RESEARCH STARTER

Latent heat flux

Latent heat flux is a crucial concept in thermodynamics, referring to the rate at which energy is absorbed or released when water transitions between its various states—liquid, solid, and vapor. This energy exchange is distinct from sensible heat flux, which involves temperature changes in the air without phase changes in water. Latent heat flux encompasses processes like evaporation, melting, and sublimation, which absorb energy, and freezing, condensation, and deposition, which release energy.

The significance of latent heat flux extends to climate dynamics, particularly in the context of rising greenhouse gas concentrations, which increase energy availability at Earth's surface. Most of this additional energy is utilized for evaporation, resulting in an enhanced latent heat flux, especially over the vast expanses of water covering the planet. This process affects atmospheric temperatures by transferring dormant heat energy through moisture-laden air, which can influence climate patterns even in distant regions. Consequently, changes in latent heat flux can play a significant role in global warming and climate variability. Understanding this concept is essential for grasping the complexities of Earth’s energy balance and climate system.

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Definition

Water can exist as a liquid, solid, or vapor. The latent heat flux (expressed in joules per kilogram) is the rate at which energy is released or absorbed when water changes from one to another of these three states. It is latent by contrast with the sensible (or dry) heat flux caused by the movement of air. The energy involved in the various phase changes of water are known as the latent heat of vaporization (evaporation and condensation of gas and liquid), latent heat of fusion (melting of ice) and latent heat of sublimation (transition of ice directly to vapor). Evaporation of water, melting of ice, and sublimation of ice are heat-absorbing processes. The heat energy used is held in a latent or dormant (hidden) state until it is released back into the atmosphere when freezing, condensation, or the sublimation of vapor to ice occurs.

Significance for Climate Change

A rise in the concentration of greenhouse gases (GHGs) in the atmosphere makes available more energy at the Earth’s surface. This additional energy can be used either to heat the atmosphere by way of the sensible heat flux (increased warming) or evaporate water at the Earth’s surface via the latent heat flux. If water is present, the latent heat flux will always be larger than the sensible heat flux, meaning that energy otherwise available to heat the atmosphere is used in evaporation. Given that over 70 percent of the Earth’s surface is water, most of the additional energy at the surface due to GHG radiative forcing is used in an enhanced latent heat flux. The resulting warming of the atmosphere would be less in this case than if all the additional available energy was accounted for by the sensible heat flux alone.

The heating of air by the sensible heat flux at the Earth’s surface causes the air to become buoyant and, as a result, to rise and mix with the cooler air above. Moisture is entrained in this rising air. As a result, the mean annual global climate impact of increased energy available due to an enhanced greenhouse effect will manifest itself both as an increased sensible heat flux (warming) and increased evapotranspiration via the latent heat flux. The latter is dormant heat energy that can be transported great distances by wind and over time without loss. The impact on climate may occur in regions far from its source. This dormant heat energy does not radiate back to space until condensation occurs and returns the heat to the air.


Cheng, Minghan, et al. "Up-Scaling the Latent Heat Flux from Instantaneous to Daily Scale: A Comparison of Three Methods." Journal of Hydrology: Regional Studies, Apr. 2022, doi.org/10.1016/j.ejrh.2022.101057. Accessed 19 Dec. 2024.

Fernandez, Pablo, et al. "On the Mechanisms Driving Latent Heat Flux Variations in the Northwest Tropical Atlantic." AGU, 2 May 2024, doi.org/10.1029/2023JC020658. Accessed 18 Dec. 2024.

Ritter, Michael. "The Energy Balance." LibreTexts, 24 May 2024, geo.libretexts.org/Bookshelves/Geography_(Physical)/The_Physical_Environment_(Ritter)/04%3A_Energy_and_Radiation/4.03%3A_Radiation_and_Energy_Balance_of_the_Earth_System/4.3.02%3A_The_Energy_Balance. Accessed 19 Dec. 2024.

Full Article

Definition

Water can exist as a liquid, solid, or vapor. The latent heat flux (expressed in joules per kilogram) is the rate at which energy is released or absorbed when water changes from one to another of these three states. It is latent by contrast with the sensible (or dry) heat flux caused by the movement of air. The energy involved in the various phase changes of water are known as the latent heat of vaporization (evaporation and condensation of gas and liquid), latent heat of fusion (melting of ice) and latent heat of sublimation (transition of ice directly to vapor). Evaporation of water, melting of ice, and sublimation of ice are heat-absorbing processes. The heat energy used is held in a latent or dormant (hidden) state until it is released back into the atmosphere when freezing, condensation, or the sublimation of vapor to ice occurs.

Significance for Climate Change

A rise in the concentration of greenhouse gases (GHGs) in the atmosphere makes available more energy at the Earth’s surface. This additional energy can be used either to heat the atmosphere by way of the sensible heat flux (increased warming) or evaporate water at the Earth’s surface via the latent heat flux. If water is present, the latent heat flux will always be larger than the sensible heat flux, meaning that energy otherwise available to heat the atmosphere is used in evaporation. Given that over 70 percent of the Earth’s surface is water, most of the additional energy at the surface due to GHG radiative forcing is used in an enhanced latent heat flux. The resulting warming of the atmosphere would be less in this case than if all the additional available energy was accounted for by the sensible heat flux alone.

The heating of air by the sensible heat flux at the Earth’s surface causes the air to become buoyant and, as a result, to rise and mix with the cooler air above. Moisture is entrained in this rising air. As a result, the mean annual global climate impact of increased energy available due to an enhanced greenhouse effect will manifest itself both as an increased sensible heat flux (warming) and increased evapotranspiration via the latent heat flux. The latter is dormant heat energy that can be transported great distances by wind and over time without loss. The impact on climate may occur in regions far from its source. This dormant heat energy does not radiate back to space until condensation occurs and returns the heat to the air.


Cheng, Minghan, et al. "Up-Scaling the Latent Heat Flux from Instantaneous to Daily Scale: A Comparison of Three Methods." Journal of Hydrology: Regional Studies, Apr. 2022, doi.org/10.1016/j.ejrh.2022.101057. Accessed 19 Dec. 2024.

Fernandez, Pablo, et al. "On the Mechanisms Driving Latent Heat Flux Variations in the Northwest Tropical Atlantic." AGU, 2 May 2024, doi.org/10.1029/2023JC020658. Accessed 18 Dec. 2024.

Ritter, Michael. "The Energy Balance." LibreTexts, 24 May 2024, geo.libretexts.org/Bookshelves/Geography_(Physical)/The_Physical_Environment_(Ritter)/04%3A_Energy_and_Radiation/4.03%3A_Radiation_and_Energy_Balance_of_the_Earth_System/4.3.02%3A_The_Energy_Balance. Accessed 19 Dec. 2024.

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