In the last post, I wrote about how if the major cycles for weather in the middle East were holding true, the Arabian Peninsula would be receiving more rain than it has in the recent past, while the opposite is actually true. This post will explore the first major edge where development needs to take place to start greening the Arabian Desert, and explain my hypothesis for why rain isn’t coming, and how desertification is a self-perpetuating loop of death.
I live in Jeddah, which lies on the Tihama plain–a narrow strip of land between the red sea and the mountains of the hijaz, which run about 45-60 kilometers off the west coast of the Arabian Peninsula. Humidity on this plain is very high because of evaporation from the red sea. This can frequently be seen live thanks to the University of Madison-Wisconsin’s global composite water vapor map. While the Arabian Peninsula is often black (very dry) there is frequently a band of grey along the hijaz and in Yemen & Oman. Below is a picture of what the moisture looks like at the time of this writing:
Some of this moisture is coming off of the ITCZ (you can see the white band of clouds moving across Africa and the Indian Ocean), and some of it is coming off the Red Sea. On the tihama plain, where I am, we should frequently be experiencing an orographic lift, wherein the moisture is driven up into the atmosphere by the mountains, causing clouds to form and rain to fall. I have witnessed this in Al Baydha, which is located about 45 KM south of Makkah, and West of Taif. When it rains in Al Baydha (which it has twice, since 2010), the clouds do not come from the sea in the west, but from the mountains to our east. The clouds form east of us and then are pushed west, causing rain to fall in our area.
So if there is moisture in the air, and if that moisture is experiencing orographic lift, then why isn’t it raining more often?
The fact is, there are 3 factors inhibiting greater rainfall on in the hijaz that we need to tackle if we’re going to green Arabia.
2: Surface Temperatures & the Dewpoint
3: Availability of Nuclei for Raindrops to Form
“When desert dust reaches heights above 5 km, it absorbs and reflects back to space some of the solar radiation, and so warms the mid-troposphere (Kishcha and others 2003) at the expense of cooling the lowest levels. This generates a downward airflow that exacerbates desert conditions. The added dryness can lead to more desert dust, thus amplifying the initial effect. Desert dust particles can impair precipitation from potential rain clouds, and keep the desert drier, dustier and even less favourable to precipitation in a reinforcing feedback loop, which further increases dust generation by deserts and the likelihood of its transport to non-deserts. Far away from deserts, the transported dust may suppress precipitation from convective clouds by inhibiting the formation of raindrops (Rosenfeld and others 2001)” –Cited from the United Nations Global Deserts Outlook”
I’m going to connect these dots a little more: When large sand particles strike bare ground, they release tiny pieces of dust (20 microns or less) into the high atmosphere, which inhibit raincloud formation, warm the part of the atmosphere where clouds would form, and increase downward wind, thus increasing the amount of sand that strikes the desert ground, and releasing more tiny pieces of dust. Thus we have a feedback loop that increases with intensity as desertification progresses, making it harder and harder for precipitation to occur, which in turn makes it harder for plants to live, and thus exposing more bare ground to the buffetings of the winds and dust. As bare ground increases, the land becomes less and less able to absorb and retain moisture, thus increasing the rate of evaporation. In short, dust is enough by itself to kill the rainfall and the land’s fertility, and once the cycle of desertification is underway, it will grow on itself until everything is dead. (More scientific information on this cycle can be found here.)
This process, specifically in the Arabian Peninsula, has been observed and recorded by Craig Dremman, rather convincingly in his post here. The picture below is a teaser:
SURFACE TEMPERATURE AND DEWPOINT
The dew point is the temperature at which water in the air condenses at the same rate at which it evaporates. If the temperature of the air falls below the dew point, you could precipitation (assuming pressure stays the same) while if the temperature stays above the dew point, you could no precipitation. When bare soil (one of the fundamental descriptions of a desert) is exposed to sun the temperature rises, creates convection, and makes it very difficult for the temperature to fall below the dew point.
This is compounded by the lack of water in the soil. NASA’s Earth Observatory has a page on the physical processes that cause drought that talks about wet soils vs. dry soils and the impact they have on drought. When soils are wet, the evaporation from bare soil can help create clouds, whereas when soils are dry, there is no evaporation taking place and thus no cooling effect.
Availability of Raindrop Nuclei
For water to move from a gas to a liquid, it needs something to condense on. If you ever have a couple of cold beverages outside on a humid day, the water collecting on the outside of the can or bottle is being pulled out of the air and condensing on your drink. The same process is required for clouds to form–water in the atmosphere condenses on tiny (1/100th the size of a raindrop) particles. This is initially why scientists used to think that dust in the atmosphere contributed to cloud formation (though they were wrong) because they thought the dust could act as a condensing nucleus. This fact is the basis for the idea behind cloudseeding.
Here is the tricky thing: Not all cloud condensing nuclei are equal. Kenya’s tea plantation area of Kericho has the highest number of hailstorms per year on the planet. After some studies done to try to figure out the cause of all these ice storms, it was concluded that the tea trees themselves were the source of the hail storms. The tea litter would be stirred up by workers picking tea leaves, and waft up into the atmosphere. Here is the kicker: the ice nuclei was able to form around tea litter at a temperature 5 degrees warmer than litter from the surrounding forest. The atmosphere could be warmer and the ice would still form as long as the nuclei for that ice was tea litter. Do you see those italics? This is crucial! Ice forms around tea litter nuclei between -3C and -5C. Silver iodide, the material used in cloud seeding to create rain, requires temperatures of -9 to -10C. The point is, rain will form around organic materials created by some trees more easily than around the best material used by geoengineering companies whose job it is to create rain through cloud seeding.
There is one more gaping lack of raindrop nuclei to be mentioned here: that created by volatile organic compounds emitted from trees. A study from 2008 in the Amazon watershed noted that “the forests emit a large amount of volatile organic compounds (VOCs) which contribute to produce shallow and relatively warm clouds, very efficient to induce rains in the region.” Aside from organic litter, the VOC’s created by the respiration of a forest efficiently lead to cloud formation and rainfall. In a desert, where there are no trees, almost no organic material at all, and since dust is a major inhibitor of cloud formation, the climate must depend on nuclei being brought in from other places–carried by advection or wind.
So there you have it. If the weather in the Arabian Peninsula were following the typical pattern of increasing rainfall as the ITCZ moves North, rain would be increasing. Instead rain is decreasing, and there are 3 principle obstacles to overcome before that can change: dust, temperature, and the lack of rain nuclei. These three factors feed on each other–leading to decreased precipitation, a decrease in the soil’s ability to retain water, less plant life, more bare soil, more dust, higher temperatures, less nuclei, going on and on and on until the land is dead and incapable of supporting life except for those that are the most specialized and extreme. This cycle must be stopped.
At this point, if you are following all of my posts in this series (which you should!) you have a basic understanding of the natural cycles of rainfall in the Arabian Peninsula as well of the destructive cycle of desertification. After the next few posts, the solutions–the things that we as people can do to reverse desertification and restore productivity to these denuded lands should become obvious. If we understand the problem well enough–so well that we comprehend its context and its causes and its effects–the solutions will present themselves.