Thursday, December 29, 2016

Rain for deserts near cold seas

Deserts near cold sea, such as the Namib Desert, appear to me to be ideal candidates for rain making. With the Namib there is a good strong sea breeze bringing in air from the sea as land warms up, and this breeze starts fairly early in the morning. Sometimes mists form and when the sun heats these mists up the water droplets evaporate, thus cooling the air. The cool air from the sea and the cooling by evaporation cause a stable situation - that is the air does not rise because of cold air beneath warmer air, or same temperature air. If the air were warmer than the surrounding air, it would rise. Looking at the air around Walvis Bay, one may note that relative humidities are high, but lack of rising air, that would cool on ascent to form rain, causes dry conditions. Because air requires contact with warm surfaces to heat up and only makes fairly superficial contact with hot ground, there is no efficient heating of the air. But if one could use huge solar air heaters in the Namib desert (there is plenty of space) one would have air rising and hopefully causing rain. Using average sorts of figures from Wikipedia for Walvis: Jan T=17.5 deg C, RH= 80%. Now if the air is cooler than surroundings, it will not rise. Say the surrounding air is at 17.5 deg C. Now heat air up to 22.5 deg C, using huge solar air heaters, and we find that the air could rise 1515 m and it only needs to rise 1060 m before clouds form (used general sort of lapse rates and Espy's equation). The high relative humidity makes for very good rain prospects. These are general figures and actual lapse rates would have to be determined, but things look good. It might be interesting to note that whilst air does not significantly heat up with solar radiation (it has to be in contact with a hot surface), clouds do heat up with sunlight. In fact clouds absorb all thermal infrared and at Earth's surface roughly 50% of "sunlight" is infrared radiation. Mist is a sort of low level cloud. So clouds or mist will heat up, water droplets will evaporate, cooling things, and so on.
See also  
 http://www.homepages.ed.ac.uk/v1ewaveg/rain%20making/shs%20rain%20paper%20Feb.pdf on why Saudi Arabia is dry.
I must point out that when temperatures are low the air cannot hold much water vapour. The graph below shows the maximum number of metric tons 1 cubic kilometre of air can hold at different temperatures (number of metric tons when the air is saturated).

Wednesday, December 28, 2016

Cooling Earth using solar air heaters.

Ice reflects solar energy fairly well, keeping Earth cool. When ice melts, dark solar energy-absorbing surfaces sometime take the place of the ice surfaces. 

https://nsidc.org/cryosphere/seaice/processes/albedo.html says: Sea ice has a much higher albedo compared to other earth surfaces, such as the surrounding ocean. A typical ocean albedo is approximately 0.06, while bare sea ice varies from approximately 0.5 to 0.7. This means that the ocean reflects only 6 percent of the incoming solar radiation and absorbs the rest, while sea ice reflects 50 to 70 percent of the incoming energy. The sea ice absorbs less solar energy and keeps the surface cooler.
The whole process of rain formation takes heat from the lower levels where evaporation cools things and takes it higher up where clouds form from condensation. This condensation releases heat again, so that the heat has been transferred from lower to higher regions. The clouds then generally reflect solar energy, cooling Earth, but high level cold clouds might actually warm Earth.. Low level clouds are especially associated with cooling of Earth (for one reason, they are warmer and emit more energy by radiation than high level clouds). We could create something similar in the following manner. When ice melts, shade the area with huge solar air heaters. The solar air heaters prevent ground from heating up, because of the shading and absorbing of the solar energy, and they are cooled to some extent by the air passing through them. This hot air rises, allowing for the possible formation of low level clouds, which could cool Earth by reflection and radiating to space. So this process again takes energy from lower regions to higher regions as with rain formation. But generally, one could put solar air heaters on the roofs of buildings in hot climate regions. This would cool buildings and enhance cloud and rain formation. Low level clouds in warm climate zones are especially associated with cooling of Earth.

Tuesday, December 27, 2016

Pieces of art to bring rain

See how to build a solar air heater at
http://www.builditsolar.com/Experimental/PopCanVsScreen/PopCanVsScreen.htm
Your piece of art can be built in the shape of a solar air heater and the dark painting inside can absorb solar energy.
To heat up air well requires hot surfaces for the air to come into contact with. Black objects absorb solar energy well and so if you have black gauze or a black wire mesh that has a large surface area that air can come into contact with and the surface is hot, you will heat the air well. When a black object heats up it radiates heat and this heat will generally be lost to the surroundings. The glass (or other glazing) on the solar air heater keeps heat in, but even without it you can heat up the air with a piece of art made of black material, such as a wire mesh, that stands in the sun.
For those interested: The book Heat Transfer by JP Holman tells us that ordinary window glass transmits radiation up to about 2.5 microns (the energy goes through the glass if its wavelength is less than about 2.5 microns).
Now the question is: How much solar energy has wavelength of less than 2.5 microns?
The answer is 'about 97% of solar energy.' So about 97% of the solar energy passes through the glass and is absorbed by the black surface of the solar air heater (assuming a perfect absorber).
Now how much of this energy escapes? Well if the absorber heats up to 50 deg C, then radiation from it that is above 2.5 microns will not escape. The answer is that far less than 1% of this radiation can escape through the glass. The reason is that the energy of wavelengths greater than 2.5 microns radiated from a black body at 50 deg C is more than 99% of the total.
If there is no glass and your piece of art is at 20 deg C, it will radiate 419 W per square metre. If your piece of art is at 60 deg C it will radiate nearly 700 W per square metre. You do not want heat loss by radiation. Instead you want the heat to heat the air so there can be convection - so it is better to have your dark piece of art in the solar air heater. "Cool roofs" can reduce rainfall and "dark art in greenhouses" can increase rain - see https://www.scientificamerican.com/article/cool-roofs-may-have-side-effects-on-regional-rainfall/

Air land temperatures and sea temperatures and rain

Looking at sea temperatures near Jeddah, I see they are very high (about 30 deg C). So why is there not rain? Literature says that it is because the expanse of the Red Sea is too small for moisture to have been picked up by air, but it also could partly be because the waves are fairly low and generally calm conditions prevail, so there is not much evaporation from spray. I have noticed that when the sea temperature is higher than the land temperature there is sometimes a dramatic increase in rainfall - see graph. The graph shows the sea and land temperatures and (although it in the wrong units of mm) shows the rainfall in mm (bottom curve).The graph starts out with sea temperature being greater than land temperature (and rainfall is relatively high). Later the land temperature is higher than the sea temperature (and rainfall is close to zero). Later the sea temperature becomes higher than the land temperature and the rainfall increases dramatically. Of course if sea temperatures are lower than land temperatures, the air heats up on going to land areas and relative humidity decreases, making rain less likely. 
For Los Angeles, etc, it would be good to investigate this: If one had shallow pools of seawater with dark bottoms to absorb solar radiation one could increase water temperatures to more than land temperatures. The second graph is for Cape Town. The upper two curves show sea and land temperatures. The lowest curve (generally) shows the rainfall in mm. The land temperature starts out being higher than the sea temperature and rainfall is low. Then the land temperature is less than the sea temperature and rainfall is high, etc. Third graph is for Los Angeles Basin. Usually high land air temperatures bring in air from the sea. But if land is hotter than sea air then the relative humidity of the sea air will decrease on being heated by the land. Here is a graph for Los Angeles basin. It uses mean sea temperatures and land air temperatures. The rainfall is in inches and the temps are in deg C (not really right to do T and rainfall on one axis, but still), The sea is hotter than land air T up to month 4. Then sea temp is cooler than land air temp up to month 9, then sea is warmer for 10, 11 and 12. Do not know why it works so dramatically in some cases, but it seems generally higher sea temps than land air temps mean much more rain.
GRAPHS:
Jeddah graph below: The graph shows the sea and land temperatures and (although it in the wrong units of mm) shows the rainfall in mm (bottom curve).The graph starts out with sea temperature being greater than land temperature (and rainfall is relatively high). Later the land temperature is higher than the sea temperature (and rainfall is close to zero). Later the sea temperature becomes higher than the land temperature and the rainfall increases dramatically. 


The second graph (below) is for Cape Town. The upper two curves show sea and land temperatures. The lowest curve (generally) shows the rainfall in mm. The land temperature starts out being higher than the sea temperature and rainfall is low. Then the land temperature is less than the sea temperature and rainfall is high, etc. 


Here is a graph for Los Angeles basin (below). It uses mean sea temperatures and land air temperatures. The rainfall is in inches and the temps are in deg C (not really right to do T and rainfall on one axis, but still), The sea is hotter than land air T up to month 4. Then sea temp is cooler than land air temp up to month 10, then sea is warmer after 10, 11 and 12. I have an idea of how it might work - hotter humid air blowing onto land would enhance rain chances.

Graphs by me (blog owner)


Wednesday, December 7, 2016

Melting snow faster

There may be snow in cold months. The Japanese use biochar on the snow to melt it. The biochar makes the snow darker and it can therefore absorb solar energy and melt days sooner than without the biochar. This could help animals trapped in the snow. One could also use ordinary dark earth dust to decrease albedo and get snow to melt earlier. Snow reflects sunlight during the day and keeps cool during the day in this manner. Snow has a high emissivity (a measure of its closeness to a blackbody in thermal properties) and gets very cold at night (releases its heat to surroundings and space).
By my calculations, roughly 1/3 of the energy emitted on a clear night by snow can radiate out to space through the 8 to 14 micron "atmospheric window" (the fraction of the total energy radiated by the snow that has wavelength of between 8 and 14 microns is about 1/3 - I used Planck's law and integrated). If one plowed the snow the albedo would decrease (more solar energy would be absorbed).

Even by making furrows in the snow one decreases the albedo of the snow (increases absorptivity for solar energy). This is because sunlight entering into a furrow can get reflected back and forth and eventually be absorbed by the sides. This is well known in agriculture where plowing decreases albedo.
http://costsnow.fmi.fi/workshops/24%20-%2025.08.2016,%20Helsinki/presentations/COST_2016_Manninen.pdf
says, " Surface roughness reduces the albedo of the surface due to
multiple reflection and in some cases by trapping the incoming
radiation completely. "