Terms & Concepts

Here are just some of the terms and concepts I mention in my blog:



Meteorologists typically do not use local time (ET, CT, MT, or PT), unless they are issuing out watches or warnings of severe weather to the public, and/or if it’s mainstream media.  Instead, we keep track of time using Zulu time (Z).  Zulu time is the exact same as Coordinated Universal Time (UTC) AND Greenwich Mean Time(GMT).  The reason that we use this time is because there is less confusion dealing with time around the United States, as well as around the world.  A universal time is only ONE time, used everywhere around the world.  Like local time, Zulu time also changes in coordination with standard time/daylight saving.  UTC runs on a 24 hour time-run, just like military time (00Z – 23Z).  Here is a conversion of local time vs. Zulu time for the 4 main timezones in the U.S.:

From UTC to Local Time(Standard):
Eastern Standard Time (EST)                                               UTC – 5 hours = EST
Central Standard Time (CST)                                                UTC – 6 hours = CST
Mountain Standard Time (MST)                                           UTC – 7 hours = MST
Pacific Standard Time (PST)                                                  UTC – 8 hours = PST

From Local Time(Standard) to UTC:
Eastern Standard Time (EST)                                               EST + 5 hours = UTC
Central Standard Time (CST)                                                CST + 6 hours = UTC
Mountain Standard Time (MST)                                           MST + 7 hours = UTC
Pacific Standard Time (PST)                                                  PST + 8 hours = UTC

From UTC to Local Time(Daylight):
Eastern Daylight Time (EDT)                                               UTC – 4 hours = EDT
Central Daylight Time (CDT)                                                UTC – 5 hours = CDT
Mountain Daylight Time (MDT)                                           UTC – 6 hours = MDT
Pacific Daylight Time (PDT)                                                  UTC – 7 hours = PDT

From Local Time(Daylight) to UTC:
Eastern Daylight Time (EDT)                                               EDT + 4 hours = UTC
Central Daylight Time (CDT)                                                CDT + 5 hours = UTC
Mountain Daylight Time (MDT)                                          MDT + 6 hours = UTC
Pacific Daylight Time (PDT)                                                  PDT + 7 hours = UTC



Meteorologists measure and look at levels of the atmosphere in terms of pressure (in millibars = hectopascals), instead of height.  Pressure decreases as height increases.  Below are approximate height levels corresponding to their pressure level .  I will start with the surface and make my way up in the atmosphere.  Depending on altitude at any point in the world, pressure vs. height will be different, therefore, the heights are only APPROXIMATE of each pressure…

Surface:  Ground level

1000 millibars(mb):  ~364 feet above ground level (AGL)

925 mb:  ~2,500 feet AGL

850 mb:  ~4,781 feet AGL

700 mb:  ~9,882 feet AGL

500 mb:  ~18,289 feet AGL

300 mb:  ~30,000 feet AGL

250 mb:  ~34,000 feet AGL

200 mb:  ~38,662 feet AGL



Synoptic:  Large-scale, on the order of  a few thousand kilometers. (e.g. North America)

Mesoscale:  Medium-scale, on the order of a few hundred kilometers. (e.g. small state)

Microscale:  Small-scale, on the order of a few kilometers or less. (e.g. size of a city or smaller)

Gradient:  A change in the value of a quantity (temperature, pressure, vorticity, etc)

Advection:  A horizontal (north, south, east, west) movement of a mass substance, usually transported by the wind. [Example: temperature advection & vorticity advection]

Air Temperature:  Often referred to as temperature.  It is a quantity measured by a thermometer.  Temperature represents the molecular kinetic energy.

Dewpoint Temperature: Also referred to as, simply, dewpoint.  Dewpoint temperature is the temperature at which the air needs to be cooled in order for saturation to occur.  The higher the dewpoint, the “stickier” it feels outside.  Thunderstorms feed off of high dewpoint temperatures.

Relative Humidity:    Relative humidity is a ratio, often expressed as a percent.  It is a function of moisture in the air and temperature.  Its definition is:  the amount of atmospheric moisture present relative to the amount that would be present if the air were saturated.  This sounds a bit confusing but, saturation is simply when the dewpoint temperature is reached.

Isobars:  On surface maps only! These are connecting points which form lines of constant (equal) pressure at the surface.

Isoheights:  These are lines of constant height found on 250mb, 300mb, 500mb, 700mb, 850mb and 950mb maps that I post.

Isotherms:  Lines of constant temperature.

Isodrosotherms:  Lines of constant dewpoint temperature.

Isotachs:  Lines of equal wind speeds.

Cyclone:  Large-scale, circular fluid motion that rotates counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.  Low atmospheric pressure is usually at the center of cyclonic circulations.

Anticyclone:  Large-scale, circular fluid motion that rotates clockwise in the Northern Hemisphere and counter-clockwise in the Southern Hemisphere, centered around high atmospheric pressure.


Surface Low Pressure:  At the surface, pressure is measured because elevation is different around the world.  Pressure is a result of temperature differences (temperature gradient).  In the Northern Hemisphere, wind flow around a low pressure system is counter-clockwise.  Typically, the southeast quadrant of a low advects warm air, or warm air advection(WAA), since wind is blowing from south to north.  The northwest quadrant of a low advects cold air, or cold air advection (CAA), since the wind flow is from north to south.  The flow around a low pressure system creates a temperature gradient and thus, frontal boundaries.  They are: 

Cold Front:  A cold front is the leading edge of a temperature drop-off.  It is usually associated on the west side surface low pressure.  On a weather map, a cold front is indicated by a blue line with triangular pips.  The points of the pips indicate the direction of motion of the front.  A cold in the Northern Hemisphere usually blows from northwest to southeast and brings a change in wind and a drop in temperatures.

Warm Front:  A warm front is the leading edge of warm temperatures. It is usually associated on the east side of a surface low pressure and advects warmer air from the south towards the north.  On a weather map, it is indicated by a red line with half-circle pips.  The pips point in the direction the warm front is moving.

Occluded Front:  Forms when a cold front overtakes a warm front.  It is viewed as a purple line with two different types of pips on a surface weather map.

Stationary Front:  A frontal boundary that is not classified as a cold front nor a warm front because it is not moving (stationary).  Once it begins to move, it then becomes a cold front, warm front, or occluded front.

Low pressure diagram. Source: http://www.trewern.org

Low Pressure system diagram 2 with occluded front. Source: http://www.trewern.org


Trough vs. Ridge

The primary characteristic of  a trough is that a trough is a region of lower heights.  Height is a function of the average temperature of air below that height surface.  For example, if you are looking at 500 mb heights (on a 500 mb map), then you are looking at 500 mb heights based on the average temperature from the surface to 500 mb.  You may or may not know that air density changes with temperature.  As the air temperature cools, it becomes heavier, compacted, and more dense, and thus takes up less volume.  Therefore, as air cools, it becomes more dense and the height lowers.  This would be classified as a trough.  A ridge is the opposite of a trough.  As air sinks from above, it warms.  Warm air expands and is less dense than cool air, thus, heights are raised.  Ridges tend to bring warmer, drier weather.  Troughs and ridges are usually associated and more visible in the middle and upper levels of the atmosphere.  Below are examples of both ridges and troughs, description in caption:

Trough (left) and Ridge (right). These are what you would typically see on an upper-level weather map (850-250mb). Solid black lines are isoheights (lines of constant height). Black arrows indicated general wind flow around a trough and ridge.

Shortwave Trough, a.k.a Shortwave:  Embedded waves within the longwave trough/ridge pattern.  Shortwave troughs tend to be associated with a upper-level front or a cold pool aloft.  They also tend to move twice as fast as the longer-wave pattern.  In Laymen’s terms, shortwave troughs are associated with bad weather at the surface.  Below is an example at 500 mb of a shortwave embedded in a long wave from a 12Z map December 12, 2006.

12Z 500mb map from 060212. Two shortwave troughs are circled, embedded in a longwave system.


Wind Barbs

Wind Barbs:  Wind barbs display both wind speed and direction.  In meteorology, wind speeds are not measured in miles per hour (mph), they are measured in knots (kt).  On a weather map, they look like sticks with flags coming out of them.  Below are a few, simple descriptions of wind barbs.

Wind Speed & Direction: 

Description of what wind barbs are.  Source: rap.ucar.edu

 Wind Direction: 

Wind barb describing wind direction in the Northern Hemisphere.  Source: svgopen.org

Wind Speeds:

Wind barbs describing wind speeds. Half barbs are 5 knots, full barbs are 10 knots, flags are 50 knots. Source: nasailor.com



Jet Stream:  The jet stream is a current of air flowing in the middle and upper-levels of the atmosphere.  It is the wave-like pattern the encircles the Earth in the mid-latitude region.

Jet Streak:  A jet streak is a segment of the jet stream with higher velocity winds.  Jet streaks influence troughs and ridges by amplifying them.  Jet streaks can energize the trough and make an upper-low deepen.

Example of Jet Stream vs Jet Streak at 300mb.


Convergence & Divergence

Convergence:  Convergence is air streams flowing into one another around a single area OR stronger wind moving into weaker wind.  Convergence at the surface is forced to move up, which then lifts the air.  If there is enough moisture, this will create clouds and possibly precipitation.  Below are the two types of convergence

Types of Convergence

Divergence:  This is the opposite of convergence.  It occurs when air streams move away from one another OR when stronger wind moves away from weaker wind.  Upper-level (300mb-200mb) divergence is a result of rising air at or near the surface.  This means anytime you see strong divergence on a 300 mb map, either clouds or precipitation are occurring near the surface. Below are types of divergence.

Types of Divergence

The image below is a generalization of how convergence and divergence work in the atmosphere.  Again, it’s not as simple as the diagram depicts, however, the basic principles are nonetheless true.

Figure describing convergence vs. divergence in the upper atmosphere and lower atmosphere



Also known as soundings, skew-T log-P is an instantaneous, vertical snap-shot of the atmosphere from the surface to about 100 mb.  At least twice a day, meteorologist at various weather stations around the United States, and around the world, launch weather balloons, similar to image below.

Weather balloon in launch. The balloon is filled with either helium gas or hydrogen gas. Below the actual balloon is a parachute for after the burst of the balloon. The senors are at the very bottom of the string. PHOTO COURTESY OF: NOAA

Closer look at the box of sensors attached at the end of the string. PHOTO COURTESY:  NOAA

These balloons have sensors on them that measure temperature, dewpoint, and wind speed and direction.  From there, computers do many calculations and plot the data as a graph.  Soundings are great to predict mesoscale weather phenomena, such as severe weather.  It can determine the strength of a cap inversion.  More on cap inversion below. Finally, all the soundings taken around the U.S. are used to plot weather conditions for 00Z and 12Z, as well as produce the synoptic scale forecasts through computer models.  An example of a weather sounding is below.  The red line is the temperature plot, green line is the dewpoint temperature and the wind barbs are located at the right hand side of the graph, plotting wind speeds and direction.

Weather sounding from Dodge City, KS at 12Z 110519. Computer calculations are at the bottom. These calculations determine how much energy there is above the surface at the time of the weather balloon launch, mainly for severe storm potential.

Parcel sounding (dotted red line on graph above):  This is a theoretical plot based off of surface conditions and the LCL.  The parcel starts to rise, vertically, from the surface on a dry adiabat (parcel is dry). Then, once it reaches the LCL, it travels vertically on a moist adiabat (parcel is considered moist).

Capping Inversion:  This is also known as a temperature inversion which happens just above ground level in the atmosphere.  Normally, temperature cools as you go up in the atmosphere.  However, in a temperature inversion, temperature sharply goes up (gets warmer) for about 100-200 mb.  The sharper and warmer the layer is, the stronger the cap.  Since soundings are a vertical snap-shot, and clouds build vertically in thunderstorms, it is difficult for storms to develop with a cap inversion in place.  The storms need a lot of energy to “break” the cap in order for thunderstorms to get going.  Below, is an example from the NWS Brownsville of a capping inversion.

Circled in yellow is a cap inversion from Brownsville, TX 00Z 110519


CAPE:  acronym for Convective Available Potential Energy, measured in J/kg.  It is what it sounds like, available potential energy for thunderstorms.  The more CAPE, the more energy the atmosphere has for storm development. However, this does NOT mean that, if a lot of CAPE is present, strong storms have to and will develop.  This is a huge misconception.  The atmosphere needs much more than just energy to get storms going. CAPE can be calculated a few ways, but can be visible on a weather sounding.  It can be difficult to explain the total process on how to get the CAPE from a sounding, but all you need to know is where it is on the sounding.  The image below shows you an example of a sounding where CAPE is shaded.  Notice the CAPE is an area that is shaded between the temperature and the parcel plots.  The parcel plot is an imaginary, theoretical sounding if the atmosphere was completely moist once it reached the cloud base. If you want to know what you’re looking at or how/why this is CAPE, feel free to leave my a comment.

Example sounding showing CAPE (white shading). Parcel sounding (yellow line) has to be to the right of the temperature sounding (red line) in order to be considered CAPE.  Solid green lines are dry adiabats, dashed purple lines are moist adiabats, dashed blue lines are mixing ratios.

CIN:  Pronounced “sin”.  Another acronym which stands for Convective Inhibition.  If CAPE is the “positive area”, CIN would be the “negative area” (negative CAPE) of the sounding, also measured in J/kg.  This is the region where a parcel of air, if raised would sink back down.  This would be the region of capping, meaning it would be hard for air to break through this region for thunderstorm development.  Below is an example of CIN on a skew-T log-P diagram.

CIN is shaded in red. Blue line represents the parcel sounding. PHOTO COURTESY: geochief.org

LCL:  Lifted Condensation Level.  This is simply knowns as the level where the cloud base is.  The air parcel will be considered theoretically “moist” from this point up in the atmosphere.

LFC:  Level of Free Convection.  This is where CAPE values start.  This is also the level at which the air parcel and environmental temperature values are equal.  Once the air parcel reaches this level, it will begin to rise easily.

EL:  Equilibrium Level.  This is the level at which CAPE values end.  This is also the point at which a buoyantly rising parcel is equal to the environmental temperature (balloon).



Vorticity is plotted on a 500 mb weather chart, which indicates the clockwise and counterclockwise spin in the troposphere.  This spin is in relation to the z-axis (vertical axis).

Troughs and ridges, along with height centers cause vorticity.  Essentially, vorticity is the measured speed and directional wind shear from the surface to 500 mb.  Wind flow through a vorticity gradient will produce regions of Positive Vorticity Advection (PVA) and Negative Vorticity Advection (NVA).  PVA is associated with rising air (clouds, precipitation), while NVA is associated with sinking air (subsidence, stable air).

Example of vorticity plotted on a 500mb map. Positive vorticity areas are in red, Negative vorticity areas are in blue/purple. Troughs are associated with PVA and ridges are associated with NVA.


Computer Forecasting Models

Computer forecast models help aid meteorologists with making their forecasts.  However, none of the models are ever 100% correct, but close enough to help.  Therefore, meteorologists don’t just copy forecasts from a particular model(s).  A lot of the models can’t pick up on smaller and more defined areas, such as mesoscale and microscale phenomena due to the fact that their resolution is so broad.  Here are a few example of some computer forecasting models:

  • Global Forecast System (GFS):
    -Forecast out to 384 hours
    -Updated every 6 hours (00z, 06z, 12z, 18z)
    -27 km horizontal resolution up to 192 hours, then ~50 km to 384 hours
    -Decent vertical Boundary Layer resolution
  • North American Mesoscale Model (NAM) :
    -Forecasts out to 84 hours
    -Updated every 6 hours (00z, 06z, 12z, 18z)
    -12 km horizontal resolution
    -4 km high resolution NAM/WRF also available
  • Rapid Refresh Model (RAP) :
    -Forecasts out to 18 hours
    -Updated at every hour
    -20 km horizontal resolution
  • Canadian Model (CMC/GEM):
    -Forecasts out to 144 hours (standard), or 240 hours(extended)
    -Updated every 12 hours (00z, 12z)
    -Can vary in resolution, but standard is 80 km
  • European Model (ECMWF):
    -Forecasts out to 240 hours, monthly forecasts up to 32 days
    -Updated every 12 hours (00z, 12z)
    -16 km high resolution, 32 km standard horizontal resolution
  • United Kingdom Met. Office (UKMET):
    -Forecasts out to 72 hours
    -Updated every 12 hours (00z, 12z)
    -75 km horizontal resolution

…..and many, many more 
For more information about models, click here.

For more terms and definitions, click here.


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s