In changes of state you saw that melting and boiling take in energy without raising the temperature, and freezing and condensing give it back. In the atmosphere this hidden energy, carried in water vapor, is one of the biggest energy flows on the planet. It builds clouds and powers storms. It follows the warming of water you met in why coasts stay mild, but it is far larger.
🔑 Key idea
Evaporating water takes in its latent heat of vaporization, about $L_v = 2.5 \times 10^6$ J/kg near 0 °C ($2.26 \times 10^6$ J/kg at 100 °C), and condensing vapor releases the same amount: $$Q = mL$$ Melting ice takes in $L_f = 3.34 \times 10^5$ J/kg, and freezing gives it back.
💡 Why it works
In liquid water, molecules cling to each other with hydrogen bonds. To escape as vapor, a molecule must break free, and that takes energy, which comes from the surroundings and cools them. That is why sweating cools you and a wet towel feels cold. The energy is not lost: it rides along in the vapor. When the vapor condenses into cloud droplets, the bonds form again and the energy is released into the air around the droplets, warming it.
✏️ Worked example: How big is latent heat?
Compare the energy to evaporate 1 kg of water with the energy to warm 1 kg of water by 1 °C.
Warming by 1 °C: $Q = mc\Delta T = 1 \times 4186 \times 1 = 4186$ J.
Ratio: $2{,}500{,}000 \div 4186 \approx 600$. Evaporating a kilogram of water takes as much energy as warming it by about 600 degrees, if it could stay liquid.
Answer: About 600 times as much
✏️ Worked example: Warming a cloud
In a rising cloud, 2 g of water vapor condenses in each kilogram of air. How much energy is released, and by about how much could it warm that kilogram of air? (For air, $c_p = 1005$ J/(kg·°C).)
Warming: $\Delta T = Q/(mc_p) = 5000 \div (1 \times 1005) \approx 5$ °C.
That is why saturated rising air cools more slowly than dry air.
Answer: 5000 J, about 5 °C
⚠️ Common mistake
Thinking condensation cools the air because it happens in cool air. Condensation happens when air cools to its dew point, but the condensing itself releases heat into the air. Evaporation cools; condensation warms. Check: a puddle drying takes energy from the ground (cooling); a cloud forming gives energy to the air (warming).
A parcel of moist air rising. Below the cloud base it cools at the dry rate; inside the cloud, condensation releases latent heat and it cools more slowly, so it stays warmer than the air around it and keeps rising.
Latent heat moves enormous amounts of energy around Earth. Averaged over the globe, evaporation is the biggest way the surface sends energy up into the air, roughly a quarter of all the sunlight arriving at the top of the atmosphere (NASA). The energy travels as vapor, sometimes thousands of kilometers, and comes out where clouds form. A hurricane is a machine that runs on it: warm ocean water evaporates, and the vapor condenses in the storm's towering clouds.
Practice
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