Questions: Latent Heat and Water Phase Transitions

Question 1

A kilogram of water evaporates from the ocean surface. Compared to raising the same kilogram of liquid water by 1°C, approximately how much more energy does evaporation require?

• A. About 6 times more (latent heat of vaporization ≈ 2,500 J/kg vs. 4,186 J/kg·°C × 1°C)
• B. About the same — both processes require roughly 4 kJ/kg
• C. About 600 times more (latent heat of vaporization ≈ 2,500 kJ/kg vs. ~4.2 kJ/kg per °C)
• D. About 60 times more — latent heat is significant but not dramatically larger

Answer: (C)

The specific heat of water is ~4.2 kJ per kg per °C, so raising 1 kg by 1°C requires ~4.2 kJ. Evaporating 1 kg requires ~2,500 kJ — roughly 600 times more. This enormous ratio is why evaporation is such a powerful energy transport mechanism: a thin layer of ocean water evaporating can transfer as much energy as heating a far larger mass of air by many degrees.

Question 2

When water vapor condenses in a rising air parcel to form clouds, the latent heat released is lost from the atmospheric system and does not contribute to any further weather processes.

• T. True
• F. False

Answer: False

Latent heat released during condensation directly warms the surrounding air parcel, making it more buoyant and causing it to rise further. This is why cumulonimbus clouds grow so tall — each layer of condensation releases heat that drives the next layer of ascent. The energy is not lost; it converts from latent (stored in water vapor) to sensible (measurable temperature increase), which then drives convection. This feedback loop is the engine of thunderstorms and tropical cyclones.

Question 3

Why do tropical cyclones (hurricanes/typhoons) rapidly weaken when they move over land or cooler ocean water?

Model answer: Tropical cyclones are sustained by latent heat released from the condensation of water vapor that evaporated from warm ocean surfaces. Warm water provides the continuous evaporation needed to keep supplying vapor. When the storm moves over land or cooler water, evaporation rates drop sharply, cutting off the latent heat supply that drives the convective updrafts at the cyclone's core. Without this energy input, the circulation weakens and the storm dissipates.

This question connects the abstract concept of latent heat to a concrete, large-scale phenomenon. The warm ocean is not just a moisture source — it is an energy source, and that energy is delivered to the atmosphere specifically as latent heat embedded in water vapor. The phase transition from vapor to liquid inside the cyclone's deep convective towers releases this energy at altitude, where it sustains the storm's outflow and maintains the pressure gradient that drives inflow at the surface.