Mike Godsey’s SF Bay Area Weather Glossary

Advection
A horizontal movement of any weather condition by the movement of air. For example along the West Coast summer fog occurs when warmer moist air moves over the cold upwelled waters near shore.

Advection Fog, marine layer clouds
Fog or clouds that develops when warm moist air moves over a colder surface. As that air cools to below its dew point water vapor condenses into tiny water droplets and is visible as fog.

Air mass
A large body of air with similar temperature and moisture levels. For example a marine air mass.

Aleutian low, Gulf of Alaska low
A semi-permanent area of low pressure located in the Gulf of Alaska. This low gives birth to storms and passing lows get stronger in this area. In winter and spring it sends storms towards the West Coast. During the summer this low weakens and retreats towards the North Pole. During this time the North Pacific High dominates the north pacific.

Anemometer, wind sensor
An instrument used to determine wind speed. Wind sensor

Anticyclone, North Pacific High, Pacific high, Bermuda high.
A large area descending air creating high pressure. The hich winds blow outward from this high and are turned clockwise in the Northern Hemisphere. On the west coast the North Pacific High is commonly found over the offshore waters of the eastern pacific.

Backing Wind
Wind that changes its direction in a counter clockwise fashion. For example a Northwest wind changing to a West wind. See veering wind.

Barometric Pressure (see pressure gradient)
The pressure exerted by the atmosphere over a given point. This pressure is expressed in millibars (mb) or in inches of mercury (Hg). The greater the barometric pressure between two points the stronger the wind. See isobars.

Blocking High
A high pressure area, often over the south central USA, that stays in place for a prolonged period. This acts to stop the normal progression of weather systems from west to east for long periods of time. This tends to maintain the current wind pattern

Boundary Layer
The layer of air near the earth’s surface within which the effects of friction are significant. It is roughly the lowest one or two kilometers of the atmosphere. Within this layer the speed and direction of the wind is affected by friction with the earth’s surface

Buoys/Weather buoys
Buoys placed in the near shore coastal waters to transmit information weather information. The wind at the buoys may be an indicator of the wind at the beach later in the day. Along the West Coast the buoys are 9-13 miles from shore. They are useful for looking at hourly wind strength, directions, pressure, swell Ht. and swell direction. In certain conditions, especially in the spring, they are a good indicator of the winds to be expected later in the day at the coast. In other conditions especially in the summer it can blow at the buoys and be light at the beach. Generally the buoys have stronger winds than at shore but when the upper level winds are NW and very strong the beach will have stronger winds.

Catalina Eddy
This eddy like wind pattern is common in Southern California in the summer. Wind sweeping from the NW past Pt. Conception curves over the Southern California bight and creates SW to ESE winds along the shore. This is most likely to happen the the inland valleys are warm. This eddy tends to deepen the marine layer and fog. A strong Catalina Eddy will keep the coast foggy and windless most of the day.

Ceiling
The bottom of the lowest cloud layer above the surface.

Cirrus
High clouds made of ice crystals usually around 18,000 feet. The upper level wind direction and speed can very roughly judged by these clouds.

Climate
The long term record of average daily and seasonal weather events

Closed Low
See Cutoff Low

Cloud Train
Commonly seen near the Hawaiian islands as long line of puffy cumulus clouds carried along by the NE trade winds from the North Pacific High.

Cold Front
The leading edge of an advancing cold air mass associated with a surface low-pressure area. As a cold front passes the pressure rises and the wind clocks from southwest to northwest. Rain or snow is usually along or ahead of the front.

Combined Seas
The combined height of swell (generated by strong winds hundreds or thousands away) and wind waves (generated by local daily winds)

Convection
Upward motion of air caused by surface heating. If the air is moist this process may cause rain or thunderstorms. Convection is often associated with gusty up and down winds.

Convergence
Horizontal movement of air into a particular region. Convergence at lower levels is associated with upward motion.

Coriolis Force
A force produced by the earth’s rotation. This force acts to change the wind direction. The strength of the coriolis force depends on the latitude and speed of the wind. In the Northern Hemisphere the air is deflected to the right. For example the North Pacific High winds rotate outward from the high making a turn to the right. This produces NW winds along the California coast and NE trade winds in Hawaii.

Cumulonimbus Cloud
A large vertically developed cloud often capped by an anvil shaped cloud producing tornadoes, hail, lightning, strong winds and heavy rain. These are an important wind factor in some parts of the USA.

Cumulus Clouds
A cloud in the shape of individual detached domes, with a flat base and a bulging upper portion resembling cauliflower.

Cutoff Low
An area of low pressure cut off from its associated jet stream. These low will wobble about in a region in an unpredictable fashion making forecasting very difficult. In the winter along the pacific coast they can cause prolonged rain and long periods of strong southerly winds. In the summer they can cause deep fog and long periods of fog and wind in the S. F. Bay Area

Cyclonic Winds
A large area of closed pressure circulation with rotating and converging winds. Due to coriolis force the circulation around the low is counterclockwise in the Northern Hemisphere.

Depression
See Low Pressure System

Dewpoint
The temperature to which the air must be cooled for water vapor to condense.

Diablo Wind
Found in the S.F. Bay Area these winds are similar to Santa Ana winds in southern California. Powerful Diablo winds occur below canyons in the East Bay hills near Mt. Daiblo when a high pressure develops over Nevada and lower pressure along the central California coast.

Divergence
A horizontal outflow of air away from a particular region. Divergence at lower levels is associated with a downward movement of air from aloft.

Doppler Radar
Specialized radar that measures the direction and speed of rain and wind.

Ebb Current
Movement of a tidal current away from shore or down a tidal. When the wind opposes this current chop and swell will become steeper.

ECMF model
European Centre for Meteorology Forecast model.

El Nino
A cyclical warming of East Pacific Ocean seawater temperatures off South America. During an El Niño event warm equatorial Pacific waters displaces the colder waters along the South America coast and slowing the normal upwelling of cold water. Strong El Niños can result in significant changes in weather patterns in the United States and elsewhere. El Niño increases the number of storms and rain over California. For windsurfers this means more winter sailing and warm waters. But in the summer it means a warmer ocean poor thermals especially at Rio and the North Bay

El Norte
A strong N to NE winds that roars down the Sea of Cortez in the winter when a high settles into the 4 corners region of the USA.

ETA model
A forecast model generated every 12 hours by NMC.

Fetch
The area and distance over which ocean ground swell is generated by the wind. The greater the fetch the longer the distance or period from crest to crest and the size of the swell.

Foehn
A warm dry wind blowing down the lee side of a mountain range. Air is compressed during its drop so its temperature increases. This is a type of katabatic wind.

Fog
An area where vast numbers of visible tiny water droplets suspended in the air and limit visibility. Fog occurs when the temperature and the dew point of the air have become nearly the same. This often occurs on the west coast when warm moist air from the south is carried over colder ocean water.

Front
The transition zone between two air masses of different densities and different temperatures. The basic frontal types are cold fronts, warm fronts and occluded fronts. Cold fronts are often associated with rain and wind.

Gale
Wind speeds from 39 to 54 mph (34 to 47 knots).

Geostationary Satellite
A satellite that rotates at the same rate as the earth so it remains over the same spot above the equator.

Geostrophic Wind
The theoretical horizontal motion of air along parallel isobars in a pressure or contour field. It is assumed that there is no friction, that the flow is straight with no curvature and there is no divergence or convergence with no vertical acceleration.

GOES Satellite
A geostationary satellite weather satellite that holds the same position over the equator. GOES stands for Geostationary Operational Environmental Satellite. These provide critical weather imagery.
GOES West Visible Imagery Loop

Ground Swell
This is swell generated by distant storms. Along the west coast ground swell comes from the S. hemisphere or the Gulf of Alaska. The time it takes for each swell crest to pass a point is over 15 seconds so we say it has a long period. The longer the period the bigger the surf when the swell encounters the bottom in the surf zone. Ground swell has cleaner and well-defined sets.

Gulf Stream
A swift, relatively narrow ocean current off the east coast of the United States.

Gust
A sudden major but temporary increase in wind speed. Technically a gust has a duration of less than 20 seconds and an increased speed greater than 10 mph. The iWindsurf.com sensor records the peak gust not the avenge of the gusts.

Gust Front
The leading edge of the gusty surface winds produced by thunderstorm downdrafts.

Haze
A suspension of fine particles in the air that reduce visibility and give the air a milky appearance.

Heat Wave
A period of abnormally hots weather lasting from several days to several weeks. On the west coast a marine surge and SW winds may occur at the end of a heat wave.

High Clouds
Cirrus type clouds composed of ice crystals and generally above 20,000 feet. The main types of high clouds are cirrus, cirrocumulus, and cirrostratus.

High Pressure System
An area of relative pressure with diverging winds that are turned by coriolis force At the surface the North Pacific High plays a major role in the spring NW clearing winds and the Gorge and Bay Area summer winds.

High Wind
Sustained winds greater than or equal to 40 mph.

Humidity
The amount of water vapor in the air.

Hurricane
A tropical cyclone or low with sustained winds of 65 knots or greater. Known as a typhoon in the western Pacific.

Insolation
The heat of the sun received at the earth’s surface. Insolation comes from INcoming SOLar radiATION.

Instability
Conditions when spontaneous convection can occur and air parcels will accelerate upward. Associated with gusts.

Inter-Tropical Convergence Zone
The equatorial region where the northeasterly and southeasterly trade winds converge forming a world-circling band of clouds and thunderstorms. These massive thunderstorms pump air into the upper atmosphere. This air returns to the surface thousands of mile away in the temperate zones creates permanent highs like the North Pacific High. The location of the ITCZ changes north and south with the seasons. Likewise the highs average location moves.

Inversion
A temperature increase with altitude which is opposite of the usual decrease in temperature with height. Often found capping a marine layer fog. A tight inversion is associated with good thermal winds if there is a strong pressure gradient.

Isobar
Lines on a surface weather map connecting points of equal barometric pressure. Useful in locating areas of low and high pressure. The tighter the isobars are together the stronger the wind.

Jet stream
Strong upper level winds concentrated in a narrow band flowing around the world from west to east. The jet stream forms as cold polar air moving towards the equator meets the warmer equatorial air moving northward toward the poles. The jet stream often “steers” surface features such as front and low-pressure systems. In some conditions these strong winds can transfer momentum to the surface producing very strong winds.

Jetstreak
A zone of maximum wind speed along the route of a jet stream.

Katabatic Wind
A wind created by air flowing downhill. When the air is warm, it may be called a foehn wind or Chinook, or Santa Ana. When this air is cool, it is called a drainage wind, mountain breeze or glacier wind.

Knot
A nautical unit of wind speed used by sailors and meteorologists. 1 knot = 1.151 statute miles per hour. At low wind speeds below 20 knots the difference between knots and mph is not significant to sailors. At higher speeds the difference becomes more noticeable.

La Niña
La Niña in many ways is the opposite of El Niño. Easterly trade winds increase in intensity cooling water in the central and eastern equatorial Pacific Ocean since there is more upwelling along the western shores of South America. In La there is a different change in large-scale pressure patterns. The colder water along our coast mean better thermal winds in the North Bay and Rio.

Land Breeze
A daily coastal breeze that blows from the land to the sea because the sea is warmer than the adjacent land. Most common during the night and peaks around dawn.

Lapse Rate
The change of an atmospheric variable such as humidity, wind or temperature with increasing height. A steep lapse rate implies a rapid decrease in temperature with height.

Lee
The side of an object furthest away from the wind.

Low Clouds
Low clouds with a base below 6500 feet. Types include stratus, stratocumulus, cumulus and cumulonimbus.

Low Level Jet
Strong winds that are concentrated in relatively narrow bands in the lower part of the atmosphere. These are hard to forecast but can cause strong surface winds to develop fast.

Low Pressure System
A large area of closed pressure circulation with rotating and converging winds. The circulation around the low is counterclockwise in the Northern Hemisphere.

Macroscale
The meteorological scale covering an area ranging from the size of a continent to the entire globe.

Mare’s Tail
Thin, wispy cirrus clouds composed of ice crystals that appear as veil patches of strands, often resembling a horse’s tail.

Marine Layer
Along the west coast this is a layer of moist marine air close to the surface and capped by warmer air that creates an inversion. Fog is often but not always associated with marine layer. The depth of the marine layer can range from several hundred feet to over 3000 feet. This depth is determined by many variables including: subsidence from a high, reduction of the 925mb relative humidity above the marine layer, a trough offshore or in Southern California a Catalina Eddy. Some time strong NW wind will simply erode the marine layer. The deeper the marine layer the greater its intrusion inland. Since this air is cool and dense it acts as a local surface high. If there is a pressure gradient from this high to an inland point there will be wind close to the leading edge of the marine layer. So the deeper the marine layer the more inland the wind will penetrate.

Marine Surge
In the Bay Area this is a rapidly developing S to SW to WSW to flow of fog laden marine air coming up the coast. This will often weakens the Peninsula wind. Depending on its directions it can make for great winds at Sherman Island and San Luis in the AM or PM and at Larkspur or Berkeley or Pt. Isabel. Often the fog will be too deep at Crissy. A marine surge can be triggered by a strong SFO to SBA south to north gradient and/or an upper level trough offshore and/or a summer time Cut-Off low to the north.

Maritime Air Mass
Moist air mass originating over the ocean.

Mean Sea Level
The average height of the sea surface water level.

Mesoscale
A scale ranges in size from several kilometers to around 100 kilometers. Smaller phenomena are classified as microscale while larger are classified as synoptic-scale. iwindsurf.com forecasters forecast at a microscale level but look at Mesoscale and synoptic scale variables to make those forecasts.

Meteorology
The science of atmospheric phenomenon.

Microburst
A severe but brief localized wind blasting down from a thunderstorm.

Microscale
A scale of weather phenomena that range in size from a few centimeters to a few kilometers.

Middle Clouds
Clouds with bases between 6,000 and 18,000 feet.

Millibar (MB)
The standard unit of measurement for atmospheric pressure used by the National Weather Service.

Mixing out
The break up of the marine layer and any fog associated with it. This is caused by a low or a trough coming overhead as cold air advection occurs. It also happens when the marine layer becomes much thicker than 3000 feet. Mixing out will often weaken the surface wind.

Models
Computer numerical models that subject huge amounts of data input from around the world to equations of hydrodynamics to extrapolate future weather conditions. The resolution of the models range from world wide to regional. The temporal range of the models goes from a few hours to over a week.

Monsoon
On the west coast the monsoon is associated primarily with the moisture that is carried by upper winds from Mexico to the southwestern United States or even as far as the Bay Area.

MRF
Medium Range Forecast model generated every 12 hours.

National Weather Service (NWS)
A branch of the National Oceanic and Atmospheric Administration responsible for observing and forecasting atmospheric conditions.

NEXRAD
NEXt Generation RADar. A NWS network of about 140 Doppler radars being installed nationwide. Useful for tracking rain and the wind associated with rain.

NGM
Nested Grid Model generated every 12 hours.

NOAA
National Oceanic and Atmospheric Administration. A branch of the US Department of Commerce, NOAA is the parent organization of the National Weather Service

Nor’easter
A cyclonic winter storm off the east coast of the USA. Wind gusts and waves from these storms can exceed hurricane force in intensity. A nor’easter has strong northeasterly winds blowing in from the ocean ahead of the storm.

North Pacific High
A huge dome of high pressure that resides in the pacific offshore of the west coast. It plays a critical role in the spring and early summer NW clearing winds after a storm passage. In the summer it plays a role in the thermal winds in the Gorge and the Bay Area. In Southern California in plays a role in the dreaded Catalina Eddy.

Occluded Front
A complex frontal system that occurs when a cold front overtakes a warm front. Also known as an occlusion.

Offshore breeze
A wind that blows from the land towards a body of water. Also known as a land breeze.

Offshore forecast
A marine weather forecast for the waters between 60 and 250 miles off the coast. Sometimes this can give a hint of the wind to expect in the future along the coast.

Omega Block
See Blocking High

Onshore wind
A wind that blows from a body of water towards the land. Also known as a sea breeze.

Orographic Uplift
The vertical movement of air up terrain features such a hills or mountains. This can create orographic clouds and/or precipitation.

Overcast
Sky condition when greater than 9/10 of the sky is covered.

Partly Cloudy
Clouds are present but do not completely cover the sky.

Period
The time it would take for two adjacent crests of ocean swell to pass a fixed point. On the west coast a short period in around 10 seconds a long period is around 17 seconds or longer. The longer the period: the further the swell has traveled, the cleaner the sets, and bigger the resulting surf at beaches facing the swell direction.

POPs
Probability of Precipitation

Precipitation
Any and all forms of water, liquid or solid, that falls from clouds and reaches the ground. This includes, drizzle, freezing drizzle, freezing rain, hail, ice crystals, ice pellets, rain, snow, snow pellets, and snow grains.

Pressure
The force exerted by the interaction of the atmosphere and gravity. Also known as atmospheric pressure.

Pressure Gradient
The amount of pressure change that occurs over a fixed distance at a fixed altitude. The stronger the pressure gradient the stronger the wind. The area with the isobars most tightly spaced on a weather map has the strongest pressure gradient.

Prevailing Wind
A wind that blows from one direction more frequently than any other during a given period, such as a day, month, season, or year.

Profiler
A type of doppler radar that typically measures both wind speed and direction from the surface to 55,000 feet in the atmosphere. Used to monitor winds with height in determining severe weather threat.

Quasi-stationary Front
A front which is nearly stationary. Also known as a stationary front.

Radiation Fog
Fog that is created when radiational cooling at the earth’s surface lowers the temperature of the air near the ground to or below its dew point. Formation is best when there is a shallow surface layer of relatively moist air beneath a drier layer, clear skies, and light surface winds. This primarily occurs during the night or early morning.

RAOB
An instrument attached to a weather balloon used to measure pressure, temperature, humidity, and winds aloft. Observations are made when the radiosonde is aloft and emits radio signals as it ascends.

Relative Humidity
Usually expressed in percentage. The amount of water vapor in the air compared to the amount the air could hold if it was totally saturated.

Retrogression
The movement of a high or a low opposite the normal west to east progression of weather systems.

Ridge
An elongated area of high-pressure in the atmosphere. The location of the ridge from the North Pacific High is critical in producing the west coast NW winds and the thermal winds of the Gorge.

RUC
Rapid Update Cycle model generated every 3 hours.

Santa Ana Winds
The hot, dry easterly winds that funnel through the Santa Ana river valley south of the San Gabriel and San Bernadino Mountains in southern California most commonly during the winter but can happen during other seasons. These winds are caused by an anticyclone/high located over the high deserts of California or Nevada. The warmth and dryness are due to compression heating. See also Diablo winds.

Saturation Point
The point when the water vapor n the atmosphere is at its maximum level for the existing temperature.

Scattering
The process by which small particles suspended in the air diffuse a portion of the incident radiation in all directions. This is a primary reason for colors, such as blue skies, rainbows, and orange sunsets.

Scud
Low fragments of clouds, usually stratus fractus, that are unattached and below a layer of higher clouds, either nimbostratus or cumulonimbus. They are often along and behind cold fronts and gust fronts, being associated with cool moist air, such as an outflow from a thunderstorm. When observed from a distance, they are sometimes mistaken for tornadoes.

Sea Breeze
A diurnal coastal breeze that blows onshore, from the sea to the land. It is caused by the temperature difference when the surface of the land is warmer than the adjacent body of water. Predominate during the day, it reaches its maximum early to mid afternoon. It blows in the opposite direction of a land breeze.

Sea Breeze Front
A coastal phenomena, it is restricted to large bodies of water and their immediate coastlines. This is usually the landward extent of the sea breeze. Due to the imbalance of heating between land and water, a region of maximum upward motion or convergence occurs by mid-afternoon in the summer some 10 to 15 miles inland.

Sea Fog
A type of advection fog which forms in warm moist air-cooled to saturation as the air moves across cold water. Related Arctic Sea Smoke

Sea Level
The height or level of the sea surface at any time. It is used as a reference for elevations above and below. Related mean sea level

Sea Level Pressure
The atmospheric pressure at mean sea level usually determined from the observed station pressure.

Sensor
See Anemometer

Shear
It is the rate of change over a short duration. In wind shear, it can refer to the frequent change in wind speed within a short distance. It can occur vertically or horizontally. Directional shear is a frequent change in direction within a short distance, which can also occur vertically or horizontally. When used in reference to Doppler radar, it describes the change in radial velocity over short distances horizontally.

Short Wave
An upper level wave of smaller amplitude, wavelength, and duration than a long wave trough or ridge it is embedded in. Short waves move in the same direction as the larger wave or trough. Short waves may cause upward vertical motion ahead of it. These disturbances often responsible for triggering surface cyclone development and winds at the surface. In summer in the Bay Area and Southern California they can cause a marine surge and deepen the marine layer. If they are strong enough they max mix out the marine layer.

Skew T
A thermodynamic diagram, using the temperature and the logarithm of pressure as coordinates. It is used to evaluate and forecast air parcel properties. The Skew T has may uses in weather forecasting but in the summer on the west coast it is especially useful for forecasting the marine layer dept.

Small Craft Advisory
An advisory issued warning that winds over 20 knots may cause hazardous conditions for operators of small vessels

Squall
A sudden onset of strong winds associated with dark clouds and often rain. The intensity and duration is longer than that of a gust. In Hawaii there will be increasing winds and strong gusts as a squall nears then fading winds after it passes. It may take 5 or more minutes for the normal wind to build back after the passage of a squall.

Squall Line
A narrow band of line of active thunderstorms or squalls that is not associated with a cold front.

Stable or Unstable Air
When the surface air is heavier than the air aloft it is stable air and will follow the contours of the land and is less gusty. This effect is especially pronounced when there is an inversion with a layer of warm air above cooler surface air. When surface air is about the same temperature as the air aloft or it is warmed by the land it becomes unstable and lifts easily from the surface. This makes gusty or ON and Off conditions.

Stratiform
Clouds composed of water droplets that exhibit 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.

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.

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.

Subsidence
A sinking or downward motion of air, often seen in anticyclones. It is most prevalent 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 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.

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.

Surface Winds 850mb level
Winds aloft near the surface. These are useful in forecasting the temperature later in the day and they can sometimes directly influence the surface wind.

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 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.

Thermal High
A thermal high is a local surface level stationary high that does not move from W to E with the jet stream. Along the west coast it is formed by the cold ocean water that rises from the ocean depths during the strong NW winds of spring. The strong NW spring winds blow the warmer surface water to the south. Cold water rises from bottom making for a mass of cold water of the Golden Gate. This process is called upwelling and is responsible for the chilly waters off much of the California coastline. By summer these cold waters cool the ocean air producing a local area of high pressure. As warm moist ocean air streams over this cold water it reaches the dew point and fog droplets form. If there is a pressure gradient from West to East thermals wind will develop.

Thermal Low or Trough
This is an area of low pressure due to the high temperatures at the land surface. Thermal lows tend to remain stationary over its source area. These stationary lows in the California Central Valley and in Eastern Oregon and Washington play a critical role in the strong winds common in the Gorge and Bay Area. These low may show weak cyclonic or counter clockwise circulation.

Thermal or Thermal wind
A rising mass of air due to surface heating. On a local scale thermals can create afternoon sea breezes when inland air near a body of water rises and marine air rushes inland to fill the lower pressure left by the rising air. On a large scale thermals in huge inland valley like the California Central Valley or the deserts to the east of the Gorge can cause powerful winds as cold ocean air is sucked inland towards the valley.

Thunderstorm
Produced by a cumulonimbus cloud it is of short duration and may produce by thunder, lightning, gusty surface winds, hail, icing, precipitation, strong up and downdrafts.

Tide
The periodic rising and falling of the earth’s oceans. It is the result of the tide-producing forces of the moon and the sun acting on the rotating earth.

Trade Winds
Two belts of prevailing winds that blow easterly from the subtropical high pressure centers towards the equatorial trough. Primarily lower level winds, they are characterized by their great consistency of direction. In the Northern Hemisphere, the trades blow from the northeast, and in the Southern Hemisphere, the trades blow from the southeast.

Transfer of momentum
As the sun heats up the land in the morning rising unstable surface air can cause strong upper level winds to drop towards the surface transferring energy to the surface winds usually as gusts of wind.

Tropical Air Mass
An air mass that forms in the tropics or subtropics over the low latitudes. Maritime tropical air is produced over oceans and is warm and humid, while continental tropical air is formed over arid regions and is very hot and dry.

Tropical Cyclone
A warm core low pressure system which develops over tropical, and sometimes subtropical, waters, and has an organized circulation. Depending on sustained surface winds, the system is classified as a tropical disturbance, a tropical depression, a tropical storm, or a hurricane or typhoon.

Tropical Depression
Tropical mass of thunderstorms with a cyclonic wind circulation and winds between 20 and 34 knots.

Tropical Disturbance
An organized mass of tropical thunderstorms, with a slight cyclonic circulation, and winds less than 20 knots.

Tropical Storm
An organized cyclone in the tropics with wind speed between 35 and 64 knots.

Tropical Wave
Another name for an easterly wave, it is an area of relatively low pressure moving westward through the trade wind. Generally, it is associated with extensive cloudiness and showers, and may be associated with possible tropical cyclone development.

Tropics/Tropical
The region of the earth located between the Tropic of Cancer, at 23.5 degrees North latitude, and the Tropic of Capricorn, at 23.5 degrees South latitude. It encompasses the equatorial region, an area of high temperatures and considerable precipitation during part of the year.

Trough
An elongated area of low atmospheric pressure that is associated with an area of minimum cyclonic circulation. Surface troughs may produce wind at the surface or may help form a pressure gradient to nearby high pressure areas. Upper troughs may have winds that influence surface conditions.

Turbulence
Disrupted flow in the atmosphere that produces gusts and eddies. caused by random fluctuations in the wind flow. It can be caused by thermal or convective currents, differences in terrain and wind speed or variation in temperature and pressure.

Unstable/Instability
Occurs when a rising air parcel becomes less dense than the surrounding air. Since its temperature will not cool as rapidly as the surrounding environment, it will continue to rise on its own. Often associated with very gusty winds.

Updraft
A small scale current of air with vertical motion. If there is enough moisture, then it may condense, forming a cumulus cloud, the first step towards thunderstorm development.

Upper Air/Upper Level
The portion of the atmosphere which is above the 850 millibars level around 5000 ft. Upper lows, highs, troughs, ridges and wind all occur above the 850mb level. These upper level phenomenon can sometimes have a marked effect on the surface especially on the west cost.

Upper Level Wind: 500mb
Winds aloft at around 18,000 feet. These winds help steer passing lows and highs. In certain situations they can transfer energy to our surface winds. They also can influence the flow of the marine layer along the coast in the summer.

Upslope Effect
The cooling of an air flow as it ascends a hill or mountain slope. If there is enough moisture and the air is stable precipitation may form. If the air is unstable, there might be an increased chance of thunderstorm development.

Upwelling
Along the California coast this process is critical in producing the summer thermal winds. This process in caused by the strong NW winds that blow along the shore and at the buoys during the spring and summer. As this wind blows the warmer surface water southward cold waters from the ocean depths rises to the surface. This creates a body of cold surface water which in turn produces cool marine air and a local high-pressure area. When conditions are right the thermal low in the Central Valley will make a pressure gradient causing this marine air to funnel through gaps in the coast range.

Vapor Trail or Contrail
A cloud-like trail seen behind aircraft flying in clear, cold, humid air. Can sometime give you an off the cuff idea of what the direction and speed of upper level winds.

Veering Wind
A wind that changes its direction in a clockwise motion. An example would be a west wind changing to a northwest wind.

Vertical Temperature Profile
A series of temperature measurements taken at various levels in the atmosphere. All major airports collect this data via balloons. The same data may be extrapolated from computer models. This real or modeled data is graphed on a skew t diagram.

Vertical Wind Profile
A series of wind direction and wind speed measurements taken at various levels in the atmosphere that show the wind structure of the atmosphere over a specific location. Obtained through a rawinsonde sounding or comparable method, and exhibited in a skew t-log p diagram.

Virga
Streaks or wisps of water or ice particles that fall from clouds but evaporate before reaching the ground.

Vortex
Any circular or rotary flow in the atmosphere that possesses vorticity.

Vorticity
The measurement of the rotation of a body of air in the atmosphere. In forecasting it is the rotational motion about an axis that is perpendicular to the earth’s surface that is most important. The greater the rotation the greater the vorticity. In the Northern Hemisphere, the vorticity is positive when the parcel of air has a counterclockwise rotation. It is negative when the parcel has clockwise.

Vorticity Max
The area of maximum vorticity in a moving body of air. These can sometimes have a dramatic effect on surface wind.

Warm Advection
The horizontal movement of warmer air into a location.

Warm Front
On a weather map a warm front is depicted as a line with periodic red half circles. This type of front is the leading edge of an advancing warm air mass that is replacing a retreating relatively colder air mass. Following the passage of a warm front the temperature and humidity increase, the pressure rises and the wind shifts slightly from southwest to northwest. Warm fronts are common in most of the USA but rare on the west coast.

Waterspout
A small, weak tornado like wind that occurs over water and sucks water into its column. They may be observed beneath cumulonimbus or towering cumulus clouds. Small waterspouts are sometimes seen off Coyote Pt. in the S. F. Bay Area following to the passage of large airliners.

Weather Radio
Continuous, 24 hour a day VHF broadcasts of weather observations and forecasts directly from regional National Weather Service office. In marine regions these forecasts are aimed primarily at boaters and are often two vague for windsurfers.

Westerlies
Broad patterns of persistent winds with a westerly component covering huge geographic areas. These winds are centered over the mid latitudes of each hemisphere.

Wind
Air that flows in relation to the earth’s surface. There are four aspects of wind: direction, speed, consistency (gusts and lulls) and shifts. Surface winds are measured anemometers. Upper level winds are detected through balloons, satellites or aircraft reports.

Wind Advisory
Sustained winds 25 to 39 mph. By tradition these advisories are not issue in areas where such winds are common (e.g. in summertime in the San Francisco Bay or the Gorge).

Wind Chill Index
The calculation of temperature that considers the effects of wind and temperature on the human body.

Wind gauge or sensor
See Anemometer

Wind Shear
The rate of wind speed or direction change with distance. Vertical wind shear is the rate of change of the wind with respect to altitude. Horizontal wind shear is the rate of change on a horizontal plane.

Wind Vane
An instrument that determines the direction from which a wind is blowing.

Windward
The direction from which the wind is blowing. Also the upwind side of an island or point. The opposite of the downwind or leeward side.

Zonal Flow
The flow of air along a line of latitude, normally from west to east. Usually indicates the absence of troughs or ridges.

Zulu Time, GMT, UT
One of several names for the twenty-four hour time which is used throughout the scientific communities.