Precipitation basics

Precipitation is a liquid or solid form of water falling from a cloud.  This means that before precipitation can occur, clouds must first be present - and these require rising moisture-laden air.  The process of growing cloud droplets to become rain drops depends on the temperatures within the cloud. Cold clouds involve ice and possibly liquid water, and warmer clouds only have liquid water.  This webpage is focused on the effects of precipitation rather than the mechanisms of the process.  These articles are good starting places to explore how precipitation occurs: Precipitation: Formation to Measurement (Plymouth State University) and Precipitation Processes by Roland Stull.

Precipitation affects the parcel of air it fell from in two ways.  First, water molecules are leaving the parcel so there is less evaporative cooling when the air sinks.  Second, the latent heat of condensation released within the air parcel remains with the molecules still within the parcel.  The air parcel is now quite different from when it left the ground - it has more heat and is drier.  If air sinks back to the ground, it will be warmer and drier compared to when it started its upward journey.  Precipitation creates a nonadiabatic process affecting the air parcel since energy and mass have been exchanged with the environment.

Deserts receive precipitation - sometimes in the form of snow! Location: Sonoran Desert, New Mexico.

Deserts receive precipitation - sometimes in the form of snow! Location: Sonoran Desert, New Mexico.


Precipitation and The Water Cycle

Water covers three quarters of the Earth's surface, and the oceans contain 97% of the water available at and near Earth's surface. Evaporation and precipitation are the two processes that move water from the oceans to the other reservoirs.  Evaporation from the ocean's surface allow water vapor to form clouds and precipitation over land and oceans.  Wind transports water vapor over land, creating precipitation  that falls in liquid and solid forms.  The liquid form flows is rivers and  into the soil and rocks, eventually returning to the oceans.  Snow may melt or sublimate, or it may be compressed into ice, remaining in this form for many thousands of year.  

The diagram from NASA JPL (above) illustrates how water moves between various reservoirs. How long a water molecule stays in a reservoir is called its residence time.  Water may be locked up in one reservoir for hundreds of thousands of years.  For example,  humans living in dry regions are removing underground water thousands of years old, and it will take thousands of years to replenish by natural processes.

Water Cycle Dice Game

This seemingly simple game illustrates the many processes that move water between its primary reservoirs and how long it remains there.  Residence time is important to consider when processes are affecting matter cycles.  Ice can store water for hundreds of thousands of years.  When polar climates warm and ice melts, it takes a long time by a human's perspective for it to form again when the climate cools, especially given the poles experience some of the lowest annual precipitation totals on Earth (see the Convergence section of the Atmosphere, Clouds, and Precipitation software).


Precipitation, Life, and Climate Regimes

Water appears to be a "simple" molecule consisting of two hydrogen atoms bonded to one oxygen atom, yet it has unusual properties compared to all others that make it vital for life to exist and thrive while also playing a critical role in moderating and regulating Earth's energy budget. It is the molecule at the heart of Earth Systems since it is critically involved in all three of the  big components: energy flows, matter cycles, and life webs.  Water is quite common in the universe since nearly three quarters of all molecules are hydrogen and just over 1% are oxygen, but a planet with all three phases (solid, liquid, and gas) readily available is not common.  

If precipitation is available for much of the year, rain and snow forests can be supported.  These areas have an abundance of life in an array of diverse forms. Whereas, persistent sinking of dry air creates deserts that have minimal amounts of life..  The global circulation cells create both of these climate regimes since rising parts of the cells creates consistent rains and the dry air leaving the tops of the rain eventually sinks and creates deserts (see image above).  Explore the global circulation cells at the Global Winds section of the Earth, Wind and Forces software and these global bands of wet and dry conditions using the Convergence section of the Atmosphere, Clouds, and Precipitation section.

Notice the positive feedback mechanism between evaporation and precipitation within the circulation cells when they occur over the oceans.  The copious precipitation creates a large supply of warm, dry air to sink at bands 30 degrees from the equator.  This warm, dry air will evaporate more ocean water which feeds more water vapor to the bands of upwelling air at the equator and 60 degrees from the equator, making greater precipitation.

Minimal life is found in the Mohave Desert, the driest place in North America.

Minimal life is found in the Mohave Desert, the driest place in North America.


Deserts over Oceans?

A desert is an area with very little annual precipitation, and they aren't confined to land areas.  Even though there is plenty of water over the oceans, there are large regions with very little precipitation.  Since only water evaporates from the oceans, salt minerals are left behind, making the surface waters denser.  Precipitation is fresh water, so if it falls on the oceans, it mixes with the surface waters, decreasing the salinity.  

Since the oceans are the primary source for precipitation over land, this means water is leaving the oceans but the salt stays.  The precipitation over land helps break down rock and transport the chemicals to the oceans, and these chemicals become the salt in the oceans.  This creates a positive feedback mechanism where the oceans become more saline over time.  There is a threshold of ocean salinity for supporting life, but we know they haven't exceeded this since complex life began 600 million years ago.  It turns out that these ocean deserts help remove salt from the oceans - but only when these deserts are bodies of water with outflow to the oceans, such as the Mediterranean Sea and the Dead Sea today.  The high salinity, high evaporation, and minimal precipitation create conditions where the salts chemical precipitate from the water and become rocks called evaporites.  These evaporites remove salt from the oceans for long periods of time (residence time) allowing the oceans to stay within salinity values that support life.

Salty Ocean Deserts

The oceans are not uniformly salty, as illustrated in the image above from NASA's Aquarius Mission website.  Go to their site to compare contoured monthly data fields of a number of variables to determine the factors that create saltier and fresher waters in the Atlantic Ocean.

If not familiar with contour maps, see the Contouring activities.


Rain Shadows and Mountains

Air being pushed (wind) into a mountain range cannot go through it, it must go over it.  If the rising air contains enough water vapor and condensation, clouds and precipitation form - so water molecules leaving the rising air parcel, but the latent heat of condensation stays with the air molecules.   The air sinking on the downwind side of the mountain is warmer and drier than when it began moving up the mountain.  If the winds are consistent through much of the year, the dry conditions create a desert.  To determine the seasonal patterns of wind for a given latitude, see the Global Winds section of the Earth, Wind, and Forces software.

Notice that the rain occurs on the upwind side of the mountain (or windward side) but not on the downwind or leeward side, but clouds may be one both sides of the mountains.  Sinking air does not cause the clouds to immediately disappear, rather, the condensed water droplets begin to evaporate.  The latent heat of evaporation cools the air, offsetting nearly half of the heating caused by contraction of air parcel (Ideal Gas Law), which slows the rate of evaporation.  The cloud disappears only when all of the condensed water in the parcel evaporates.

Note that this is a very simplified model of precipitation, and the results indicate trends observed with air rising and sinking along mountain ranges, not accurate values observed in nature.

Explore Rain Shadows

Use Mountains section of the Atmosphere, Clouds, and Precipitation software to determine  the conditions that create the most intense rain shadows.  

Where are these conditions found on Earth?  Use the terrain map and your knowledge of global wind patterns from the Earth, Wind, and Forces software to identify these rain shadows.

But can mountains cause too much of a good thing?  With the advent of water vapor sensing satellites, atmospheric rivers have been discovered, that, when blown into mountain ranges, can create catastrophic flooding.

Rain Shadow Effect Changed North America ** MOVIE Coming **

Explore how ice sheets covering large parts of North America changed since 21,000 years, when ice extended furthest south.  Notice the ice-free corridor east of the Canadian Rockies caused by the rain shadow effect.  Because water was stored in land ice, sea level was lower, North America and Asia were connected until 11,000 years ago, and people walked from Asia into North America. Source: Williams, et. al., 2004, Ecological Monographs.

Roles of Precipitation in Earth's Energy Budget

Land Plants

Evaporation, precipitation, and wind move water from the oceans to land, which supports life thriving in what would otherwise be a very inhospitable place.  Plants alter the reflectance of the land surface, absorbing visible light for photosynthesis (the radiation is converted to chemical energy rather than heat) and reflecting visible and infrared wavelengths  not used, helping to cool the land during the day.  The cooler land radiates less thermal radiation at night, helping to slow the nighttime cooling.  The water within the plant cell's also moderate temperature change due to its high specific heat (see the Radiation Budgets section of the Blackbody Radiation software).   Plants lose heat more slowly at night compared to bare rock and soil surfaces.  Finally, plants evaporate and transpire water vapor, which is a greenhouse gas, so more heat is retained in the nighttime atmosphere than a drier atmosphere.  So one of the primary effects of precipitation on land is driven by the support of plant life.

Bodies of Water on Land

Precipitation on land may be stored in lakes, which act similarly as oceans in moderating the local to regional climate.  Due to its high specific heat, water changes temperature slowly as it gains/loses a tremendous amount of heat.  Diurnal and seasonal temperature changes are decreased over water and along the coast.  Lakes are also a significant source of evaporated water over land, increasing the precipitation over land, helping to maintain this moderating effect on Earth's energy budget.  Combining both the stored heat and availability for evaporations, lakes create lake-effect snows during the late fall to early winter.

Precipitation in Global Circulation Cells

Due to the consistent and relatively intense solar warming of the tropics, the global atmospheric circulation cells help transport the warmer air toward the poles.  But precipitation at the rising portions of the cells adds the latent heat of condensation to the poleward moving air.   If the miraculous qualities of water have not been heralded enough, water has the highest latent heat of condensation/evaporation of any natural substance, meaning we have the most potent material to help warm the higher latitudes through precipitation.

Water supports life that has completely changed the chemistry and the energy budget of Earth.

Water supports life that has completely changed the chemistry and the energy budget of Earth.