Competition between sensible heat and latent heat

Sensible heat by definition is the energy required to change the temperature of a substance with no phase change. Latent heat in simple words is the energy released or absorbed, by a body during a constant-temperature process for a phase transition.

Wherever there is moisture and heat both sensible and latent heat transfer processes are happening and one is competing with the other. The most common example for us is an exchange of energy between the earth's surface and atmosphere.

[1] The earth's surface is receiving solar energy in the form of electromagnetic radiation, absorbing the radiation energy and emitting back from the surface to the atmosphere by means of the same electromagnetic radiation.

[2] In the ground heat energy is transferring from one place to another by thermal conduction.

[3] Turbulent transfer of heat energy towards or away from the surface within the atmosphere.

[4] Evaporation of water stored in the soil or condensation of atmospheric water vapor onto the surface.

Each of these processes is associated with an energy flux density that is the rate of transfer of energy normal to a surface of the unit area as J/ m2-sec. This is equivalent to Watt/ m2. 1 watt = 1 joule/ sec.

Let us imagine a ground of certain depth, The energy balance of a surface layer of finite depth and unit horizontal area can be written as, dQ/dt = Rn ? G ? H ? λE , ------------ [1] Q is the total heat energy stored in the surface layer. Rn is the net surface irradiance (commonly referred to as the net radiation). It represents the gain of energy by the surface from radiation. It is a positive number when it is towards the surface. G is the ground heat flux. It is the loss of energy by heat conduction through the lower boundary. It is a positive number when it is directed away from the surface into the ground. The value at the surface is denoted G0. H is the sensible heat flux. It represents the loss of energy by the surface by heat transfer to the atmosphere. It is positive when directed away from the surface into the atmosphere. λE is the latent heat flux. It represents a loss of energy from the surface due to evaporation. (λ is the specific latent heat of evaporation, units J kg?1 and E is the evaporation rate, with units kg m?2s?1).

For an infinitely thin surface layer, the heat storage Q in Eq. 1 is zero and reduces to,

Rn ? G0 ? H ? λE = 0 ------------ [2]

Or, [Rn ? G0] = H + Λe --------[3]

The quantity [Rn-G0] is known as the available energy.

The way in which the available energy is partitioned between the sensible and latent heat flux can be quantified by taking the ratio of the sensible to latent heat flux, which is known as the Bowen ratio. It is the ratio of sensible to latent heat fluxes from the earth's surface up into the air.

B0 = H/λE --------[4] H is the sensible heat flux and λE is the latent heat flux.

The Bowen ratio is non-dimensional and depends on the availability of water at the surface.

For surfaces where water is freely available B0 is small, and most of the available energy is transferred to the atmosphere in the form of latent heat.

For surfaces like deserts, B0 is large, and most of the available energy is transferred to the atmosphere in the form of sensible heat, which warms the air close to the surface.

Vegetation is a significant influence on the Bowen ratio.

Typical values of BO are 5 over dry land regions, 0.5 over grasslands and forests, 0.2 over irrigated orchards or grass, 0.1 over the sea, and negative in some advective situations such as over green area in a desert region s where sensible heat flux can be downward while latent heat flux is upward.

Competition of sensible heat and latent heat in melting of ice and vaporization of water

Competition between the latent heats of fusion and vaporization becomes a factor if energy is supplied to the snow surface in substantial amounts from radiation or the atmosphere. Evaporation is then in competition with melting. The partition of available energy between them depends on the vapor pressure in the top atmosphere. When atmospheric vapor pressure is very low, evaporation may claim a sizable fraction of the total heat available, but pressure must be less than 6 mb, which is the vapor pressure at a surface of melting snow if there is to be net upward movement of water molecules from the snow surface. At times when the air is warm enough to support a flow of sensible heat to the snow, it is likely also to be humid and will therefore supply vapor to the snow. The major source of heat/temperature in the snowy sea is the ocean. Oceanic airstreams are both warm and humid relative to the 0°C and 6-mb limits of a snow surface. Evaporation based on the sensible-heat flux from air to snow is, therefore, a rare event.

Credit: Google