Rain with sea temperatures higher than land temperatures

I was interested to read about "The Slow Food Movement" and about "Campesina" in an "The Conversation" article, who are helping small scale farmers. But drought can affect small scale farmers badly. UAEREP (can be found on Facebook) is working with rain enhancement methods and I have my own rain enhancement methods. I hope what I say below will help countries lessen the effects of drought:
With global warming land is heating up faster than the sea and air blowing from the sea heats up more and the relative humidity (RH) drops more when air blows from sea to land.
Consider two cases:
1) If land temperatures are higher than sea temperatures, then air blowing from land to sea is cooled by the sea, its RH increases and water can condense out and it could rain over the sea. With high RH evaporation into air is reduced and the air will not pick up much moisture. When the air blows back with sea breezes it will have very little extra moisture in (it could have less).
2) If the land temperatures are cooler than the sea, then when air blows from land to sea the sea heats up the air and RH drops and the air readily takes up moisture from the sea, so that it has more moisture than it started with and when it blows back with sea breezes it will decrease in temperature over cooler land and and rain can occur from condensation.
Solution: So here is a solution. Make more spray (mist) over the sea with floating spray pumps operated by wave motion. Solar energy will be absorbed by the spray mist and evaporation will occur as the mist heats up. Then this moist air could supply rain when it blows to land. If possible, reflect solar energy from the land onto the mist that is generated over the sea, using mirrors.  
It takes less than 1 kWh to evaporate 1 litre of water. Now in sunny areas, every day, we can get more than 8 kWh of solar energy falling on every square metre. If the mist absorbs 1/8th of this, there will be 1 kWh of solar energy every day on every square metre, to evaporate the mist and heat the air that the mist is in. It seems we could evaporate 1 litre on every square metre every day. This is a significant amount of water to put into the air, and a 1 km by 1 km square could supply 1000x1000 = 1000000 litres per day to the air. By comparison, at 25 deg C and a relative humidity of 50% a column of air with base of 1 square metre and a height of 200 m has 2.3 litres of water in (evaporated water actually). At the same temperature, but with a relative humidity of 72% this column has 3.3 litres in (1 extra litre). Humid air has two advantages I can think of: 1) Humid air is less dense than dry air at the same temperature and pressure and rises (which can result in convectional rain). 2) Humid air can be heated by infrared radiation from the ground, causing it to rise - water is a greenhouse gas and this heating is part of the greenhouse effect.

Frost prevention, rain enhancement, air pollution dilution

Although I do not drink wine, I like grapes, and I read that grapes had been damaged by frost in France. Here is an idea: Have a long pipe running through the vineyards, with holes in to let out water vapour. Boil water at one end of the pipe (or at points along the pipe) and water vapour will come out all along the pipe into the vineyards. This will increase the sky temperature by increasing relative humidity and the greenhouse effect and help reduce frost.
Hot water vapour actually radiates heat. The book "Fundamentals of Thermal-Fluid Sciences" by Cengel and Turner says that a 1-metre thick layer of water vapor at 1 atm pressure and at 100 deg C emits more than 50% of the energy that a blackbody would emit at the same temperature. Energy will therefore be radiated to the vines.
Regarding cold lower valleys, what usually happens is that the cold air comes down the sides of a valley and sinks below the warmer air (which could be only 10 metres above it). Helicopters are used to mix the warmer air above with the colder air to warm up the air immediately above the ground and higher. This method would also cause mixing because the vapour coming out the pipe would be less dense than the air, causing it to rise and mixing warmer air above with the colder air. One could also ensure that the vapour comes out at fairly high pressure to ensure mixing. The system could be used to enhance convectional rain or dilute pollution in cities by means of convection (dirty air rises out of the city).

Bring rain and reduce air pollution.

Clean up cities and bring rain with solar energy: With global warming the land is heating up faster than the ocean in terms of temperature (although about 90% of the solar energy absorbed by Earth is going into the oceans). When this happens the relative humidity of the air decreases more when blowing from sea to land. One can have floating spray pumps in the ocean to increase relative humidity (the pumps could be operated by wave motion). Another method to get more rain is to have this sort of system near the sea: Where there is land not being used (say on rocky hills, etc) erect pipes a few metres in diameter and a few hundred metres high and reflect sunlight onto these pipes using mirrors. The air in the pipes will be heated and rise and convectional rain is far more likely than usual to occur. It should be very cheap system to implement without the need for machinery to operate it. The bottom part could consist of a greenhouse. Often one will get 6 kWh or so of sunshine per day on a one square metre horizontal surface. Theoretically at 101.325 kPa this could heat 3600 cubic metres of air by 5 deg C (just the solar energy on one square metre could do this) starting off with 24 deg C air. With a 5 deg C temperature difference one can expect about 600 cubic metres per second out of the pipe (tower) if it is 400 m high or so (to research this Google stack effect draft, etc). To supply the energy for heating the 600 cubic metres coming out one needs roughly 5x1.2x600 kJ of energy per second (volumetric heat capacity of air is about 1.2 kJ per cubic metre per deg C and temp rise is 5 deg C and volume is 600 cubic metres). If sun power is 0.5 kW/ sq metre then we need 5x1.2x600/0.5 square metres of radiated surface. ie we need 7200 square metres of radiated surface. Therefore we need an area of about 84 m by 84 metres to supply energy to heat 600 cubic metres of air by 5 deg C every second. Of course there will be heat losses, so a bigger area is needed. This device can also be used to move polluted air out of cities by convection.
Links: 1) Cool roofs could be reducing rainfall https://www.scientificamerican.com/article/cool-roofs-may-have-side-effects-on-regional-rainfall/
2) Colour of land could affect rainfall: https://eos.org/articles/more-intense-rains-in-u-s-midwest-tied-to-farm-mechanization?utm_source=Eos%20Primary%20List&utm_medium=email&utm_content=more-intense-rains-in-u-s-midwest-tied-to-farm-mechanization&utm_campaign=5022c1d8f8-Weekly_All_Content_Digest&utm_term=0_f923f18da4-5022c1d8f8-522497757
3) Regarding cool roofs, etc, http://iopscience.iop.org/article/10.1088/1748-9326/11/6/064004/meta says, 
"The lowered wind speeds and vertical mixing during daytime led to stagnation of air near the surface, potentially causing air quality issues." 

If space is a problem then one could build a greenhouse with black floors to absorb solar energy and holes in the floors to let hot air rise through. See following diagram:

Artificial heated lagoons for rain

 With colder sea temperatures than land temperatures (eg Western Cape in summer, UAE, etc), cold air from the sea does not hold a lot of moisture and when it blows onto land and heats up, relative humidity decreases. Also this cold air does not easily rise by convection to form rain. One of my ideas is artificial lagoons with solar energy reflected into them to heat the water.
With deep ocean one has a limitless supply of heat from the water (evaporation cools the surface, but there is plenty of heat from the water to heat it up again), but the surface can only be heated to about the sea temperatures below the surface. About 43% of energy from the sun is of visible light frequency. Visible light has the distinct property of being able to penetrate seawater to some depth, whereas the infrared generally gets absorbed quickly. So if you have shallow pools with dark bottoms you suddenly have 43% of the solar energy available in a shallow pool that normally would have heated a huge body of water a little, but there is very little energy that can be used from the water to heat the surface when it cools from evaporation. As I am looking for high temperatures I have to think about shallow pools rather than deep sea. With a shallow pool the energy comes mainly from the solar energy (not the water). If one wants high temperatures (with evaporation and radiation the shallow pool can cool quickly), one needs ways of getting solar energy to the pool or lagoon, etc. So my proposal has been shallow lagoons made by bulldozing out of the sand and with mirrors to reflect solar energy into the lagoon. Say the sun provides 700 W of energy per square metre of water surface. Eventually the temperature of the lagoon water gets so high (at about 26.5 deg C) that all the 700 W per sqare metre is needed for evaporation and so mirrors will be needed for more energy. My graph below shows the 700 W straight line (0.7 kW), the evaporation rate on a 1 sq metre surface (upper curve kg/hour) and the power needed for the evaporation (lower curve kW). Not correct to have kW and kg/hour on the vertical axis, but it works out. The conditions are a 40% relative humidity, a 3 m/s wind over the surface and an atmospheric pressure of 100 kPa.

Are cool roofs reducing rainfall and increasing pollution?

Are cool roof paints reducing rainfall? 
Regarding pollution, http://iopscience.iop.org/article/10.1088/1748-9326/11/6/064004/meta says, 
"The lowered wind speeds and vertical mixing during daytime led to stagnation of air near the surface, potentially causing air quality issues." 
Why not distribute a solar air heater that can be attached to roofs? The solar air heaters will shade roofs during the day and heat air, but also be cooled by the air passing through. This should cause cloud formation which will keep the city warm at night.  Paint manufacturers could team up with solar air heater manufacturers to produce this air heating, roof cooling and rain enhancing solar air heater on roofs situation.
See http://www.builditsolar.com/Experimental/PopCanVsScreen/PopCanVsScreen.htm for information on solar air heaters.
Drought in coastal cities: I am wondering if it would not be better to make buildings in, say, Los Angeles a dark colour to heat up more and enhance chances of rain (or use solar air heaters on roofs as mentioned above). One hears about a 10% increase in rainfall due to urban heat island effect. I have done some calculations:
Assume the air from the sea has a relative humidity (RH) of 70% and is at 18 deg C and it blows onto land and heats up over the city, but remains at 18 deg C over the rest of the area (a simplification to make calculations easier). My calculations use an environmental lapse rate of 6.5 deg C per km rise and a dry adiabatic lapse rate of 9.8 deg C per km rise. The dew point of the sea and land air remains at 12.5 deg C whether heated or not, but the RH changes on the air being heated. My calculations show the number of degrees the air heats up over the city (1 deg means it heats up to 19 deg C from the 18 deg sea air temperature). Then they show the change in RH of the air after heating. Then they show the height to which the heated air can rise and the height to which it needs to rise for clouds to form: 0) Heats up 0 deg over city, RH remains 70%, the air can rise 0 m and it needs to rise 694 m for clouds to form. 1) Heats up 1 deg over city, RH is now 65.8%, the air can rise 303 m and it needs to rise 819 m for clouds to form 2) Heats up 2 deg over city, RH is now 61.8%, the air can rise 606 m and it needs to rise 944 m for clouds to form 3) Air heats up 3 deg over city RH is now 58.1%, the air can rise 909 m and it needs to rise 1069 m for clouds to form 4) Air heats up 4 deg over city RH is now 54.6%, the air can rise 1212 m and it needs to rise 1194 m for clouds to form 5) Air heats up 5 deg over city RH is now 51.4%, the air can rise 1515 m and it needs to rise 1319 m for clouds to form 6) Air heats up 6 deg over city RH is now 48.4%, the air can rise 1818 m and it needs to rise 1444 m for clouds to form After step 4 the air is heated enough for clouds to form - see the graph. Note that more cloud formation could cool Earth overall.
Here is another very good design to get air moving upwards: 
http://www.houstoncoolmetalroofs.com/cool-roof-information/cool-roof-design-texas/

See https://www.scientificamerican.com/article/cool-roofs-may-have-side-effects-on-regional-rainfall/ which says, "However, they shift rainfall patterns by reducing evapotranspiration, the process by which water evaporates from the ground and enters the atmosphere. In the maximum expansion scenario, cool roofs led to a 4 percent decline in rainfall."
One could increase convection by using solar air heaters made by placing a black piece of corrugated iron roof sheet a few centimetres above a silver sheet of corrugated iron. Another method is to make the soil dark with biochar - plow biochar into the soil. You can make your own biochar by heating wood in a barrel. 
You can also paint rocks black: Rocks have a high heat capacity - they hold a lot of heat. If cool roofs reduce rainfall, rocks painted black can increase rainfall. 
Just as cool colours can decrease rainfall, so can dark colours incease rainfall by increasing convection. See http://www.xcmag.com/2007/10/thermal-flying-part-2-thermal-generators-and-triggers/
When clouds develop, because of convection, they will help with global warming by cooling the Earth. 

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).

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)

Bringing rain with seawater spray pumps

My guess is that with global warming there are going to be more droughts. The land heats up faster than the sea and if the land is warmer than the sea then air coming from the sea heats up and its relative humidity falls. Example: Air comes off the sea with relative humidity of 70% and temperature at 20 deg C and heats up to 25 deg C over land. The relative humidity drops to 52% and low relative humidity is associated with low rainfall. One needs to get air from the sea flowing over land as it brings moisture. Cold air tends to be denser and so cold air from the sea has more of a chance of flowing onto land. So here is the idea: 1) Make air over the sea fairly cold and very moist so it will tend to move in over land and bring moisture. 2) Over land, heat this air up so it will rise and vapour will condense, making rain.
You can do all this as follows: Use spray pumps operated by waves in the sea to form a mist that is evaporated in sunlight, causing air to be very moist and also cool (because of evaporation). Now use a solar air heater on every rooftop to heat this air when it comes onto land, so it will rise causing rain. Every two seconds one can have 1.2 kJ of solar energy falling on every square metre in many locations and this 1.2 kJ can heat one cubic metre of air 1 deg C. You could move massive volumes of moist air up like this. Another advantage is that low level clouds (and I would think mist) is associated with cooling of Earth. So you should have cooler Earth and more rain.
EXAMPLE: Say you have a 2km stretch of coast with the spray pumps (operated with wave motion). Let the air temperature over this stretch be 18 deg C with a relative humidity of 95%. Suppose the 2 km wide air parcel from the sea blows onto land. If you heat this 18 deg parcel up to 26 deg C with solar air heaters the relative humidity of the parcel falls to 58.3%. Suppose the surrounding air over the rest of the land area (surrounding the 2 km wide parcel) is at 22 deg C. Then the parcel can rise 1212 m (using a fairly standard dry adiabatic lapse rate). Using Espy's equation it only has to rise 1102 m for clouds to form, so it could rain.