I'm studying the relationships between elevations and precipitation (Rainfall gradients). it seems that there is a linear relationship between the two parameters. I am interested in the spatiotemporal variations of this gradient. What relation are you finding in the different climatic zones and different seasons? Same question for the evolution of annual average temperature depending on altitude. Thank you.
Dear Mohamed,
For precipitation to occur, the moisture contained in the air must condense into droplets around condensation nuclei, and the latter must agglomerate to become large enough to fall out. Condensation (corresponding to a relative humidity of 100%) occurs when the capacity of the air to hold moisture reaches its maximum (called saturation). This can occur either by adding more water vapor in the air or by cooling the air, a process that is very efficient because of the strong dependency of the saturation concentration on the temperature of the air (See the Clausius–Clapeyron relation, for instance here: https://en.wikipedia.org/wiki/Clausius–Clapeyron_relation).
A mountain slope is a very efficient mechanism to cool the air, if the latter can be forced to move upslope. Conversely, if the air is running down the slope, it will warm up and exhibit a decreasing relative humidity. Hence it is not surprising to see a relation between precipitation and elevation, though this relation may not need to be linear.
It should be clear from these elements that many different factors can stimulate or tamper precipitations, including terrain slopes and elevation, and that you can expect different relations in different seasons or in different locations. There is an abundant professional literature on this topic, which you will be able to explore by searching the Web with keywords like "precipitation slope elevation". Basic books on meteorology should also contain useful materials and pointers to that theme. You will find a study specifically looking at this relation in the US here:
http://www.prism.oregonstate.edu/documents/Daly2008_PhysiographicMapping_IntJnlClim.pdf
An older but more globally distributed study is available here:
http://journals.ametsoc.org/doi/pdf/10.1175/1520-0442(1994)007%3C1305:SRBTAP%3E2.0.CO%3B2
I hope this helps. Cheers, Michel.
Dear Prof. Saidi,
You can find some explications about air temperature variability in the High Atlas mountain (Rheraya catchment) in this paper:(https://www.researchgate.net/publication/306415889_boudhar_at_al_2011_CRGeoscience_343_2011_32-42).
We developed a model "MSPAT", using thermal data from Landsat image and measuring temperature, that takes into account the physical components of the basin, such as elevation, slope and exposition and the solar angle.
For precipitation, the link between topography is very complicated, especially in the case of the localized and/or snowfall events.
Abdelghani
Data boudhar at al 2011 CRGeoscience 343 (2011) 32–42
Dear Mohamed
There is also another important phenomenon which could also explain rainfall gradient in relation with elevation. It is called the seeder-feeder mechanism. This is an orographic precipitation-enhancement mechanism, in which precipitation from an upper-level precipitating cloud ( so called the seeder cloud) falls through a lower-level orographic cloud (so-called feeder cloud). The phenomenon has been observed on radar and also been modelled by computer. That was actually the subject of my master thesis. I have modelled this phenomena in order to explain the rainfall gradient with elevation and applied it to Western Ghats mountains (India). A paper was published, (Robichaud and Austin, 1988 QJRMS journal), see the following link:
http://onlinelibrary.wiley.com/doi/10.1002/qj.49711448207/full
This phenomenon is important in all seasons and should be considered in order to explain enhancement precipitation enhancement and gradient in mountainous areas. The first to point this was Tor Bergeron (1965) who explained rainfall enhancement of up to 50% over modest hills in Sweeden.
You can also look my M.Sc. thesis on Researchgate web site (my profile) on this subject.
Hope it helps
Alain
Thanks for all. Excuse me to return to the topic. I want to verify, by your different experiences and studys, if the relationship between rainfall and elevation is linear at the beginning then it become logarithmic at a specific level. That means that it is imposible to exceed the potentiel maximum (of precipitation) of the area. Sometimes rainfall can even decrease as soon as the elevation reaches a very cold areas where the hygrometric capacity is low.
Thank you.