Intermittent precipitation from a cumuliform cloud. usually of short duration but often heavy.


A reservoir that uptakes a chemical element or compound from another part of its cycle. For example. soil and trees tend to act as natural sinks for carbon.


A type of precipitation consisting of transparent pellets of ice 5 mm or less in diameter. Same as ice pellets.


Originally smog meant a mixture of smoke and fog. Today. smog means air that has restricted visibility due to pollution. or pollution formed in the presence of sunlight-photochemical smog.

SNOpack TELemetry (SNOTEL)(8)

A near real-time hydrometeorological data collection network in the West that collects SWE. precipitation. and temperature data from nearly 600 remote high-elevation stations.

Snow flurries(6)

Light showers of snow that fall intermittently.

Snow grains(6)

Precipitation in the form of very small. opaque grains of ice. The solid equivalent of drizzle.

Snow pellets(6)

White. opaque. approximately round ice particles between 2 and 5 mm in diameter that form in a cloud either from the sticking together of ice crystals or from the process of accretion.

Snow rollers(6)

A cylindrical spiral of snow shaped somewhat like a child's muff and produced by the wind.

Snow squall (shower)(6)

An intermittent heavy shower of snow that greatly reduces visibility.

Snow water content (SWC)(8)

How much liquid water is contained in a volume of solid snow (in other words. how much water would be measured if a known amount of snow was melted). Snow water content and snow water equivalent are different terms for the same parameter.

Snow water equivalent (SWE)(8)

How much liquid water is contained in a volume of solid snow (in other words. the amount of water measured from melting a known amount of snow). Snow water content and snow water equivalent are different terms for the same parameter.


Solid precipitation in the form of minute ice flakes that occur below 0C.


An aggregate of ice crystals that falls from a cloud.


A horizontally layered accumulation of snow from snowfall events. which may be modified by meteorological conditions over time.

Solar constant(1)

Aka total solar irradiance. The constant expressing the amount of solar radiation reaching the Earth from the sun. approximately 1370 watts per square meter. It is not. in fact. truly constant and variations are detectable.

Solar cycle(1)

Eleven-year cycle of sunspots and solar flares that affects other solar indexes such as the solar output of ultraviolet radiation and the solar wind. The Earth's magnetic field. temperature. and ozone levels are affected by this cycle.

Solar radiation(1)

Energy received from the sun is solar radiation. The energy comes in many forms. such as visible light (that which we can see with our eyes). Other forms of radiation include radio waves. heat (infrared). ultraviolet waves. and x-rays. These forms are categorized within the electromagnetic spectrum.

Solar variability(2)

changes in the sun's radiation due to the sun's internal dynamics.


Either of the two times of the year when the sun is the greatest distance from the celestial equator. occurring about June 22 and December 22. See winter solstice and summer solstice.


Any process or activity that results in the net release of greenhouse gases. aerosols. or precursors of greenhouse gases into the atmosphere.

Southern Oscillation (ENSO)(6)

Shifting of pressure zones in the Pacific during an El Ni�o event.

Southern Oscillation Index (SOI)(3)

SOI is based on the (atmospheric) pressure difference between Tahiti and Darwin. Australia.


A characteristic that refers to a location (which may be a specific location on the Earth's surface. or relative to an arbitrary point).

Specific heat(6)

The ratio of the heat absorbed (or released) by the unit mass of the system to the corresponding temperature rise (or fall).

Specific humidity(6)

The ratio of the mass of water vapor in a given parcel to the total mass of air in the parcel.

Spontaneous nucleation (freezing)(6)

The freezing of pure water without the benefit of any nuclei.

Spring freeze date(6)

The date of occurrence in the spring of the last minimum at or below a temperature threshold.

Squall line(6)

Any nonfrontal line or band of active thunderstorms.

Station pressure(6)

The actual air pressure computed at the observing station.

Steam fog(6)

See Evaporation fog.


One of the two types of dry climate. A marginal and more humid variant of the desert that separates it from bordering humid climates. Steppe also refers to the short-grass vegetation associated with this semiarid climate.

Storm surge(6)

An abnormal rise of the sea along a shore. Primarily due to the winds of a storm. especially a hurricane.


A low cloud. predominantly stratiform with low. lumpy. rounded masses. often with blue sky between them.


The boundary between the stratosphere and the mesosphere.


The region of the atmosphere extending from the top of the troposphere to the base of the mesosphere. an important area for monitoring stratospheric ozone.

Stratospheric ozone(3)

In the stratosphere. ozone has beneficial properties where it forms an ozone shield that prevents dangerous radiation from reaching the Earth's surface. Recently. it was discovered that in certain parts of the world. especially over the poles. stratospheric ozone was disappearing creating an ozone hole.


A low. gray cloud layer with a rather uniform base whose precipitation is most commonly drizzle.

Subarctic climate(6)

A climate found north of the humid continental climate and south of the polar climate and characterized by bitterly cold winters and short cool summers. Places within this climatic realm experience the highest annual temperature ranges on earth.


The process whereby ice changes directly into water vapor without melting. In meteorology. sublimation can also mean the transformation of water vapor into ice. (See Deposition.)

Subsidence inversion(6)

A temperature inversion produced by the adiabatic warming of a layer of sinking air.


The slow sinking of air. usually associated wit high-pressure areas.


A climate zone adjacent to the tropics with warm temperatures and little rainfall. Recent theory suggests that sulfate aerosols may lower the earth's temperature by reflecting away solar radiation (negative radiative forcing). General Circulation Models which incorporate the effects of sulfate aerosols more accurately predict global temperature variations.

Sulfate aerosols(1)

Particulate matter that consists of compounds of sulfur formed by the interaction of sulfur dioxide and sulfur trioxide with other compounds in the atmosphere. Sulfate aerosols are injected into the atmosphere from the combustion of fossil fuels and the eruption of volcanoes like Mt. Pinatubo. Recent theory suggests that sulfate aerosols may lower the earth's temperature by reflecting away solar radiation (negative radiative forcing). General Circulation Models which incorporate the effects of sulfate aerosols more accurately predict global temperature variations. See particulate matter. aerosol. General Circulation Models.

Summer solstice(6)

Approximately June 22 in the Northern Hemisphere when the sun is highest in the sky and directly overhead at latitude 23.5�N. the Tropic of Cancer.

Sun pillar(6)

A vertical streak of light extending above (or below) the sun. It is produced by the reflection of sunlight of ice crystals.


A colored luminous spot produced by refraction of light through ice crystals that appears on either side of the sun. Also called parhelion.

Supersaturated air(6)

A condition that occurs in the atmosphere when the relative humidity is greater that 100 percent.

Surface air temperature(2)

The temperature of the air near the surface of the Earth. usually determined by a thermometer in an instrument shelter about 2 m above the ground. The true daily mean. obtained from a thermograph. is approximated by the mean of 24 hourly readings and may differ by 1.0 degrees C from the average based on minimum and maximum readings. The global average surface air temperature is 15 degrees C.

Surface inversion(6)

See Radiation inversion
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Americans’ health, security and economic wellbeing are tied to climate and weather. Every day, we see communities grappling with environmental challenges due to unusual or extreme events related to climate and weather. In 2017, the United States experienced a record-tying 16 climate- and weather-related disasters where overall costs reached or exceeded $1 billion. Combined, these events claimed 362 lives, and had significant economic effects on the areas impacted, costing more than $306 billion. Businesses, policy leaders, resource managers and citizens are increasingly asking for information to help them address such challenges.