Re: Which water heats the fastest ; muddy or clear?

You have asked a question that does not have a clear answer. The system is 
a very complicated one with many other factors to consider.

First, let's think about solar heating of the lake. The amount of 
reflection from the lake surface is unlikely to change very much with the 
muddiness of the water. All of the sunlight that is not reflected by the 
surface will be absorbed by the lake. None (or a negligible amount) will be 
reflected from the lake floor and make it back out of the lake again, 
whether it is muddy or not (unless you are talking about a lake that is 
shallow and crystal clear, so that its bottom is visible and bright!) So 
the total amount of sunlight energy trapped is hardly affected at all by 
whether or not the water is muddy. But the depth at which it is trapped may 
be considerably affected. The energy from visible and ultraviolet sunlight 
will only penetrate a centimetre or two into muddy water; it will be 
gradually absorbed as it travels to much greater depths in clear water. 
Water is much less transparent to infrared light; all of that energy will 
be trapped by layers close to the surface, whether or not the water is 
muddy.

So the total amount of heating will be the same. But for muddy water, the 
whole of the heating will be from the top. For clear water it will be 
mostly at the top, but there will be some heating at deeper levels also.

Second, we need to think about the severity of the winter! Why? Well water 
is a most unusual substance. It attains its greatest density at 4 deg C 
(about 39 deg F). Below that temperature it is less dense, and actually 
expands as it cools! If the winter is very severe, the lake will have a 
layer of ice on top, the water immediately below the ice will be at 
freezing point, and will gradually get warmer, reaching some value greater 
than 0 deg C but less than 4 deg C at the floor. If a lake is in this 
condition when it is heated from above, and the heat is trapped in surface 
layers, then turbulence will result as the warmer surface water sinks! Some 
of the heat energy will be carried downward by convection. But if the 
winters are less severe, then the lake will have a surface temperature of 
perhaps 5 to 10 deg C, which will gradually decrease to some value greater 
than 4 deg C at the floor. It will be stable when heated from above, there 
will be little or no convection or turbulence, and the heat energy will not 
be efficiently carried downward from the surface layers.

Third, we need to think about re-emission of infrared radiation. There will 
be a nett loss of energy from the lake by this mechanism whenever the 
surface layers of water are warmer than the surrounding air and 
countryside. This will probably mean most of the hours of darkness through 
the spring. Both clear water and muddy water are probably fairly close to 
black body radiators as far as infrared radiation is concerned. But the 
surface layer temperatures are likely to be a bit higher in muddy water, 
depending on the detail of the interactions in the first two considerations 
above.

Another point that you might consider is that if the muddiness is because 
of rainwater runoff, then what about the energy input to the lake from the 
rainwater? Is it warmer or colder than the average temperature of the lake 
when it flows in, for example? 

And finally, there is the issue of evaporative cooling. Heat is lost from a 
lake as its surface waters evaporate. The speed and efficiency of 
evaporation depend critically on the exact details of winds (unaffected by 
muddiness) and surface ripples (which may be affected by muddiness), and on 
any monolayer surfactant adsorption at the surface (which will certainly be 
affected by muddiness).

I hope I have uncovered enough complications to convince you that I cannot 
give a simple and accurate answer.

John.