night by giving off infrared radiation, cooling the air touching it. Cold, dense air drains into the valley, with warmer air above. This inversion stops convection, because the cold air can't rise through the warmer lid, so fog and smoke stay trapped near the ground.
4Why is fresh water densest at 4 °C important for lake life in winter?
The densest water (4 °C) sinks to the bottom, so the bottom of the lake stays at about 4 °C. Colder water floats on top and freezes, and the ice layer insulates the water below. Fish and other organisms survive in the liquid water underneath.
5In the sand-versus-water investigation, why must the masses be equal? Name two other variables to control.
Q = mcΔT shows that a larger mass would warm less for the same energy, so different masses would make the test unfair. Other controlled variables: distance from the lamp, cup type and colour, starting temperature, time, depth of the thermometer.
Challenge
1A 1.0 m² patch of ocean absorbs sunlight at 400 W (400 J every second) for 8 hours, and the energy warms the top 10 m of water (mass 10 000 kg). By how much does the water warm? What if the same energy went into 0.10 m of dry sand (mass about 160 kg)?
Energy = 400 × 8 × 3600 = 11 520 000 J. Water: ΔT = 11 520 000 ÷ (10 000 000 g × 4.18) ≈ 0.28 °C. Sand: ΔT = 11 520 000 ÷ (160 000 g × 0.80) = 11 520 000 ÷ 128 000 = 90 °C (in reality the sand would lose much of this heat to the air and by radiation, so it wouldn't get that hot, but the comparison shows why land heats far faster: less mass is involved and its c is lower).
2Explain why the ocean's absorption of more than 90% of the extra heat from greenhouse gases is both good news and bad news.
Good: it has slowed the warming of the air and land, giving societies time to respond. Bad: warmer water expands and raises sea level, harms coral and fish, melts ice shelves from below, and fuels stronger storms. Because the ocean releases stored heat slowly, warming will continue for a long time even after emissions fall.
3Lake breezes often trigger thunderstorms a few kilometres inland from the Great Lakes on summer afternoons. Use convection to explain why.
The cool, dense lake air pushes inland under the warmer air over the land, like a wedge (a lake-breeze front). The warm, moist air is forced upward along this front. As it rises, it cools, its water vapour condenses, and the released heat (latent heat) makes it rise even faster, building tall cumulonimbus clouds that can produce thunderstorms.
Quiz
1. How does energy from the Sun reach Earth?
3. How much energy is needed to warm 200 g of water by 5 °C? (c = 4.18 J/(g·°C))
4. On a sunny afternoon at a lake, which way does the breeze blow near the ground?
5. Why do coastal climates have smaller temperature ranges?
6. In the troposphere, the average temperature usually...
7. What does a temperature inversion do?
Big idea: Heat moves by radiation, conduction and convection. Water's high specific heat capacity makes oceans and lakes slow to warm and cool, which shapes coastal climates and breezes. This covers D3.2 and the investigation in D2.5.
How to help: Do the sand-versus-water investigation together; it shows the idea better than any explanation. At the beach or a lake, ask which way the breeze is blowing and why. Check calculations: students often forget to convert kilograms to grams when using c in J/(g·°C).
Common difficulties:
- Saying "cold" moves. Heat (energy) moves from warm to cold; cold is just less energy.
- Mixing up which way a sea breeze blows. Winds are named for where they come from: a sea breeze comes from the sea.
- Unit errors with Q = mcΔT (grams vs kilograms).
Safety: supervise lamps and glassware.