Tropical rainforest without rain wouldn't be much of a rainforest. I mean all plants need water to grow and without it they shrivel up and die. So what about the ancient Hawaiian proverb - Hahai no ka ua i ka ulula'au. – which means the rain follows after the forest. How could that be? Well all land plants lose water when the pores on their leaves open up during photosynthesis and this evaporation draws more water up through their stems. With so much rain soaking the soil in rainforest, water is nearly unlimited. And accordingly rainforest trees can afford to move and lose more water than other plants. All that water vapor rising from the forest feeds moisture-laden clouds while also causing convection. Together these effects accelerate the formation of rain which falls to the soil and gets taken up all over again. This cycle of absorption, evaporation and rain happens everywhere there are plants.
没有雨水，就没有热带雨林，意思是所有的植物生长都需要水，没有水它们就会枯萎死亡。那么，那句古老的夏威夷谚语——Hahai no ka ua i ka ulula'au，意思是先有森林后有雨，是怎么回事呢？所有陆地植物在光合作用过程中，叶片上的细孔打开时会失去水分，这种蒸发使得更多的水分通过树干上行。如此多的雨水浸泡着雨林的土壤，使得水几乎是无限的。因此相较于其它植物，雨林树木可以承受更多的水分移动和流失。所有从森林中升起的水蒸气滋养了云层，同时也带来了对流。这些因素一起加速了雨水的形成，雨水落到土壤中又被吸收。这个吸收、蒸发和降水的循环在任何有植物的地方都发生着。
However, super wet soil, fast pumping trees, and hot tropical sun make the cycle so fast in the rain forest that unlike other biomes where clouds might form in one place and rain in another, in a rainforest, all that water stays in the same region. Without the forest pumping so much water into the air, rainforests wouldn't be as raining. And without so much rain, the forest couldn't pump so much water into the air. So which came first, the rain or the rainforest? Well before rain forests, ancestors of trees like cypress, pine and spruce dominated the land, but they were conservative when it came to using and losing water. So the air tended to be dry, meaning less rain. However, around a hundred and thirty million years ago, a new kind of plant developed that took the risk of losing more water in return for souped-up photosynthesis. These were the flowering plants and their risk paid off. And their faster growth enabled them to out-compete the ancestral pine and take over the tropical regions of the globe.
These angiosperms lost so much water into the air that as they spread they brought their own rain with them. And today tropical rainforest receive more rain than if they were pine forests, in some places as much as a meter more rain each year, that's equivalent to an extra two and a half hours of heavy rain each week. Not surprisingly all that water cools off the forest too, which was why the Amazon isn't nearly as hot as the Sahara or even in East Texas Pine Forest in summer. But the hot dry tropics of the past may soon be a part of our future. In parts of the Amazon where vast swathes ofrainforest have been logged or cleared for agriculture, unusual droughts are already occurring and forest fires become more frequent. Scientists worried that these changes will lead to ever hotter, dryer and more flammable Tropics in the coming decades making things tougher both for the remaining forest and for the people who live there. So when in drought, plant a tree, seriously. Hahai no ka ua i ka ulula'au.