Rain is Life, and Holobionts Create it

 

Buying vegetables at a stall in Florence on a rainy day. My wife, Grazia, is the one with the pink umbrella. 

By Ugo Bardi

After three months of drought during the summer, Florence is now drenched in rain. It has been raining for two months and people are complaining that it is too much! But it is how things have to be: I recently discovered that rain is an "autocatalytic" phenomenon. A more humid atmosphere creates more rain, and rain creates a more humid atmosphere. And that creates long periods of static weather -- too little rain and then too much. 

I learned that from the work of Anastassia Makarieva, Mara Baudena, and several others who have studied the relationship between atmospheric humidity and rain. Look at this figure, from a paper by Makarieva et al., to be published.

The y-axis is the amount of rain, in mm/hour. The x-axis is the amount of water vapor in the "air column." Note the strongly non-linear relationship. It is a typical power law: a small increase in atmospheric humidity causes a large change in rainfall. The three red points indicate the boundary of the "abiotic regime" (no rain) and the power law region.  

As I said, it is an autocatalytic phenomenon, Rain tends to generate more rain, at least as long as it wets the land and it generates moisture transpiration, which increases the water vapor content in the atmosphere. This has very practical consequences in many senses. One is the role of forests in weather and climate. Forests generate strong evapotranspiration, that is they pump water from the soil to the atmosphere. And, also, forests tend to keep water in the soil, slowing down the runoff.

So, not only do forests generate rain, but they also tend to maintain the rain pattern. Without forests, and with the land covered with buildings, you have the typical desert climate: dry most of the time, then with short periods of heavy rain. Disasters ensue, now a common pattern in areas such as California or Italy, where deforestation has taken place. 

So, we need our fellow holobionts, the trees. Onward, fellow holobionts!

(below, some rain-loving holobionts pictured together)