rain and/or warm temperatures.
SMALL CRAFT ADVISORY
An advisory issued for marine interests, espcially for operators of small boats or other vessels. Conditions include wind speeds between 20 knots (23 mph) and 34 knots (39 mph).
SMOKE
Small particles produced by combustion that are suspended in the air. A transition to haze may occur when the smoke particles have traveled great distance (25 to 100 miles or more), and when the larger particles have settled out. The remaining particles become widely scattered through the atmosphere. It is reported as "FU" in an observation and on the METAR.
SNOW
Frozen precipitation in the form of white or translucent ice crystals in complex branched hexagonal form. It most often falls from stratiform clouds, but can fall as snow showers from cumuliform ones. It usually appears clustered into snowflakes. It is reported as "SN" in an observation and on the METAR.
SNOW ADVISORY
A statement or advisory issued when snow is expected to create hazardous travel conditions. It warns of less severe weather conditions than a winter storm warning.
SNOW BANNER
A plume of snow blown off a mountain crest, resembling smoke blowing from a volcano.
SNOW BLINDNESS
Temporary blindness or impaired vision that results from bright sunlight
reflected off the snow surface. The medical term is niphablepsia.
SNOWBURN
A burn of the skin, like a sunburn, but caused by the sun's rays reflected off the snow surface.
SNOW COVER
The areal extent of ground covered by the snow. It is usually expressed as a percent of the total area of a given region.
SNOW CREEP
A continuous, extremely slow, downhill movement of a layer of snow.
SNOW CRUST
The crisp, almost icy, surface on fallen snow, usually formed by the slight melting and refreezing of the surface snow.
SNOW DEPTH
The actual depth of snow on the ground at any instant during a storm, or after any single snowstorm or series of storms.
SNOW DEVIL
A small, rotating wind that picks up loose snow instead of dirt (like a dust devil) or water (like a waterspout). Formed mechanically by the convergence of local air currents. Also
called a snowspout.
SNOW EATER
Any warm downslope wind, or foehn, that blows over snowy terrain and melts
the snow. Generally applies to the Chinook wind in the Rocky Mountains.
SNOWFALL
The rate at which snow falls, usually expressed in inches of snow depth over a six hour period.
SNOWFLAKES
An ice crystal or an aggregate of ice crystals which fall from clouds.
SNOW FLURRY/FLURRIES
Light showers of snow, generally very brief without any measurable accumulation. May be reported as "SHSN--" in an observation and on the METAR.
SNOW GARLAND
Snow appearing as a beautiful long thick rope draped on trees, fences and other objects. Formed by the surface tension of thin films of water bonding individual snow
crystals.
SNOW GRAINS
Frozen precipitation in the form of very small, white, opaque grains of ice. The solid
equivalent of drizzle. It is reported as "SG" in an observation and on the METAR.
SNOW LEVEL
The elevation in mountainous terrain where the precipitation changes from rain to
snow, depending on the temperature structure of the associated air mass.
SNOW LINE
The lowest elevation area of a perennial snow field on high terrain, such as a mountain range.
SNOWPACK
The amount of annual accumulation of snow at higher elevations.
SNOW PELLETS
Frozen precipitation in the form of white, round or
conical opaque grains of ice. Their diameter ranges from 0.08 to 0.2 inch (2 to 5 mm). They are easily crushed and generally break up after rebounding from a hard surface, unlike hail. Sometimes it is called small or soft hail. It is reported as "GS" in an observation and on the METAR.
SNOW ROLLER
The product of moist, cohesive snow that when initiated by wind rolls down a hill,
gathering snow until it can no longer move, either due to size or lack of slope. It is shaped like a
rolled sleeping bag, some reaching four feet across and seven feet in diameter.
SNOW SHOWER
Frozen precipitation in the form of snow, characterized by its sudden beginning and
ending. It is reported as "SHSN" in an observation and on the METAR.
SNOW SQUALL
A heavy snow shower accompained by sudden strong winds, or a squall.
SOLAR DAY
The complete rotation of the earth in relation to the sun. Although it varies, an average has determined a mean solar day of 24 hours. It is universally used for civil purposes. Compare with the sidereal day.
SOLAR ECLIPSE
An eclipse of the sun occurs when the moon is in a direct line between the sun and the earth, casting some of the earth's surface in its shadow. The moon's disk shaped outline appears to cover the sun's brighter surface, or photosphere. That part of the earth that is directly in the moon's shadow will see a total eclipse of the sun, while the areas around it will see a partial eclipse.
SOLSTICE
The point at which the sun is the furthest on the ecliptic from the celestial equator. The point at which sun is at maximum distance from the equator and days and nights are most unequal in duration. The Tropic of Cancer and the Tropic of Capricorn are those parallels of latitude which lies directly beneath a solstice. In the Northern Hemisphere, the winter solstice falls on or about December 21 and the summer solstice on or about June 21.
SOUNDING
A plot of the atmosphere, using data from upper air or radiosonde observations. Usually confined to a vertical profile of the temperatures, dew points, and winds above a fixed location.
SOUTHERN OSCILLATION
A periodic reversal of the pressure pattern across the tropical Pacific Ocean during El Nino events. It is represents the distribution of temperature and pressure over an oceanic area.
SPECIFIC HUMIDITY
The ratio of the density of the water vapor to the density of the air, a mix of dry air and water vapor. It is expressed in grams per gram or in grams per kilograms. The specific humidity of an air parcel remains constant unless water vapor is added to or taken from the parcel.
SPRING
The season of the year which occurs as the sun approaches the
summer solstice, and characterized by increasing
temperatures in the
mid-latitudes. Customarily, this
refers to the months of March, April, and May in the North
Hemisphere, and the months of September, October, and November in the Southern Hemisphere.
Astronomically, this is the period between the
vernal equinox and the summer solstice.
SPECIAL MARINE WARNING
A Special Marine Warning is issued for a brief/sudden occurrence of sustained wind or frequent gusts > 34 knots, usually associated with thunderstorms.
SPRING TIDE
A tide of increased range, which occurs about every two weeks when the moon is new or full. Compare with a neap tide.
SQUALL
A sudden onset of strong winds with speeds increasing to at least 16 knots (18 miles per hour) and sustained at 22 or more knots (25 miles per hour) for at least one minute. The intensity and duration is longer than that of a gust. It is reported as "SQ" in an observation and on the METAR.
SQUALL LINE
A narrow band or line of active thunderstorms that is not associated with a cold front. It may form from an outflow boundary or the leading edge of a mesohigh.
STABLE/STABILITY
Occurs when a rising air parcel becomes denser than the surrounding air. It will then return to its original position. When the density of the air parcel remains the same as the surrounding air after being lifted, it is also considered stable, since it does not have the tendency to rise or sink further. Contrast with unstable air and instability.
STAGNATION AREA
An area that has a combination of stable stratification, weak horizontal wind speed, and little, if any, significant precipitation. It is usually associated with a warm area of high pressure.
STANDARD ATMOSPHERE
A standard atmosphere has been defined by the International Civil Aeronautical Organization (ICAO). It assumes a mean sea level temperature of 15 degrees Celsius, a standard sea level pressure of 1,013.25 millibars or 29.92 inches of mercury, and a temperature lapse rate of 0.65 degrees Celsius per 100 meters up to 11 kilometers in the atmosphere.
STANDARD SURFACE PRESSURE
The measurement of one atmosphere of pressure under standard conditions. It is equivalent to 1,013.25 millibars, 29.92 inches of mercury, 760 millimeters of mercury, 14.7 pounds per square inch, or 1.033 grams per square centimeter
STANDING CLOUD
Any type of isolated cloud, generally formed over peaks or ridges of mountainous areas, that appears stationary or standing over the terrain. An example is altocumulus lenticularis.
STANDING WAVE
An atmospheric wave that is stationary with respect to the medium in which it is embedded. A mountain wave is an example.
STATIONARY FRONT
A front which is nearly stationary or moves very little since the last synoptic position. Also known as a quasi-stationary front.
STATION ELEVATION
The vertical distance above mean sea level that is the reference level for all current measurements of atmospheric pressure at that station.
STATION PRESSURE
The atmospheric pressure with respect to the station elevation.
STEAM FOG
A type of advection fog that is produced by evaporation when cool air passes over a warm wet surface and
the fog rises, giving the appearance of steam. Also called sea smoke when it occurs over the ocean. An example is Arctic Sea Smoke.
STORM
An individual low pressure disturbance, complete with winds, clouds,and precipitation. Examples include thunderstorms, tornadoes, or even tropical cyclones. The name is associated with destructive or unpleasant weather. Storm-scale refers to disturbances the size of individual thunderstorms.
STORM PREDICTION CENTER (SPC)
A branch of the National Centers for Environmental Prediction, the Center monitors and forecasts severe and non-severe thunderstorms, tornadoes, and other hazardous weather phenomena across the United States. Formerly known as the Severe Local Storms (SELS) unit of the National Severe Storms Forecast Center.
STORM SURGE
The increase in sea water height from the level that would normally occur were there no storm. Although the most dramatic surges are associated with hurricanes, even smaller low pressure systems can cause a slight increase in the sea level if the wind and fetch is just right. It is estimated by subtracting the normal astronomic tide from the observed storm tide.
STORM TRACKS
The path or tracks generally followed by a cyclonic disturbance.
STORM WARNING
A Storm Warning is issued for 1-minute sustained surface winds of 48 kt (55 mph or 88 kph) or greater, either predicted or occurring, not directly associated with tropical cyclones.
STORM WINDS
On the Beaufort Wind Scale, a wind with speeds from 56 to 63 knots (64 to 72 miles per hour).
STRAIGHT-LINE WINDS
Any surface wind that is not associated with rotation. An example is the first gust from a thunderstorm, as opposed to tornadic winds.
STRATIFORM
Clouds composed of water droplets that exhibit no or have very little vertical development. The density of the droplets often blocks sunlight, casting shadows on the earth's surface. Bases of these clouds are generally no more than 6,000 feet above the ground. They are classified as low clouds, and include all varieties of stratus and stratocumulus. The opposite in type are the vertical development of cumuliform clouds.
STRATOCUMULUS
A low cloud composed of layers or patches of cloud elements. It can form from cumulus clouds
becoming more stratiformed and often appears as
regularly arranged elements that may be tessellated, rounded, or roll-shaped with relatively
flat tops and bases. It is light or dark gray in color, depending on the size
of the water droplets and the amount of sunlight that is passing through them.
STRATOPAUSE
The boundary zone or transition layer between the stratosphere and the mesosphere. Characterized by a decrease in temperature with increasing altitude.
STRATOSPHERE
The layer of the atmosphere located between the troposphere and the mesosphere, characterized by a slight
temperature increase and absence of clouds. It extends between 11 and 31 miles (17 to 50 kilometers) above the earth's surface. It is the location of the earth's ozone layer.
STRATUS
One of the three basic cloud forms (the others are cirrus and cumulus). It is also one of the two low cloud types. It is a sheetlike cloud that does not exhibit individual elements, and is, perhaps, the most common of all low clouds. Thick and gray, it is seen in low, uniform layers and rarely extends higher than 5,000 feet above the earth's surface. A veil of stratus may give the sky a hazy appearance. Fog may form from a stratus cloud that touches the ground. Although it can produce drizzle or snow, it rarely produces heavy precipitation. Clouds producing heavy precipitation may exist above a layer of stratus.
STRATUS FRACTUS
Stratus clouds that appear in irregular fragments, as if they had been shred or torn. Also appears in cumulus clouds (called cumulus fractus), but not in cirrus clouds.
SUBLIMATION
The process of a solid (ice) changing directly into a gas (water vapor), or
water vapor changing directly into ice, at the same temperature, without ever going through the liquid state (water). The opposite of crystalization.
SUBPOLAR
The region bordering the polar region, between 50 and 70 degrees North and South latitude. This is generally an area of semi-permanent low pressure that exists and where the Aleutian and Icelandic Lows may be found. However, a dome of high pressure may form over the cold continental surfaces during the winter, for example, the North American High and the Siberian High.
SUBREFRACTION
Less than normal bending of light or a radar beam as it passes through a zone of contrasting properties, such as atmospheric density, water vapor, or temperature. Contrast with superrefraction.
SUBSIDENCE
A sinking or downward motion of air, often seen in anticyclones. It is most prevailent when there is colder, denser air aloft. It is often used to imply the opposite of atmospheric convection.
SUBTROPICAL
The region between the tropical and temperate regions, an area between 35 and 40 degrees North and South latitude. This is generally an area of semi-permanent high pressure that exists and is where the Azores and North Pacific Highs may be found.
SUBTROPICAL AIR
An air mass that forms over the subtropical region. The air is typically warm with a high moisture content due to the low evaporative process.
SUBTROPICAL JET
Marked by a concentration of isotherms and vertical shear, this jet is the boundary between the subtropical air and the tropical air. It is found approximately between 25 and 35 degrees North latitude and usually above an altitude of 40,000 feet. Its position tends to migrate south in the Northern Hemispheric winter and north in the summer.
SUMMATION LAYER AMOUNT
The amount of sky cover for each layer is given in eighths of sky cover attributable to clouds or obscurations. The summation amount for any given layer is equal to the sum of the sky cover for the layer being evaluated plus the sky cover for all lower layers, including partial obscuration. A summation amount for a layer can not exceed 8/8ths.
SUMMER
Astronomically, this is the period between the summer solstice and the autumnal equinox. It is characterized as having the warmest temperatures of the year, except in some tropical regions. Customarily, this
refers to the months of June, July, and August in the North Hemisphere, and the months of December, January, and February in the Southern Hemisphere.
SUN DOG
Either of two colored luminous spots that appear at roughly 22 degrees on both
sides of the sun at the same elevation. They are caused by the refraction of
sunlight passing through ice crystals. They are most commonly seen during winter in the middle latitudes and are exclusively associated with cirriform clouds. The scientific name for sun dogs is parhelion and they are also known as mock suns.
SUN PILLAR
Horizontal ice crystals in the form of plates, which occur in clouds and ice fog near the earth's surface, reflect sunlight into vertical sun pillars for a spectacular
display.
SUNRISE
The daily appearance of the sun on the eastern horizon as a result of the earth's rotation. In the United States, it is considered as that instant when the upper edge of the sun appears on the sea level horizon. In Great Britain, the center of the sun's disk is used instead. Time of sunrise is calculated for mean sea level. See sunset for comparison.
SUNSET
The daily disappearance of the sun below the western horizon as a result of the earth's rotation. In the United States, it is considered as that instant when the upper edge of the sun just disappears below the sea level horizon. In Great Britain, the center of the sun's disk is used instead. Time of sunset is calculated for mean sea level. See sunrise for comparison.
SUPERCELL
A severe thunderstorm characterized by a rotating, long-lived, intense updraft. Although not very common, they produce a relatively large amount of severe weather, in particular, extremely large hail, damaging straight-line winds, and practically all violent tornadoes.
SUPERCOOLING
The reduction of the temperature of any liquid below the melting point of that substance's solid phase. Cooling a substance beyond its nominal freezing point. Supercooled water is water that remains in a liquid state when it is at a temperature that is well below freezing. The smaller and purer the water droplets, the more likely they can become supercooled.
SUPERREFRACTION
Greater than normal bending of light or radar beam as it passes through a zone of contrasting properties, such as atmospheric density, water vapor, or temperature. Contrast with subrefraction.
SURFACE BOUNDARY LAYER
The lowest layer of the earth's atmosphere,
usually up to 3,300 feet, or one kilometer, from the earth's surface, where
the wind is influenced by the
friction of the earth's surface and
the objects on it. May also be called the boundary layer or the friction layer.
SWELL
Ocean waves that have traveled out of their generating area. Swell characteristically exhibits a more regular and longer period and has flatter wave crests than waves within their fetch.
SYNOPTIC CHART
Any map or chart that depicts meteorological or atmospheric conditions over a large area at any given time.
SYNOPTIC SCALE
The size of migratory high and low pressure systems in the lower troposphere that cover a horizontal area of several hundred miles or more. Contrast with macroscale, mesoscale, and storms.
SYZYGY
The points in the moon's orbit about the earth at which the moon is new or full.