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I need help with latent and sensible heat. P.S: I have been through all Wikipedia and other pages that say phase change and temperature change.

My Question is: if there is convection, it will take the heat away from surface, what is the chances that the heat taken is latent or sensible?

Evaporation is also convection and heat transfer is also convection, so where does sensible and latent heat stands in this story.

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First some definitions:

Latent heat is the heat that leaves or enters a system during a process where the temperature of the system stays constant. Phase changes are prime examples where latent heat leaves (exothermic) or enters (endothermic) a system. Ice melts (endothermic) and water freezes (exothermic) at constant temperature. Sensible heat is the heat that enters or exits a system when the temperature of the system changes. Heating or cooling water are cases that involve sensible heat. All heat flow between two systems requires a temperature difference between the systems. Convection is heat flow between two systems by movement of mass. The two other heat flows are conduction and radiation.

Now to the questions:

During a phase change, the temperature of the system stays constant. The heat gained or lost is called latent heat. To vaporize a liquid, we must supply heat. Hence vaporization is endothermic (heat enters the system). To get the heat into the system, we must set the surroundings at a hotter temperature. Otherwise the heat will not flow. The process by which the heat flows can be any one of the three modes. We can set a pot of water in a pan on an electric heater and realize conduction from the burner to the pan to the water. We can set use a hair dryer to blow hot air over the pot and realize (forced) convection from the air to the pan. Finally, we can set the pot a small distance above the glowing electric burner and realize radiation from the electric burner to the pot.

By contrast, consider the case of a day where the outside temperature is below freezing and the air is still. A duck pond in the yard is not frozen over. Clearly, it’s water temperature is above freezing. Should we jump in the duck pond to warm up? Of course not! The natural convection coefficient of stagnate water is higher than the natural convection coefficient of stagnate air (and the difference only becomes greater in flowing water versus flowing air). The water pulls heat from us by convection faster than the air does at the same temperature, and our body temperature will drop faster than when we would stay standing in the still air. Also, while the conduction coefficient of water is larger than that for air, the main heat transfer in a fluid is convection, even in stagnate cases. The heat that flows out of our body as our body temperature drops is sensible heat.

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  • $\begingroup$ I haven't seen latent heat particularly defined that way before... do you have a source? $\endgroup$
    – JeopardyTempest
    Commented Aug 2, 2018 at 13:27
  • $\begingroup$ The definition is general. Here is an equivalent. en.wikipedia.org/wiki/Latent_heat I would be lax to say this is a certifiable reference. Rather, it is how I would explain it in a course on thermodynamics. We have latent heats of vaporization, fusion, solidification, and so forth. My preference in that case would indeed be to use the term enthalpy rather than latent heat. $\endgroup$
    – Jeffrey J Weimer
    Commented Aug 2, 2018 at 14:15
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    $\begingroup$ Oops! I just fixed a mistake in my statement. Heat gained or lost during a phase change is latent not sensible. $\endgroup$
    – Jeffrey J Weimer
    Commented Aug 2, 2018 at 14:18
  • $\begingroup$ Reference works fine, it just gives a place to say it. I just always thought of latent heat as having the central definition of being the energy transfer during a phase change, so though it didn't have huge implications, interesting to see it's defined that more general way instead. :) Thanks $\endgroup$
    – JeopardyTempest
    Commented Aug 2, 2018 at 15:01
  • $\begingroup$ Sir, i have a follow up question. In case these process occur in nature, during sensible heat the planetary boundary layer (PBL) is extended and so does the air temperature is affected. during the latent heat transfer the vice versa at PBL level occur. So during the Sensible and Latent heat transfer the water vapor transferred is of same amount through convection or latent heat will take more than Sensible heat. Regards $\endgroup$
    – khan
    Commented Aug 2, 2018 at 15:44
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Sometimes in meteorology, at least here in the US, we actually call thunderstorms convection, which can confuse the terminology a bit.

There is moist convection (e.g., storms) and dry convection (e.g., thermals). Sensible heat (and/or dynamical processes like fronts, orographic lift, or positive vorticity advection increasing with height) initiate the rising motion to start such convection. All air has moisture, it's just that in moist convection, latent heat subsequently becomes important as a byproduct of the initial rising air, and the result becomes is something wet!

If the atmosphere is sufficiently moist, and the lift is enough to reach the LCL... then within the rising air the water vapor will begin to condensate/deposit... which thereby releases its latent heat into the air... and this additional energy help the air to rise further (since it will aid it in being warmer [and thus less dense] than surrounding air).

So convection itself is a sensible process, basically warmer air moving up into cooler air.
But latent heat release from condensing rain drops (or other precipitation forming) will add extra energy to help the air convect even more, and is vital in the formation of the strong updrafts in thunderstorms.

The initial energy that convection is taking upwards... will generally be from sensible heat. This is because the only form of latent heat transfer which would add energy to the atmosphere... would be condensation/freezing/deposition. But those basically only happen in air that is cooling down... since temperature drops are the most effective way to bring air nearer to saturation quickly.
And if near-surface air undergoes cooling, it will generally be unable to rise.
There's really one time there's condensation/deposition from the near-ground air layer: towards dawn. Coming as a result of the surface air layer losing energy via conduction to the colder ground.

(Note that in meteorology, though all motion of air of different temperatures can be legitimately termed convection... we also typically use the term primarily in regards to vertical motion. Horizontal motion of air masses is called advection. But advection can certainly move energy just the same, it's just often driven by somewhat different atmospheric processes)

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    $\begingroup$ Thank You @Jeopardy Tempset. The information you provided is too much for me, i will take my time to process and absorb it. I will come back to you with follow up discussion in a separate question or in Chat. Thank You for nice suggestions and information. $\endgroup$
    – khan
    Commented Aug 3, 2018 at 13:59
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    $\begingroup$ Thank you for clarifying from the perspective of meteorology. In engineering analysis, convection moves heat from one body to another using a flowing fluid. The bodies are always at different temperatures. We do not use the terms sensible and latent heat. We distinguish energy content using terms such as enthalpy and distinguish the process of energy flow (system <-> surroundings) as mechanical work or heat flow. Finally, we would likely consider vertical convection as just natural convection and advection either as forced convection or as part of a natural convection flow cell. $\endgroup$
    – Jeffrey J Weimer
    Commented Aug 3, 2018 at 20:52
  • $\begingroup$ @JeffreyJWeimer Yeah, certainly in meteorology much advection is forced as well. But with it a spectrum of how much vertical motion is being forced vs natural, (I think) we often use it for all (vigorous) vertical motion. We certainly do use mechanical work/enthalpy/etc in research, the terms just aren't as common in operational work as convection/advection. Interesting how different fields adapt their own language slightly (even as it sometimes leads to confusion... wind shear is another example of a term I've seen some confused by, with slightly different uses in aeronautics vs meteorology) $\endgroup$
    – JeopardyTempest
    Commented Aug 3, 2018 at 22:59

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