Intro: Do Forests "make" Rain and Can We Prove It or Not? the Biotic Pump.
Dry air weighs an average of about 29 grams per mole. Water vapor weighs about 18 grams per mole. So if you add water vapor to dry air the air gets lighter! (Rimar from instructables told me that years ago and I never thought of it before that)., Light moist air rises because it is .... lighter than dry air.. This means that generally the air over the sea is lower pressure than the air over the land, (because there is a lot of water to evaporate in the sea!) However, trees transpire lots of water vapor (if it is available to the tree) and in theory they could transpire more than can evaporate from the oceans. This is because they have larger surface area and photosynthesis requires transpiration anyway. So the air around the trees gets lighter and it rises. This sucks in more air for outside the area. The rising air gets colder as it rises until the dew point is reached. The water vapor condenses, turns to clouds and n goes down. PV= nRT is the universal gas equation. When n goes down high in the sky, pressure is reduced up there. So we have a PUMP that has a stroke length that is a couple of miles high, clouds are the "piston rings" and the trees act like a kettle on simmer boiling up more water" Then it rains and most of the water vapor is removed from the air. The biotic pump theory claims that extremely intensive very efficient transpiration from trees and efficient condensation and rainfall creates a continuous day and night low pressure area over the forest and this "wins out" and sucks in more moist air from over the sea. Positive feedback loop, sucks in more and more moisture and creates a prevailing wind! Towards the land! As long as the treed area is big enough and growing actively, the land will win the battle of low pressures, suck in air and create the prevailing wind!
Step 1: The Amazing Biotic Pump Controversy. Smoke in a Bottle! Can We Test the Theory?
The amazon rainforest is pretty amazing. It is so darn wet! And it rains so much. Clouds form in the morning, they rise (convection clouds) and grow and then it rains by midday or late afternoon. Almost like clockwork nearly every day. Actually meteorologists cannot account for why it rains so incredibly much. Their theories say that the amazon river should be much smaller than it actually is. So, reality versus theory? THEORY wins! Reality must be wrong. In the early 2000's two Russian nuclear physicists came along and took a look at the equations and said, PV= nRT (ideal gas law) Meteorologists are treating n incorrectly! n is number of moles. All this time the meteorologists have said that clouds create high pressure, "it is obvious, they rise and get bigger so they must be pushing out" But the Russians said something different. Something like, you are watching a change of state, you are watching condensation occur. You cannot see what the water vapor is doing because water vapor is invisible. The cloud edge might be a convergence zone. It might be sucking in! Because when condensation occurs n in the gas equation changes. Water vapor has turned into water and left some empty space and that means that pressure might be going down instead of up! "No no no" say the meteorologists, when condensation happens, heat is given off, and this causes the remaining air to expand.
So, who is right? Can we test it.
Step 2: Biotic Pump Theory, Design a Proof of Theory!
This instructable outlines a theoretical problem in meteorology where 2 factions are at loggerheads over something quite silly. But this dispute must be resolved correctly because depending on the result, water resources must be treated much differently.
Step 3: Clouds Create High Pressure or Low Pressure? Cloud in a Bottle Evidence.
The bottle experiment is pretty conclusive. Squeeze the bottle and the cloud disappears, release pressure and it returns! So a cloud only forms if pressure and temperature are FALLING. How does this Jive with the competing theories?
Step 4: How Can We Model the Competing Theories?
Any ideas? Perhaps something that is an analogue for this type of system? Here is my more or less complete explanation of the theory.
Step 5: CALIFORNIA Needs a Rethink (IF the Biotic Pump Theory Is Correct!)
If the biotic pump theory is correct, we can prevent the California drought by planting tall trees from the coast to at least 40 miles inland and WASTING water! This is so counter intuitive but maybe it will work! The key is to transpire and evaporate as much water as possible so that it transpires quickly, condenses, falls as rain and the the ground gets wet again. Virtuous loop sucks in more water from over the ocean! And don't forget all the cloud cover! It will keep California from getting hotter than death valley! On an even bigger scale, those States with water rights (where you are not allowed to have rainbarrels or "impede" the progress of water to the ground, would need a rethink. Because if the biotic pump theory is correct, the most important thing to do is get that water to re-evaporate as quickly as possible, preferable through vegetation transpiring the water as they grow.
Step 6: Condensation Nuclei. Alkemy of Fungus and Tree!
For the theory to be correct we must find the condensation nuclei that the forest generates! Or the theory will fall flat. AND it turns out that condensation nuclei are made! Fungi eject spores in the late night and early morning. But they also let out tiny particles of potassium salts WITH THE SPORES! and these rise in the buoyant damp air. AND the tree leaves release turpenes as they photosynthesize. The turpenes are the key. As the salt rises, the air becomes a tiny bit like a smog in the sunlight and the turpenes condense on them creating perfect condensation nuclei. How the condensation nuclei form
So we can look at this process. Can it even happen without tall old trees? Fungi on grass probably also release potassium salts, but near the ground and it probably stays in the air only a few seconds. I think a key is to have tall trees as a "substrate" for the fungi.
Step 7: Life Is a Plant War. Moss, and Ferns and Grass Versus Trees! Choose Your Weapons!
Once upon a time there was no grass, just little plants like moss and ferns and big plants like trees. Eventually the trees won. They spread inland from the coasts and almost the whole world was covered in trees, it was a warm lush world. Then BOOM! Permian extinction event. Trees were wiped out by who knows what? The Triassic was totally different, Crazy dry interiors of continents, low oxygen levels, (perhaps dinosaurs evolved and won out because they had superior lungs to mammal like reptiles) but over the millions of years of the period, it slowly got greener and nicer again and the oxygen levels rose right up It took 7 million years or more for trees to invade the interiors again! Maybe it was a slow priming of the biotic pump that was creating the nicer climate? But the desert plants of the triassic would have resisted, wouldn't they? How can a little seasonal plant possibly fight against a mighty tree? And the answer might be that they use proxy fighters. They used seasonal sacrifice (burning up every dry summer to clear the young trees) But the trees eventually seem to win over much of the earth and it gradually becomes wetter and more conducive to tree growth. BOOM, another extinction event at the end of the Cretaceous. The usual slow recovery occurs, first ferns, then trees and then something goes wrong. Grasses evolve, and they have a great weapon against trees. They have mixed seasonal fire hazard with underground growing tips. So they are designed to burn easily and massively and recover really quickly. Once they dry up and burn, photosynthesis in that area would stop completely for several weeks. That kills off that part of the biotic pump. So it seems to me that grasses are designed to create a bit of random weather chaos every year. If a huge grass fire happens, the winds in that area might change or the rain will cut deep gulleys and race to the sea or the black carbon will cause heat islands. Trees have a different strategy, they are trying to create order. They need the season to happen the same every year. Or ideally, no seasons at all! And building on that order, you get extraordinary diversity in forested areas. Trees created beavers to block the rivers more effectively, to impede the progress of water to the oceans , and grasses responded by creating people (and feeding them with grain). People love fire, and people burn down trees. So we see that grasses have started to win the battle, all over the world by using us to kill off the trees!