- When the percentage of water vapor reaches a saturation point (also called dew point) in the air it condenses into liquid water drops
- This saturation point decreases with temperature decrease.
- Clouds form when water vapor cools below the saturation point.
- The most common way for water vapor to cool is when air rises from the ground. A parcel of air expands in size as it rises, while not losing energy to its surroundings (called an adiabatic process). In the process the temperature of the air cools.
- If there is a low pressure up in the atmosphere at the troposphere height, this will cause the air below to rise. (Thats why low pressure systems are associated with rain)
- When a water condenses to form a cloud, it creates an area of lower pressure, and amplifies the updraft, bringing up more water vapor.
- The amount of : water vapor in the air = the amount blown in + the amount being evapotranspired from the vegetation and ground – amount blown out
- In humid areas it is easier than in dry climates, for the evapotranspiration of water to help the amount of water vapor in the air go above the saturation point, and thus create clouds. Thus growing vegetation in a humid area will have more effect on the local rainfall.
- When water condenses it forms small liquid droplets that stay up in the air, ie clouds. The droplets weigh too little to fall to the earth. They have to increase in size a million times before they are heavy enough to fall.
- A warm cloud is a cloud above 0 degree celsius. Larger droplets can form through air turbulence that brings together the droplets. The process is called collision and coalescence. Larger droplets can also form when droplets of different electrical charges attract each other.
- A cool or cold cloud has part or all of its temperature below 0 degrees celsius. Water vapor that remains water vapor below 0 degrees is called supercooled. Both ice crystals and liquid water drops form. Under certain conditions the ice crystals can grow larger from the supercooled water vapor. They then start falling, and as they get to warmer temperatures on the way down they can turn into rain.
- Particles in the air, called aerosols, can help the droplets nucleate and become larger droplets.
- Aerosols also reflect sunlight and so increase the temperature of the troposphere. Temperature increase increases saturation point, which means condensation is less likely.
- The combination of the nucleating effect and temperature-raising, sunlight-reflecting effect of aerosols means that its complex whether aerosols increase or reduce rain. {The IPCC considers the aerosol-cloud-climate coupling one of the least understood areas of climate change}
- Air pollution is made of small aerosols, the water droplets would become larger than if no aerosols, but not large enough to fall as rain. These become haze, which increases the temperature. In dry climates, the net effect is that there is less stratocumulus and small cumulus clouds, and there is less rainfall downwind of where air pollution is released. [ See Case Study: California air pollution]
- Aerosols increase rainfall in deep convective clouds. Deep convective clouds can release large amounts of rainfall [Ref: Khain, Moshe, Pokrovsky 2008]
- Aerosols in general may decrease rainfall in dry climates, and increase rainfall in humid ones [Khain,Moshe,Pokrovsky 2008]
- Bacteria act as aerosols, and can nucleate water droplets at higher temperatures. These bacteria have been found more in the atmosphere over row crops and desert areas. The percentage it makes up of aerosols, and whether the bacteria makes a significant contribution to rain is debated. {ref: Morris 2013}
- Lichen and moss release biogenic volatile organic compounds (BVOC) that may help seed rain. {ref: Fang et al}
- Fungi release fungi spores that may help seed rain.
- There is a soil moisture – precipitation coupling. Wetter soil creates more rain, and dryer soil creates less rain. As soil moisture increases more evapotranspiration happens, which leads to more rain, which leads to more soil moisture. This is the local small water cycle {see Kravcik et al}. The small water cycle is defined to be the movement of water in a closed loop in the local area. Conversely as soil moisture decreases , this leads to less evapotranspiration, which leads to less rain. So there are two attractor points for the system. {There is evidence for this in a number of papers Kim Wang 2012, but also there are other papers that say there is not a soil moisture – precipitation coupling, Ford, Rapp,et al, 2015}
- Vegetation can harvest rain to bring it into a small water cycle. Vegetation can transpire enough water vapor, to help push the total amount of atmospheric water vapor to a point where it condenses and forms rain. Incoming water vapor from ocean winds, can then be brought in to increase soil moisture. If this effect is large enough, it can shift the soil moisture – precipitation coupling to the wet soil-more rain feedback loop. { see case study Nebraska, Central Valley California}
- Trees can help slow the wind speed down, which keeps evapo-transpired water vapor over the vegetation where it came from, and thus help create a small water cycle.
- Water can also move in a wandering small water cycle , where it moves from soil to atmosphere and back, but while shifting its location. So the water can get blown long distances in the air. It descends as rain, and can then evapo-transpire back up, or it can then flow down rivers for long distances, and onto river banks, floodplains, or wetlands where it then traverses back up.
- Wetlands have high evapotranspiration rate compared to normal vegetation. Historically a significant percentage of land on earth was wetlands. Restoring rivers and allowing them to overflow, can create large amounts of wetlands. Wetlands increase evapotranspiration, and thus precipitation.
- Restoring rivers by taking out levees and concrete river banks, taking out dams and hydropower, increasing beavers, and restoring wetlands, can increase the wandering small water cycle and shift a continental region into a wetter attractor point, where there is more rain.
- Increasing soil moisture, by increasing its sponginess with polycrops, non-tilling, compost teas, and by increasing the amount of water diverted it into it with animal footprint indentations, dead biomass and earthworks (swales, keyline, gabion, berms, terraces, ponds) can then increase the amount of small water cycles and medium water semi-cycles.
- Earthworks, lakes and wetlands can help guide water into the groundwater.
- There is a groundwater-precipitation coupling. Water underground helps soil and plant stay hydrated, which water then evapotranspires to create rain, which can then seep back into the soil and groundwater. Aquifers can feed springs, which feed rivers to keep them running into dry season. And rivers can help keep the ecosystem hydrated.
- Slowing down the rate at which freshwater flows back out to the ocean, keeps more freshwater over land, which then leads to more water available for evapotranspiration and precipitation.
- The earth is a thermodynamic non-equlibrium system. The small and large water cycles are fluctuations that form dissipative structures.
- The water cycles are more ordered systems ( dissipative structures) with lower entropy, which means that the earth has to dissipate greater entropy by the second law of thermodynamics. This entropy dissipation is done by turning uni-directional inward coming solar radiation turning into multi-directional (higher entropy) outward going infrared radiation.
- The fourth law of thermodynamics by Fleck and Morel says “Systems increase entropy at the maximum rate available to them”. A small water cycle increases the rate of entropy generated in the outgoing infrared radiation. So the earth system seeks to maximize small water cycles.
- Rainfall vs area size of rainfall follows a fractal scaling law that mimics the scaling law of earthquakes. Smaller earthquakes trigger larger earthquakes. In a similar way smaller rainfall events, can through air turbulence trigger larger rainfall events. This kind of behavior is called self-organized criticality. {ref: Wang, Huang 2012, and Peters 2002 on self-organized criticality, and Lovejoy, Mandelbrot on fractal rain, Minkel }
Borneo : Willie Smits lead a reforestration effort that planted about 4 square miles, and increased the amount of rain by about 25%, and cloud cover 10%
Amazon: Scientists looked at the water isotopes in the atmosphere. Some of the isotopes corresponded to the water transpired by the trees rather than from the ocean. These isotopes increased just before the onset of rain. https://climate.nasa.gov/news/2608/new-study-shows-the-amazon-makes-its-own-rainy-season/
Central Valley California : Aquifers have been draining to feed water for crops, which feed significant amount of USA. California has decreased rainfall, while Central Valley is relativity constant. So crop transpiration may be playing a role in create rain.
Loess Plain : A desert the size of France was restored to more growth in China. The goats and sheep were constrained to stop eating all the vegetation as it grew. Terraces and berms were built to guide rainfall into soil. And large replanting process happened using thousands of villagers. Rain came back to region
Al Baydra : Neal Spackman led project to replant a desert area in Saudi Arabia with very little rainfall, by creating swales. Because the area had its atmosphere constrained by the mountains, when the plants transpired water into the air, it stayed in the area, so it could then rain in same area, and feed the small water cycle https://soundcloud.com/user-193856180/episode-002-neal-spackman
Fang et al An increase in the biogenic aerosol concentration as a contributing factor to the recent wetting trend in Tibetan Plateau https://www.nature.com/articles/srep14628
Morris et al 2013 Bioprecipitation: a feedback cycle linking Earth history, ecosystem dynamics and land use through biological ice nucleators in the atmosphere https://onlinelibrary.wiley.com/doi/full/10.1111/gcb.12447