Consider the case of two weather systems of different temperatures hitting the Andes Mountains from the west: After traveling a long distance over the Pacific ocean I would expect the air in both systems to be water saturated (i.e. Relative Humidity of 100%). However, the warmer system will be carrying more water and it will drop more water over the mountains (correct me if I got something wrong up to this point).

Now: Will the precipitation happen over the same elevation range? Or it will start to happen at lower/higher elevations?. Will the point of maximum precipitation stay the same? Or it will migrate upwards/downwards?

In case particular details are needed to answer this question, I'll describe now my main area of interest, that correspond to the Patagonian icefields, in the southern tip of South America between latitudes 46 to 51° South, roughly 500 km in the north-south direction. The area corresponds to the red box in the following world map:

enter image description here

The size of the area would put it in the the upper boundary of the mesoscale meteorology. However, topography gradients can vary quite a bit within the domain.

The following image show how the area looks from above, with a sat image on the left (where the ice extent is clear) and the topography on the right:

enter image description here

I have averaged the above elevation model in the north-south direction to obtain the average topography profile that Westerlies will face when hitting the mountains from the West (predominant winds come from between W and NW direction). This is how it looks:
enter image description here

The red lines (40 km apart) indicate roughly the currently ice covered longitudinal band. To the west of it, liquid precipitation dominates and there is no permanent ice cover. To the east, there is very little precipitation (dry pampas) and therefore no ice cover either.

The tree line in the area is usually between 400 and 600 m of elevation, represented by the green lines in the graph above.

The mean wind speed (from a 22 year average dataset) when the winds hit the land is between 7 and 8 m/s (25-29 km/h), as show by the shading and contours in this figure: enter image description here

From the same 22-year average dataset. The typical 10-meter air temperature on the ocean just before hitting the land is between 6 and 7 °C.

  • $\begingroup$ I work on orographic precipitation. Please clarify the scale of the system - mesoscale or synoptic. If mesoscale the physiography and topography will also play a role $\endgroup$
    – user1066
    Commented Feb 28, 2018 at 0:52
  • $\begingroup$ You probably are considering a simple upward ascent model from that image. Even still is that slope barren or is there vegetation ? That will definitely affect the lapse rate. Also how do you want to tackle the lower boundary ? $\endgroup$
    – user1066
    Commented Feb 28, 2018 at 1:43
  • $\begingroup$ @gansub I'm very glad to have your input here. I'll update the question with those details for the area of my greatest interest. $\endgroup$ Commented Feb 28, 2018 at 2:15
  • $\begingroup$ @gansub I've added a fair amount of info of my main area of interest. Have a look. I'm a glaciologist planing to work in that area and this question is of great interst to me. Thanks a lot! $\endgroup$ Commented Feb 28, 2018 at 6:31
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    $\begingroup$ @CamiloRada - This answer should help this question as well - earthscience.stackexchange.com/questions/14808/… and if you have any questions I am happy to answer them $\endgroup$
    – user1066
    Commented Aug 9, 2018 at 5:03

1 Answer 1


Examining two extreme cases, one very cold and the other very warm, the amount of precipitable water in the very cold one will be far less than that in the very warm one. Even fully saturated, the very cold air mass would need to be lifted quite high to form large enough ice crystals or water droplets to have enough mass to begin falling. Meanwhile, the warm air mass would produce larger droplets very quickly. Thus the warm air mass would begin to produce precipitation at a lower altitude, and with greater intensity.

Because the air masses begin precipitating at different altitudes, the point of maximum intensity would also be lower for the warm air mass.

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    $\begingroup$ welcome to ESSE and great background. I am sure your answers will be well received in the future. $\endgroup$
    – user1066
    Commented Feb 28, 2018 at 13:49

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