People prefer simple rules. The sun comes up and it gets warm. The sun goes behind a cloud and it gets cooler. On a flat surface, in a vacuum this is absolutely true all the time. I recently posed a question, a statement really, to a podcast that I listen to. I stated that people think that rain melts the snow and when it rains the snow melts and therefore rain melts the snow. I suggested that there was something else involved, the condensation of water vapour on the snow was actually a bigger driver of snow melt. The result of this conversation was a segment on the aforementioned podcast, which I have not mentioned.
I looked on there discussion page and saw the flurry of differing viewpoints. People suggested the most inane things that were not part of ice melting like the heating caused by the kinetic energy of the raindrop. The person suggested that kinetic energy converted to heat energy one for one, while ignoring empirical evidence, that moving objects move other objects when they impact each other; the snow moves when the rain hits it, it compacts or the rain bounces, splatters. Some of this energy must convert to heat energy, but only if the rain does not splatter or compact or move the substance it impacts, the snow.
The best dissenting response was that the Sun melts the snow, and this is one hundred percent true in the sense that the sun is the only energy input into the Earth's closed system energy budget. The problem is how to express the way I see the world so that you see the world as I see it. I once wrote about wind, how it is similar to light. Light is both a wave and a particle, everyone has heard that, if they don't understand it. So is wind: it is a solid force blowing air over the land in one direction for high pressure to low pressure, but it is also packets of air moving at different speeds caused by turbulence; it is of different sizes and you can see them when it hits the water and fields of grass or dusty parking lots.
What causes turbulence? Physical structures cause it. I am stretching my memory to remember the diagram that I saw nineteen years ago while researching an essay that I never wrote, that turbulence caused by physical structures is affected by the height and the width of the structure. Typically five times e height and three times the width of the structure down wind. Of course wind speed would also have a factor in this. Other factors that I can think of that would affect turbulence are different air masses with different densities interacting and different air speeds or directions. Air density can be affected by temperature, warmer air is less dense and humidity, humid air is less dense. Humidity could be changed into composition differences, that would make it universal. Because water is so common on the Earth, it is humidity that is most common influence on air, but increase of any substance in the air over the average composition would have a measurable effect.
As I said because Water is so common on Earth, it is one of the main drivers in the atmosphere. The amount of water that air can hold is defendant on the temperature of that air mass and the abundance of the water in the environment. And then it gets complicated. Climate is any region's weather range over the course of a large period of time, macro scale. Large areas and large times. Weather is the what is happening at any given moment, but climate is broader and deeper. Climate is about what extremes of weather you can expect in an environment. Micro Climate is how the physical environment impacts the greater climate of the region. The scale of a micro climate is human terms, or the effects that things in our personal world. For example, a mountain affects the general climate, one side may get more sun than the other, one side may get more rain than the other. Ditto with hills, but generally lesser than mountains. The composition of the environment affects the micro climate, soil types affect what grows there and what the effects rain has in the area so it affects the micro climate. Something as little as a house affects the environment all around it and thus alters the micro climate of different sides of the house. That is about as small scale as people think in, but it actually is more complicated. What is growing in an area affects the climate too. So I propose the term Nano-Climate to separate the truly small environmental effects from the human scale effects.
Some people would say that I am splitting hairs, dividing things down to a degree that it has no effect on the world, but I would counter that by saying the micro climate of an area is the sum total of the smaller affects, the smaller interrelations with the environment and changing one of these factors would have a net affect on any area's Micro Climate, just as altering many micro climates would alter the overall climate of a region and then affect the entire world. I am saying that it is time to realize that tiny changes in the environment have impacts on everything. I see the tiny environments that we live in.
If this were an essay, all the above would be the introduction and only 5-10% of this post, good thing that this is not an essay, eh? I wanted to talk about melting snow, so I will try to limit myself to that.
When the Sun is relatively directly overhead it bombards the surface of the Earth with approximately 1300 watts per square meter per second in visible light. Surfaces on an angle affect this and when the sun is not directly overhead also would affect this number. The surface albedo, the scientific word for reflectivity, also affects it. Only utterly black substances have no albedo and and only pure white or perfect mirrors have total albedo. Albedo is a value that ranges from these two extremes 1-0 which could be thought of as a percentage. Snow has a low albedo and oil his a high albedo. Water actually has a really high albedo that is not affected much by the angle of light much, which is why water almost always appears dark. Wet surfaces almost always appear darker than dry surfaces. I would say always, but there might be an exception.
Asphalt, for example covered with a splash of water appears darker than surrounding asphalt, the water has lowered the asphalt's albedo and more light is being absorbed by the surface. I would say that the difference in the albedos is the amount of the light that the water is absorbing over and beyond what the asphalt is absorbing. This absorption increase is a Nano Climate effect as it could be as small as a raindrop, but in any case will have an unnoticeable affect on any human but be a contributing factor to the whole. If the asphalt is grey and is reflecting, say 50% of the light and the addition of water decreases the albedo to reflect only 10% of the light, then we could say that that extra 40% is being directly absorbed by the water film on the surface. For the record I am looking at some dry and wet asphalt right now; the road that passes my window.
If the sun was directly overhead the water would be absorbing about 520 w/s per square meter, or 520 Joules of energy. If it were in a vacuum, then we might say that the energy would only go into heating the water. Assuming that the water was only just covering the asphalt, say 1/10th of a millimeter thick, then the amount of energy needed to raise its temperature on degree Celsius would be about 420 joules. The water on the road would be boiling in about a minute and a half. But it is not in a vacuum, there is air and air will allow for two things to occur: heat transfer, the air will get warmer and evaporation or water transfer. The Latent heat of evaporation of water is 2,270 kJ/kg, so the energy needed to evaporate the square meter of slightly covered asphalt is about 227,000 joules or it would take about seven and a half minutes under ideal conditions, if there was no wind or if the asphalt was pure white. Why wind? Why white asphalt?
The albedo of the asphalt has an effect too. The energy that the asphalt itself is absorbing can contribute to the heating of the air and the evaporation of water. On any Summer day people see the heat shimmers ofer asphalt so we know it is heating the air. The grey asphalt is absorbing about 650 joules of energy per second from the sun this can be added to the water and to the evaporation of the glaze of water to cut the time needed to evaporate the water to about three minutes and a quarter. If there is a breeze, the moving air from adjacent dry asphalt to the glaze of water, increasing the evaporation again. We have all seen how quickly roads get dry after a summer thunderstorm. I have just broken it down for you.
Now consider the snowbanks beside the road way. The road is heating up and the snow is cold, pretty obvious. The sun beats down on the snow but due to its high albedo, almost all of the light is reflected, but over the road, about half of the light that hits the road is reflected, but half is absorbed and heats the air. The heat air moves the warm air over to the snow and raises the temperature to freezing and begins the process of melting. The latent heat of melting is 334 kJ/kg, so a lot of air has to be exported to the snowy region to melt the snow. But there would only have to be a little melting to have an effect. Snow has different densities depending on its age and air temperature, fresh snow has the lowest density as low as a tenth of the density of water, if only a little bit of snow is melted it would be enough to make a difference because liquid water absorbs most of the light it receives.
Here is another place that nano climate has an effect. As school children we are taught about the convection of air over land beside a body of water. The air over the land heats up faster and rises, while the air from over the water rushes in to replace the rising air. The rising air cools and then replaces that air that has left the region over the water to complete the cycle. This is a local effect, but there is also a macro effect that affects the world and a micro effect that is on any border between different surfaces everywhere. One such location is the road surface and the snow bank. The road heats up and rises, the air over the snowbanks moves in to replace it and the air above the roadway cools and moves over the snowbanks where it cools more. An nano scale convection current.
33,400 joules would be enough to melt a little of the snow, if all the air transfers it heat and it would only take a minute of intense light to melt a bit of snow, a millimeter per square meter, but that is not likely to happen. The ideal example is a simple way to explain it. Snow reflects light in every direction so it appears white, but it also reflects it into the snow pack too, so water in the snow pack would absorb the light. Thus wet snow absorbs more energy and the energy would all be used to melt more snow. What usually happens is that fresh snow falls into a cold environment and the melted snow refreezes and the result is old snow. Old snow is more granular and has a lower albedo, so it reflects less light and absorbs more. Old snow makes more melts a bit faster and in cold weather makes older snow with a lower albedo.
I was talking about snow beside a roadway, because that is where people these days see the snow melt, but snow melts far from roads too, but they do not get the imported heat from hot air and the depend on the light not reflected from the snow to melt it. That is an oversimplification, because there are trees and tree trunks heat up just like roadways do when the sun hits them and the air heated up from close contact with the tree melts the snow exactly like the roadway.
Additionally, another factor that one must not forget is black body radiation. Most people know this as heat, but in this case it is the energy that all things emit. The amount of radiation each object emits is based on their temperature and their insulation. People emit infrared radiation at about 37°C, but everything emits radiation unless they are at absolute zero. So even if a tree is -20°C, it is emitting radiation and the radiation is melting the snow. This is why if you look at a tree in old snow you will see a gap in the snow around the trunk and why the snow seems to melt faster under trees. Black body radiation is the balancing portion of the Earth's energy budget: radiation hitting the Earth/Albedo - Black Body Radiation = 0. This is how carbon-dioxide warms the earth, it reabsorbs the Black Body Radiation and reflects it back, warming the Earth.
That is snow but what about air? Besides heat, the air can carry other things, like dust which can darken the snow surface and decreases its albedo. Air can also absorb water. Above I mentioned that water on roadways heats up and evaporates into the air. Air is not something that can hold a standard amount of water, if it were so we would not have weather at all, but it is temperature dependent, the warmer that air the more water it can hold. Since it is temperature dependent there are more and interesting variables. Air does not absorb water linearly (every increase in temperature it absorbs an equal amount of water), but it absorbs water exponentially (as the air increases in temperature it absorbs an increasing amount of water). The amount of water that air can hold increases marginally when air is lower than 10°C, but increases faster when it warms past that temperature.
Often when you get a weather forecast you will see something called Dew Point Temperature, that is the temperature that air can hold only the water that it is currently holding and no more. It is the temperature that fog or mist or clouds would form and it is the temperature that would have to be reached for those clouds to rain or snow. When the water condenses out of the air it heats it up, because the energy that was needed to evaporate the water is expelled to condense it.
One of the oddities of water is that the energy needed to change its state from gas to liquid and liquid to solid is seven times the other. Any given amount of condensing water would melt about seven times as much ice. Because of the temperature difference between road ways and a snow bank, the air over the road may pick up more evaporated water. When the air cools over the snowbank, the absorbed water may exceed the carrying capacity of the air and condense out. The released energy would warm the air, but since it would be close to the snow, the snow would heat up and melt; seven times as much snow could potentially melt as the water condenses.
The effects don't just work with big things like roads and snow banks, they work with tree trunks and buildings and even dirt particles in the snow. The last creates beautiful ice structures along the edge of the road way all winter long, which everyone can see if they look: The light hits the dirt particle in the snow and heats up. The warmer air melts some snow and the dirt becomes wet, which allows it to absorb more light and heat the air more melting more snow. Soon there is a pocket of air around the particle and the air over the particle absorbs water which condenses and melts snow further away from the particle. The warmer air around the dirt meets the air outside which is cooler and the water freezes as ice. If the dirt is close to the surface the nano climate creates peaks and valleys, but if it is deeper in the snowpack, delicate ice structures form with ice walls surrounding a small air pocket.
It is easy just to look out over the landscape and see just the big things. Stop take a look at edges, and the micro regions and the nano scale of our world, see the beauty. The little unseen nuisances.
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