D. Earth and Space Science: Climate Change · Climate ChangeWater in Three States: Latent Heat, Clouds, Precipitation and Lake-Effect Snow
D2.6 D3.2 D3.1 A1.6About 70 minutes
Learning goals- I can explain latent heat and calculate the energy absorbed or released when water changes state.
- I can explain how evaporation, condensation, melting and freezing move heat through the climate system.
- I can explain how clouds form and describe orographic precipitation and rain shadows.
- I can explain how lakes, snow and ice influence local climate in Ontario, including lake-effect snow.
- I can record and analyse temperature data during a change of state.
Words to know
latent heat
Energy absorbed or released when a substance changes state, without a change in temperature.
latent heat of fusion
Energy needed to melt a substance at its melting point; for ice, about 334 J/g.
latent heat of vaporization
Energy needed to evaporate a liquid; for water, about 2260 J/g at 100 °C.
evaporation
Change from liquid to gas; it absorbs energy and cools the surface it leaves.
condensation
Change from gas to liquid; it releases energy and warms the surrounding air.
relative humidity
How much water vapour the air holds compared with the most it could hold at that temperature, as a percentage.
dew point
The temperature at which air becomes saturated and water vapour starts to condense.
orographic precipitation
Rain or snow formed when moist air is forced up over mountains.
rain shadow
A dry area on the downwind (leeward) side of a mountain range.
Let's start after a swim
You climb out of a lake on a breezy summer day and suddenly feel chilly, even though the air is 25 °C. Dry off with a towel and you feel warm again. What happened?
The water on your skin was evaporating, and evaporation takes energy. It took that energy from your skin as heat. The same thing happens on the surface of every ocean and lake on Earth, and the energy it carries away ends up powering clouds, rainstorms and even hurricanes. Water changing state is one of the biggest ways heat moves through the climate system.
Latent heat: hidden energy
When water changes state, it absorbs or releases energy without changing temperature. This energy is called latent heat (latent means hidden).
- Melting ice absorbs about 334 J per gram (latent heat of fusion). Freezing releases the same amount.
- Evaporating water absorbs about 2260 J per gram at 100 °C (a bit more at lower temperatures). Condensing releases the same amount.
Q = mL, where Q is energy (J), m is mass (g) and L is the latent heat (J/g).
Compare: warming 1 g of liquid water by 1 °C takes only 4.18 J. Evaporating that gram takes about 2260 J, more than 500 times as much.
Worked exampleExample 1: Melting a snowbank
How much energy is needed to melt 1.0 kg of snow that is already at 0 °C, then warm the meltwater to 10 °C?
- Melting: Q₁ = mL = 1000 g × 334 J/g = 334 000 J.
- Warming the water: Q₂ = mcΔT = 1000 × 4.18 × 10 = 41 800 J.
- Total: 334 000 + 41 800 = 375 800 J (about 376 kJ).
- Notice that melting takes about 8 times as much energy as warming the water by 10 °C. That's why spring snowmelt holds air temperatures near 0 °C for days, and why large ice sheets respond slowly at first.
About 376 kJ, most of it (334 kJ) just to melt the snow.
Evaporation and condensation move heat around the world
When sunlight warms the ocean surface, much of the energy goes into evaporating water rather than raising the temperature. The water vapour carries this latent heat upward and sometimes thousands of kilometres away. When the vapour condenses into cloud droplets, the latent heat is released into the air.
This is a huge heat pump: it moves energy from the surface to the upper atmosphere, and from the tropics toward the poles. The released heat makes the rising air even warmer and more buoyant, which is what powers thunderstorms and hurricanes. Hurricanes form over very warm ocean water and weaken over land or cold water, because they lose their fuel supply.
How clouds and precipitation form
Air can hold only so much water vapour, and warm air can hold much more than cold air (about 7% more for every 1 °C of warming). Relative humidity compares how much vapour the air has with the most it could hold.
When air rises, it expands and cools. When it cools to its dew point, it becomes saturated, and water vapour condenses onto tiny particles (dust, salt, smoke) to form cloud droplets or ice crystals. When droplets or crystals grow heavy enough, they fall as precipitation.
Air is forced to rise in three main ways:
- Convection: heated ground makes air rise (summer afternoon thunderstorms).
- Fronts: warm air is pushed up over colder, denser air.
- Mountains (orographic lifting): wind pushes air up a slope.
British Columbia shows this clearly. Moist Pacific air rising over the Coast Mountains drops a lot of rain on the coast; the west coast of Vancouver Island is one of the wettest places in Canada. In the rain shadow east of the mountains, parts of the southern interior (around Kamloops and the Okanagan Valley) are dry enough for sagebrush, ponderosa pine and irrigated orchards.
Ice and snow: reflecting, insulating and storing cold
- Albedo: snow and ice reflect most sunlight (Lesson 1), keeping cold places cold. When they melt, darker surfaces absorb more energy, a positive feedback.
- Insulation: snow traps air and is a good insulator. A thick snowpack keeps the soil underneath from freezing deeply, which protects plant roots and small animals that spend winter under the snow.
- Energy buffer: melting ice absorbs lots of energy at 0 °C, so ice-covered regions resist warming until the ice is gone.
- Lake ice: once a lake freezes, it no longer gives off much heat or moisture to the air above.
Water and Ontario's climate
- Moderating effect: the Great Lakes are slow to warm in spring and slow to cool in fall. Near the shores, cool lake air in spring holds back budding until most of the frost danger has passed, and fall frosts come later. This is one reason tender fruits like peaches and grapes grow in the Niagara Peninsula.
- Lake-effect snow: in late fall and early winter, the lakes are still relatively warm and unfrozen, while Arctic air is very cold. Cold air crossing the lake gains heat and moisture, rises, and dumps snow on the downwind shore. With prevailing westerly and northwesterly winds, Ontario's snowbelts lie east and southeast of Lake Huron, Georgian Bay and Lake Superior.
- Humidity: summer air masses from the Gulf of Mexico bring humid, sticky days; the humidex combines temperature and humidity into a single "feels like" value.
Worked exampleExample 2: Why evaporation is such a strong cooler
On a hot day, 2.0 kg of sweat evaporates from a marathon runner over a race. How much heat does this remove? Compare it with the heat needed to warm 2.0 kg of water by 10 °C.
- Evaporating: Q = mL = 2000 g × 2260 J/g = 4 520 000 J (4.52 MJ). (At skin temperature, L is a little higher, about 2400 J/g, so this is a slight underestimate.)
- Warming: Q = mcΔT = 2000 × 4.18 × 10 = 83 600 J.
- Ratio: 4 520 000 ÷ 83 600 ≈ 54.
- Evaporation removes far more heat than simply warming water. Oceans cool themselves the same way, sending that energy into the atmosphere.
About 4.5 MJ, about 54 times the heat needed to warm the same water by 10 °C.
Water and climate change
Because warmer air holds more water vapour, a warmer world has a more intense water cycle: more evaporation, more water vapour (a greenhouse gas, which adds to warming: the water vapour feedback), heavier downpours in many regions, and faster drying of soils between rains in others. Snow cover and lake ice seasons in Ontario are already getting shorter (Grade 9 indicators), and in some recent winters the Great Lakes had much less ice than usual, which can allow more lake-effect snow when Arctic air arrives.
Just for funRiddle: I'm the only common substance on Earth found naturally as a solid, a liquid and a gas, all at the same time, on the same lake, on the same March morning. What am I?
Water: ice on top, liquid below, vapour (and fog) above.
Hands-on activityInvestigation: latent heat in your kitchen (A1.6)
Part 1: Melting plateau. You need: a glass, ice cubes, cold tap water, a digital thermometer, a spoon, a timer.
- Fill the glass with ice and add enough cold water to cover it. Stir gently.
- Record the temperature every 2 minutes, stirring each time, until all the ice has melted and then for 10 more minutes. Note the time the last ice melts.
- Record data in a table with headings and units: Time (min), Temperature (°C), Observations.
- Graph temperature against time. Describe the shape.
Part 2: Evaporative cooling. You need: 2 thermometers, a small piece of paper towel, an elastic band, room-temperature water, a fan (or a piece of cardboard to wave).
- Wrap the bulb or tip of one thermometer in paper towel soaked in room-temperature water. Leave the other dry.
- Record both temperatures, then fan both for 3 minutes and record again.
- Explain the difference using latent heat.
What to look for: in Part 1, the temperature stays close to 0 °C as long as ice remains (energy from the room goes into melting), then rises. In Part 2, the wet thermometer reads several degrees lower. A complete answer names latent heat of fusion (Part 1) and of vaporization (Part 2) and explains where the energy comes from.
Safety: use plastic or sturdy glass; glass thermometers can break, so digital ones are safest. Keep the fan cord away from water.
Your turn
Energy absorbed or released when a substance changes state, without any change in temperature.
2How much energy is needed to melt 250 g of ice at 0 °C?
Q = mL = 250 × 334 = 83 500 J (83.5 kJ).
3Does condensation absorb or release energy? Why does this matter for thunderstorms?
It releases energy. The released latent heat warms the rising air, making it more buoyant so it rises faster and higher, building tall thunderstorm clouds.
4Explain why rising air forms clouds.
Rising air expands and cools. When it cools to its dew point, it becomes saturated and water vapour condenses onto tiny particles, forming cloud droplets.
5What is a rain shadow? Give a Canadian example.
A dry area on the downwind (leeward) side of a mountain range, where sinking air is dry and warming. Example: the southern interior of British Columbia (around Kamloops and the Okanagan), east of the Coast Mountains.
6Why does lake-effect snow happen mostly in late fall and early winter, and less in late winter?
In late fall and early winter, the lakes are still relatively warm and unfrozen while Arctic air is cold, so the air picks up lots of heat and moisture. By late winter, the lakes have cooled and may be partly frozen, so they supply less moisture.
7Why do people in the Niagara Peninsula grow peaches and grapes, which are hard to grow in much of Ontario?
Lake Ontario (and Lake Erie) moderate the climate: they keep spring cool so buds open after most frost danger, and they keep fall warm, delaying frost. This gives a longer, gentler growing season.
More practice: warm-up
1What is the latent heat of fusion of ice?
2What is the latent heat of vaporization of water at 100 °C?
3When water evaporates from your skin, does your skin warm up or cool down?
The temperature at which air becomes saturated and water vapour starts to condense.
5What do we call precipitation caused by air rising over mountains?
Orographic precipitation.
More practice: on your own
1How much energy does 50 g of water vapour release when it condenses into cloud droplets?
Q = 50 × 2260 = 113 000 J (113 kJ).
2How much energy is needed to turn 100 g of ice at 0 °C into water at 20 °C?
Melt: 100 × 334 = 33 400 J. Warm: 100 × 4.18 × 20 = 8360 J. Total = 41 760 J (about 41.8 kJ).
3Why do temperatures often stay near 0 °C for days during a spring thaw, even when the Sun is strong?
Much of the incoming energy goes into melting snow and ice (latent heat of fusion) instead of raising the temperature. Until the snow is gone, the surface and air nearby stay near 0 °C.
4In your kitchen investigation, the ice water stayed at 0.2 °C for 18 minutes, then rose to 6 °C over the next 10 minutes. Explain both parts of the graph.
While ice remained, heat from the room went into melting ice (latent heat), so the temperature stayed constant near 0 °C. Once all the ice melted, the incoming energy warmed the liquid water, so the temperature rose.
5Explain how the water cycle moves heat from the tropics toward the poles.
Strong sunlight in the tropics evaporates lots of water, storing energy as latent heat in the vapour. Winds carry the moist air poleward. When the vapour condenses into clouds at higher latitudes, it releases that latent heat, warming the air there.
Challenge
1A lake in early December is 6 °C at the surface, and the air above is −15 °C. Using what you know about latent heat, density and convection, explain step by step how this can produce a lake-effect snow squall 30 km inland.
(1) The cold air is warmed from below by the much warmer lake (conduction and radiation), making the lowest air warmer and less dense, so it rises (convection). (2) Water evaporates from the lake into the air, adding moisture. (3) As the moist air rises, it cools; at its dew point, vapour condenses (and freezes) into cloud, releasing latent heat that makes the air rise even more and build tall clouds. (4) The prevailing wind carries these clouds over the downwind shore, where the snow falls; air is also lifted further as it slows over land and climbs any hills, so heavy snow can fall well inland.
2Warm air can hold about 7% more water vapour per 1 °C of warming. If air at 20 °C can hold 17 g of water vapour per kg of air, estimate how much it can hold at 23 °C. What does this suggest about heavy rainfall in a warmer climate?
Multiply by 1.07 three times: 17 × 1.07³ = 17 × 1.225 ≈ 20.8 g/kg, about 22% more. A warmer atmosphere can carry more water, so when it does rain, downpours can be heavier, which matches observed increases in heavy rainfall in many regions.
3Snow is an insulator, and snow cover in southern Ontario is decreasing. Explain why a winter with less snow can actually mean colder soil, and why this could matter for plants and animals.
Snow traps air, so a deep snowpack insulates the soil from very cold air above. With thin or no snow, the cold air reaches the ground directly, so the soil can freeze deeper and get colder, even if the winter is warmer on average. Plant roots, bulbs, overwintering insects and small mammals (like voles living under the snow) depend on that insulation and can be harmed.
Quiz
1. Why does the temperature of ice water stay near 0 °C until all the ice melts?
2. Which process releases energy into the atmosphere?
3. How much energy is needed to evaporate 10 g of water at 100 °C? (L = 2260 J/g)
4. Where is a rain shadow found?
5. In Ontario, where are the main lake-effect snowbelts?
6. Why does a warmer climate tend to bring heavier downpours?
Big idea: Water changes state all over the climate system, and each change absorbs or releases large amounts of latent heat. Evaporation and condensation move heat from the surface to the upper air and from the tropics toward the poles; mountains, lakes, snow and ice shape local climates. This addresses D2.6 (water in its states and climate patterns) and heat transfer (D3.2).
How to help: Do the ice-water investigation together; seeing the thermometer sit at 0 °C while ice melts is memorable. Talk about lake-effect snow if you live in or travel through a snowbelt. Check that your teen uses Q = mL for changes of state and Q = mcΔT for temperature changes, and adds them when both happen.
Common difficulties:
- Thinking temperature must rise whenever heat is added.
- Mixing up which processes absorb energy (melting, evaporation) and which release it (freezing, condensation).
- Forgetting to convert kg to g.
Safety: supervise glassware and fans near water.