Clouds & Precipitation

What are Clouds and What is Precipitation?

Clouds are comprised of water droplets and ice crystals that are suspended in the atmosphere. Precipitation occurs when these 'hydrometeors' grow to a sufficient size that falls out of the cloud, which requires a settling velocity greater than the updraft associated with the cloud. At temperatures above freezing (zero degrees Celsius), these hydrometeors are always liquid, corresponding to cloud droplets and raindrops.

When temperatures are below freezing, these hydrometeors can be liquid or solid, or a mix of both, corresponding to a wide range of particle types, including supercooled cloud droplets, snow, sleet and graupel. It is important to note that all cloud hydrometeors are formed upon a subset of aerosol particles called cloud condensation nuclei (CCN) or ice nuclei (IN), and each hydrometeor therefore contains the soluble and insoluble materials from the CCN and/or IN within them. Thus, the chemical composition of the atmosphere is tied to cloud and precipitation chemistry.

History of Cloud & Precipitation Measurements at Whiteface Mountain

Whiteface Mountain (WFM) is an important sampling site for cloud chemistry research. Routine summertime chemical analysis of cloud water collected from the top of WFM began in 1986 during the Mountain Cloud Chemistry Project, followed by the Mountain Acid Deposition Program from 1994 to 1999 and the Adirondack Lake Survey Corporation (ALSC) from 2001 to 2017. These measurements have continued under the stewardship of the Atmospheric Sciences Research Center at The University at Albany starting in summer of 2018, through a five year grant from the New York State Energy Research and Development Authority (NYSERDA). More information about the current program can be found at the cloud water website hosted by Dr. Sara Lance. Precipitation sampling has also been conducted at WFM since 1984 as part of the National Acid Deposition Program (NADP), currently managed by the Wisconsin State Laboratory of Hygiene.

Cloud and precipitation chemistry research at WFM began with a primary focus on sulfate and nitrate pollution and the "acid rain" caused by these chemicals. Since the early 2000's, cloud water sulfate concentrations have dropped by more than 80% and average pH values have increased from 4 to more than 5 (a factor of 10 change in free hydrogen ion concentrations) becoming much less acidic as a result of the Clean Air Act and associated government regulations limiting gas phase SO2 and NOx emissions. This recent history is a clear success story for the long‐term health of the Adirondack ecosystems. 

Recent ongoing research has turned the focus to the organic component of cloud water. In 2009, researchers at ALSC recognized that organic molecules were becoming more important and began to measure Total Organic Carbon (TOC) in the cloud water. Carbon forms the backbone of organic molecules, so TOC provides a fundamental measure of the contribution of organics to cloud water composition. In addition to continuing the long‐term measurements of TOC and inorganic ions, ASRC added organic acids to the list of analytes that are now routinely measured at WFM, starting in summer of 2018. Organic acids are an important subset of organic molecules because they are soluble, chemically reactive and can alter the cloud water pH.

In 2017, ASRC conducted a pilot study at WFM to better understand chemical processing of organics within clouds (termed the CPOC pilot study). This "chemical processing" occurs during a condensation/evaporation cycle within clouds, which can impart permanent changes to the aerosol chemical and physical properties. The formation of sulfate from SO2 gas within cloud droplets is now a well‐known mechanism for increasing the sulfate content of aerosols. Analogous mechanisms involving soluble organic gases have been identified, but significant uncertainty remains with regard to organics, largely due to the wide variety of organic molecules present in the atmosphere (emitted both from natural and anthropogenic sources), their wide range of chemical properties and the chemical transformations they can undergo.

During the CPOC pilot study, organic matter was found to be the dominant aerosol chemical species, comprising 66-93% of the aerosol mass, a significant shift in aerosol chemical composition from decades past at WFM, reflecting the shift in cloud water composition over that time. Ongoing research will look in more detail at the differences between cloud droplets and aerosol particles to clarify the role of cloud droplets as "aqueous micro-reactors" in the chemical transformation of organic aerosol.

Why is it important to study Clouds and Precipitation?

As you have probably noticed, clouds passing in front of the sun cast a shadow that can significantly cool the ground below. This "albedo effect" is even more dramatic over the open ocean because of the contrast between the dark ocean and bright white clouds. At any given time, clouds typically cover 30% of the Earth's surface. Thus, clouds are an important part of the climate system, altering the balance of incoming and outgoing solar radiation, as well as infrared radiation.

Clouds and precipitation are also a central part of the hydrologic balance, dictating where fresh water is distributed over land. Since aerosol particles act as CCN or IN, thereby becoming encapsulated within hydrometeors (along with soluble gases), clouds and precipitation alter how air pollution is chemically processed and removed from the atmosphere and when/where it is deposited to ecosystems. Clouds and precipitation also alter atmospheric dynamics (air motions), changing how pollution is vertically transported and mixed in the atmosphere.


Learn about the importance of clouds in atmospheric chemistry!