Listen to part of a lecture in an environmental science class.

Last week, we looked at the factors that lead to cloud formation and rainfall. Despite extensive research, some aspects of rainfall are still poorly understood. For example, one thing that's been observed but has been hard to explain is that deforestation, clearing a forest of its trees, seems to lead to a decline in rainfall. Why would this be?

Well, let's review a few things. The moisture in the atmosphere is always going through a process called recycling. In the process of recycling, water from earth's surface evaporates into a vapor, the vapor in the air, then condenses into water, and this water then falls back to the region again as rain.

Recycling can occur either over the ocean or over land. The evaporation of water from the ocean produces lots and lots of water vapor. You get this very moist air. On land, you have the evaporation of moisture from plants and trees. Plants and trees don't produce as much moisture as the ocean does. But still, since recycling is occurring, you can start to get a sense of how plants factor into land based rainfall.

Now, conventional models of rainfall, explanations of how much and where rain will fall. These models are centered around recycling. But these models aren't great at explaining certain aspects of land based rainfall, especially when it comes to rain forests that stretch into the interior of continents, areas like the Amazon rainforest, which reaches from the eastern coast of South America into the interior of the continent.

You'd expect the forests inland areas to be relatively dry because they are far from that really moist ocean air. According to conventional models, the recycling of moisture from plants and trees in such rainforests simply could not by itself, provide enough water to maintain rainforest conditions in these interior regions.

So, clearly, something's missing from the models, something that explains where this additional moisture is coming from. And a newly proposed model does just that, by taking into consideration, air pressure. The researchers who came up with this model posit that what's been missing from the calculation all along is what happens when water vapor in the air, which is moisture in its gas form.

When this vapor cools and condenses into rain, because as this water vapor rises into the higher cooler levels of the atmosphere, as it condenses into water, its volume decreases. In other words, the water vapor that used to take up space in its gas form, then takes up much, less space in its liquid form as rain water. This decrease in volume leads to a decrease in air pressure. So water vapor rises. It condenses into water. Volume decreases, and so air pressure decreases.

Now, air currents near earth's surface always flow toward areas of low pressure. So, when you have places where lots of evaporation and condensation are occurring, like rain forests, you get low pressure and lots of air currents being pulled in. Because the Amazon rainforest starts at the coast. Much of the air it's pulling in is moist air that originates over the ocean. This moisture eventually condenses and falls, then evaporates again. And then it's poured further inland by other low pressure areas, even deeper in the forest. This process happens many times, and the moisture that originated over the ocean travels far inland. As long as the forest cover is continuous.

Now, this new model of rainfall does have its limitations. For example, it doesn't take into consideration variables like land formations. And when it comes to talking about the effects of land formations on weather patterns, well, I don't have to tell you it makes a big difference whether you're talking about flat land or mountainous land. But still, it's a model that can help us make informed decisions about forest conservation.

Forest cover has to be continuous in order for moist air to be pulled from the coastal parts of a rainforest into its far away interior parts. So it could be risky to clear trees in certain crucial areas. For example, clearing near the ocean could sever the flow of moist air currents that rain forests depend on, and could cause areas of forest in the interior of a continent to dry out.