# How does elevation affect the amount of rainfall received?

How does elevation affect the amount of rainfall received? Does elevation affect precipitation?

• You question is unclear. Are you talking about comparing an area with elevation H1 with another area with elevation H2 (a 'static' situation), or are you talking about a moving air mass passing areas with varying elevation (a dynamic situation). Please edit your question. – Jan Doggen Jul 3 '17 at 14:27

Elevation affects precipitation significantly, especially in a mountain environment. On the windward side of a mountain, precipitation is increased. As air parcel rises due to increasing elevation on the windward side of the mountain, the air parcel cools, condenses, and it rains. Once the air parcel (cloud) moves over the peak of the mountain, the air parcel dries and warms quickly as it flows down the leeward side of the mountain. This is known as adiabatic warming or compressional warming. In the United States and Canada, there are north to south mountain ranges. On the westward side (windward side), it is green and the areas receive plenty of precipitation. On the eastern side, there are more arid conditions. Plateaus (flat areas of high elevation) can also have an affect on precipitation. Thermal lows can form over plateaus in the subtropics (like Mexico or parts of Asia), causing precipitation. This is a form of a summer monsoon.

• But this is not really the effect of elevation itself, but of changes in elevation. We might compare the Great Plains (not much elevation change there) with the same elevations in the Sierra/Cascades. – jamesqf Jul 3 '17 at 19:05
• Right. However, the slight elevation change in the Great Plains has a drastic effect on precipitation in the area. The Great Plains dryline is a function of elevation, which then is a function of precipitation in the Great Plains. – zfjohnson Jul 5 '17 at 17:12
• Is it just elevation changing the amount of rainfall, or is it that the elevation decreases as you get further from the rain shadow effect of the Rockies? Because after you cross the Mississippi and the elevation starts increasing as you approach the Appalachians. – jamesqf Jul 6 '17 at 17:51
• @jamesqf Yes, elevation increases east of the Mississippi. There is also some variation of elevation in Missouri and Arkansas. However, the continuous (almost constant) decrease in elevation in the Great Plains from the Rockies to about -95 degrees is important to the initiation of rainfall. In the meteorological sense, the warm moist boundary layer air advected from the Gulf of Mexico is impeded by the westward increasing elevation gradient in the Great Plains. This means when a front reaches the boundary, precipitation occurs east of that north-south intersection while the west stays dry. – zfjohnson Jul 10 '17 at 15:14

Elevation affects temperature which affects the amount of rain. Most raingauges around the world are within a few hundred metres of sea-level, i.e the range in which most people live. Within this range the rainfall-elevation relationship often appears to be linear. However, as the temperature gets cooler with altitude, the maximum precipitable moisture decreases drastically, so the rainfall-altitude curve reflexes back upon itself, so that there is an elevation of maximum precipitation ('EMP'). This variation variies with several factors, such as continentality, 'tropicality', aspect, rain shadow and barrier effect. Some examples of the EMP, are (in metres): Tajikistan 3800, Alberta Canada 2360, European Alps, 2250, NW Pakistan1500, Oman 1400 to 2200, Kilimanjaro Tanzania 1500, and Hawaii 900. In the case of polar regions the EMP is ground level because the rainfall-altitude relationship is nearly always negative. Above the EMP the rainfall decreases rapidly with altitude, which is why high mountainous regions are essentially deserts.

Wherever there is significant relief in a catchment, the first step is to determine whether the catchment is all below the EMP. The second step is to process the historic record to see if a second order, or even third order rainfall altitude regression is a significant improvement upon 'linear'. If you want further refinement the seasonal rainfalls, or rainfalls from different directions, often have markedly differing rainfall-altitude relationships. If you want to go the whole hog, and be scientific about it, then the rainfall approximates a multi-variate solution, with components of altitude, altitude squared, aspect, regional trend, rain shadow and barrier effect. This can be solved as an equation of the form: $$R = aH + BH^{2} + cA + dT + eS +f'BE'$$

Areas of high elevation, such as mountain ranges, often drain the air of its moisture. As the air rises up the mountain, it cools. As the air cools, it loses its ability to hold water. The water then condenses out of the air and falls as precipitation.

The high altitudes of mountains often receive a significant amount of precipitation. Often, this precipitation falls in the form of snow, which is why many high mountain peaks are always covered in snow. In mountains near the equator, the constantly high levels of humidity form what is called a "cloud forest." Cloud forests are habitats that feature numerous mosses, algae and epiphytic plants; often, these plants are covered in water for most of the day.

While the top of the mountain may receive abundant rainfall, mountains often remain dry on the down-wind side. This is called the "rain shadow effect," and it causes the downwind side of the mountain to remain dry for most of the year. One of the best examples of this phenomenon is created by the Himalayan Mountains. As the warm, moist air blows from the Indian Ocean, it flies up the side of the Himalayas. Most of the rain falls on the south side of the mountains, and the area on the north side of the mountains remains dry.

This actual case explain more details A Preliminary Analysis of the Relationship between Precipitation Variation Trends and Altitude in China by ATMOSPHERIC AND OCEANIC SCIENCE LETTERS, 2011, VOL. 4, NO. 1, 4146