If you are lucky enough to catch those first summer storms, subsequent storms will often follow the same track. The first storm changes the ground cover (carbon levels) relative to surrounding areas, as well as increases soil moisture.
Weather and land are inseparable and interrelate with each other.
My initial interest in this subject was stimulated by a discussion I had with Robert (Bob) Leighton over tea one night. I sought his opinion, as one of Australia’s leading meteorologists, on why storms often follow where previous storms went. Often one property or part of a property would remain dry, while the one next door was having a reasonable season.
It was not his area of expertise, but he started to explain how storms required a certain amount of moisture for it to start raining and, the rain stops when the moisture drops below a certain level. If there is an additional source of moisture, then the storm would proceed instead of petering out. Creeks and rivers were discussed as a source of moisture, which is consistent with storms often performing better near them over time. In the course of conversation, he raised the issue of vegetation cover and its influence on rainfall by contributing moisture. The other issue he raised was the temperature of the landscape due to vegetation cover or lack of it and how this effected the weather.
In a previous trip to America, Bob had been told by US researchers that they had been researching the issue of vegetation and its effect on rainfall. This is the question he asked on my behalf:
I remember when I was visiting AWC over four years ago that there were discussions about the interesting weather experienced in Kansas storms, tornadoes, ice storms etc. And they told me that a major moisture input for tornadoes, storms originated from the vegetation through “tornado valley”. (Of course, I originally thought that most of the moisture would have come from the Gulf of Mexico). I was wondering if you know of any statistics or papers on the subject of moisture availability from vegetation for cloud build-up in your territory that would be available
Response from Steve Corfidi (Kansas):
I don’t think that there is much question that local evapotranspirative fluxes affect patterns of convective initiation. The role of “vegetatively-derived” moisture is, however, a comparatively new one; the importance of the subject has become more widely accepted only in the last 10 years or so. Canadian meteorologists in the prairie provinces are especially interested in the subject, as their short wheat-growing season corresponds to the time of maximum thunderstorm frequency.
Steve included a manuscript received by the Iowa State University, which in part stated;
Vegetation cover was found to promote convection, both by extraction of soil moisture and by shading the soil so that conduction of heat into the soil was reduced (thereby increasing the available energy).
Studies of 30 years of monsoon patterns over India by Purdue University scientists showed that tropical storms are sustained by moist soil, but tend to fizzle over dry ground.
The storm will have more moisture and energy available over wet soil than dry (Associate Professor Niyogi)
GRAZING PRESSURE AND RAIN
In 1998, while in South Africa, one of the groups of scientists I spent time with was at the Range and Forage Institute in Bethlehem. During discussions, it was explained to me by one of the scientists that he was convinced that lack of vegetation cover due to excessive grazing, was responsible for a drop in rainfall on the plateaus that rise up above the plains. He said that these small elevated areas were the most productive sections as they had much higher rainfall than the lower plains area. His theory was that due to greatly reduced vegetation cover (carbon levels), there was much more heat rising up from the soil and dispersing the rain bearing clouds.
DAM CHANGED RAINFALL
The concept of moisture from the landscape promoting thunderstorms, is well supported by a long term change in rainfall near Inglewood on the Qld/NSW border. A long-term producer who had a property beside the Coolmundra dam explained to me that, after the dam was built, the rainfall increased above the-long term average recorded in his family’s records. He explained that the path of the storms was over the dam before getting to his property. This gave the storm clouds an opportunity to increase their moisture level. When questioned on whether others further along the storm path had an increase in rainfall, he assured me they had not. It is likely that the dam activates clouds close to raining and so they part with much of the moisture before getting to others’ properties.
German scientist Wilhelm Ripl talks about preventing large thermal differences in the landscape:
The more evaporation processes at and within the surface and foliage of vegetation per area that takes place, the more even the temperature is distributed and the cooler are vegetation structures, markedly so at times with high-energy flow (midday in summer). Areas that are mostly better cooled show lower atmospheric pressure than the overheated surroundings. It is easy to understand that these small local areas lacking any means for an efficient temperature-regulating system are channelling energy into global weather systems and thus modulating them. Areas with an even cover of vegetation, with sufficient evaporable water, have more predictable weather events than do damaged areas without proper vegetation cover.
AN EXAMPLE OF TEMPERATURE DIFFERENCES
This is a comment by a reader of the column some time ago:
Hi Alan, The carbon flow and the outcomes you describe has some impact and can be impacted by the conditions in the ground, including temperature. I have taken temp readings here on my farm and find that on hot days the variation in temp from bare soil to trash covered soil and then to growing grass covered soil can be 5 degrees c between each, i.e. a full span of 10 degrees c
Resilient landscapes, due to better management of carbon flows over time, have the potential to attract higher rainfall, as they have different energy patterns and often have more moisture to offer weather systems.