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Heat air near ground and clouds will form?


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Say I heat air that has a relative humidity of 60% near the ground. Let the initial temperature be 10 deg C and suppose I heat it to 25 deg C. Then the relative humidity is about 23.2%. If I use an environmental lapse rate of 6.5 deg C per km and an adiabatic lapse rate of 9.8 deg C then this air could rise (25-10)/(9.8-6.5)=4.5 km before it is at the same temperature as the surrounding air (it will then not rise or fall). The dew point for an RH of 23.2 and temperature of 25 deg C is 2.4. Espy's equation now tells me that the parcel only has to rise 125(25-2.4) = 2825 m before clouds form. It seems that generally one can just heat air near the ground and clouds will form. Is this correct?

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An environmental lapse rate of 6.5c/km implies conditional instablilty (dry adiabatic > lapse rate (envir) > moist adiabatic), so the parcel will rise to the LCL (lifted condensation level) where T= Td. However due to latent heat of condensation the parcel can continue to rise (and remain warmer than) the surrounding environment provided the lapse rate in higher layers of the atmosphere is greater than moist adiabatic.

Yes, if the atmosphere is absolutely unstable (lapse rate ~ dry adiabatic) or conditionally unstable (lapse rate >= moist adiabatic), and provided there's sufficient moisture then surface warming alone can initiate convection.

The Pacific Northwest: Where storms go to die.

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  • 4 years later...

I think Swayseeker has a plausible idea. The good example might be the Salton Sea and the peak of the Oronocino? Mountain just east of the sea. Swayseekers tunnel might be a plastic covered greenhouse funnel/tunnel/chimney  that runs(10 miles ?) from the Sea surface to the peak (3500 ft?). With local heating of the sea, moisture laden air begins to rise and move through  the chimney delta that may cover several acres immediately above the surface of the sea and into the chimney stack at say 10:am and continues until near 5:pm on a summers day. The plastic framed stack winds along the ground up the mountain ridgebacks and at some point (elevation 2000' ?)the temperature of the plastic walls of the chimney stack fall below the dew point and water vapor condenses on the walls and runs back down hill to some collection point. The latent heat released keeps the chimney draft working to pull in more moisture laden air at the sea level. The pathway of the chimney will have to be maintain uphill always to keep the draft from stalling out. That covered pathway, with a cool and moist environment, could make an interesting hiking path. Didn't Swayseeker talk about this in another thread?

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The water cycle. The classic illustration that may accompany a description of the water cycle shows water vapor ascending from land heated by the sun or water vapor arising from a body of water heated by the sun. No question that the sun is the energy source that keeps the water cycle turning.

The heated land surface case is true because the water vapor and parcel of air that contains it can be at a higher temperature than surrounding air. See Swayseeker and IbrChris above discussion. The heated air and vapor parcel is buoyant and rises through the cooler air above. This can’t continue forever because soon the surface has drawn up -through capillary action- and evaporated all the available water. For the heated land case to continue, some other source must replenish the soil moisture. The water cycle illustration includes rising vapor from the ocean(s). This is the accepted source for all water  in the cycle. The heated land case is simply recycling water from the primary source; the ocean(s).

 

The ocean water is cooler than the air above because the ocean water can absorb much greater energy than the air above. No matter how intensely the sun heats the water, the air above will only heat faster. During daylight, on leaving the water surface, the water vapor retains the temperature of the cooler water. The cooler air at the waters’ surface and the combined water vapor parcel, while slightly less massive than surrounding air, is not buoyant relative to the warmer air above. An inversion has occurred and is the normal condition over the ocean. During night, the air above will cool and the combined parcel can become buoyant. In most cases over the oceans the parcel will rise until it cools and becomes neutral and before reaching the dew point. In colder waters, the parcel rises and can create fog as the temperature falls to the dew point. There are many places over the ocean where the inversion produces fog. But there are few places over the ocean where there is no inversion and water vapor can rise to great heights and condenses to create rain clouds. The vast expanse of the South Central Pacific is an example of this inversion over a large ocean desert. The region of ocean off the southwest coast(s) of North and South America and Africa are more examples of the seasonal desert created by the natural inversion of ocean and air. 

 But at the equator, two opposing air masses of the north and south merge and create a pressure differential such that the air and water vapor in the two masses is forced to rise. The ITCZ is one of the places where the low pressure above the equator prevents the inversion seen over the rest of the oceans. Here, the merging circulation of the southern and northern Hadley cells force the air aloft.The ITCZ/Hadley cell  circulation is initiated by tropical rainforests that pump huge quantities of water into the atmosphere. Without the thunderstorms provided by rainforests, the Hadley cell(s), both north and south has no sustained fuel for continued circulation. In addition to the massive anvil headed thunderstorm, the rainforest creates a river of water vapor-a low level atmospheric river that the ITCZ low pressure through that allows the vapor to rise through the unstable air above. Without rainforests at the equator, the Hadley cells would not exist as sources of the earth’s water. 

 A canopy of forest leaves acts just as the land example in the water cycle; the sun heats the leaves just as it heats bare earth. The trees of the rainforest remove the heat by continuous transpiration. In evaporating, the water vapor expelled via the leaf stomata  absorbs the energy in the form of the heat of vaporization and a higher temperature than the surrounding air; the leaves of the tree do not get hot. Warmer than the surrounding air and less massive, the parcel of air/water vapor is buoyant and rises in the unstable low pressure trough  of the equator.

The rainforest canopy can pull water at great depths from the soil and rock of the land through the tree roots. Unlike the bare earth that is limited to the first few inches of moisture at the surface, the rainforest tree has a vast reservoir of water as the tree roots go down many feet into the earth. The underground reservoir is refreshed by a daily downpour of rain.

 When ocean temperature rises during summer months, night air temperature can fall enough that the water vapor/air parcel may become buoyant and rise. This is the time of hurricanes and monsoon. Both are still reliant upon the wetter Hadley cell circulation. These rainmakers are sporadic and short lived.

 Over the equatorial ocean, the shallow but extensive low level moisture not yet buoyant, is pulled into the ITCZ depression created by thunderstorms developed from tropical rainforests as the storms move off the continents over the open ocean. This ocean surface low level moisture is swept up by the westward flowing atmospheric river generated by the rainforest. The greater the westward flow of the rainforest river, the greater the thunderstorm activity along the ITCZ  and greater the Hadley cell transport of water to the north; less drought for California.

 If global warming is creating slightly warmer seas, one might think the evaporation rates from the oceans should increase as well to create a wetter earth. But in creating warmer ocean water, global warming creates warmer air and the inversion over the oceans remains widespread. 

 Local and regional weather impacts the distribution of the inversion. When cold air invades oceans from the poles, obviously the inversion will be temporarily reversed if the temperature differential is great enough. Moisture can rise; storms are created. In the Pacific, the Northern Hemisphere is more active with regard to the cooler air invasions than the Southern Hemisphere because of the relation of the continents; Alaska and Siberia are separated by a few miles while  Tierra del Fuego at the tip of South America and Australia are separated by thousands of miles. The proximity of the rapidly heating and cooling land masses of the north allow more opportunity for cool air intrusions that can upright the ocean inversion. In most cases , after a few days, the energy from the ocean equalizes the lower level temperature of the northern air and the inversion returns to normal. Oceans alone cannot consistently provide an unstable environment for making rain. Tropical rainforests, acting as catalyst for the ITCZ, can do so continuously. The earth enjoys three equally spaced regions of rainforests that circle the globe at the equator; Amazon, Indonesia, and Africa. Without those three zones, the earth would be very dry and a very different environment.

 

  A program of subsidizing the planting of vast acreages of tropical native trees in the Amazon should be considered  by US California and western states.

 The US should ban all imported beef from the Amazon basin nations. Grass lands for cattle created by destroying the Amazon tropical rainforest also destroys the flow of the rainforest atmospheric river. All palm oil from Indonesia and the Amazon basin should be banned.

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