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The NE Georgia Weather Center
Meterological Terms

Weather terms, listed below, are used by the National Weather Service and the Northeast Georgia Weather Center.  Knowledge of the terms will aid in your understanding of maps, radar, weather bulletins and warnings especially on the Radar Severe Weather page.

Click on the below picture (Air Masses) for a video to aid you in understanding air masses that move through our area's mountains and how these air masses have a dramatic affect on our area's weather.  Find out how our mountains are capable of changing anticipated weather conditions.  In addition, there are terms used on NEGAWX.com describing air mass conditions that are over the mountains or on their way to dramatically change current calm weather to stormy conditions or vice versa. 

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Meteorlogical Terms: 
 

Hail = Ice Balls of Various Sizes and make up. The size of hail is always determined by the strength of the up draft and height of the parent cloud.  The up draft must be strong enough to support the weight of the ice and when the up draft can no longer support the weight of the ice, the ice falls to the ground and we call it hail. Another aspect that helps create a large hail stone is the up draft pushing upwards drops of rain up where it freezes and falls, caught by the up draft and push upwards again and again and this process continues until the updraft cannot support the weight of the ice.  If you see a stone the size of a golf ball or soft ball, you can be assured that winds loft are very strong and there will be severe weather on the ground.  Also, you should report hail the size of a quarter or larger to the National Weather Service, Peachtree City, Georgia.

MESO = Cloud Rotation. The rotation in the cloud is usually horizontal. As the rotation strengthens, the horizontal rotation begins to extend itself to a vertical rotation and depending on other factors, the super cell can drop the rotation as a tornado.  The doppler radar sees the rotation as winds moving in one direction and immediately there is wind moving in the opposite direction.  These two winds moving in opposite directions begins rotation.

TVS = Tornado Vortex Signature (Doppler Radar sees a Hooked Image). At this time, the Peachtree City, Georgia, National Weather Service or other NWS facilities, will issue a tornado warning based on factors they see on radar and ground reports.  NWS also requests the public to phone in a report of any sighting of a Vortex touching the ground or hail the size of a quarter or larger. 

CAPE = Convective Available Potential Energy. A measure of the amount of energy available for convection. CAPE is directly related to the maximum potential vertical speed within an updraft; thus, higher values indicate greater potential for severe weather.  Observed values in thunderstorm environments often may exceed 1000 joules per kilogram (J/kg), and in extreme cases may exceed 5000 J/kg.  Hence, when you read or hear a report of CAPE's of 1 there is a slight chance of severe thunderstorms.  With a report of 5 almost any weather event is possible.  The greater the number from 1 through 5, the more dangerous a storm could become.

LIFT  = Lift is necessary for the development of thunderstorms.  Lift can be created by several factors such as a cold front moving cooler air into a warm moist air mass.  Cooler air is heavier than the the warm moist air and the warmer air will ride (lift) over the cooler air causing a possible out break of severe weather.   Mountains may also provide lift.  If moisture is available to be warmed and to rise upward and over the face of the mountains (friction) and along with high temperatures a thunderstorm could be born.  Also, without mountains aiding (lift) in thunderstorm development, just very warm surface themperatures and moisture, without a strong high pressure cap, can also rise into the atmosphere and cause a lightning laden thunderstorm to develope.   With a high lift factor, noted below,  in place and with  warm air rising into much colder air that is high in the atmosphere, a severe thunderstorm is likely.   The above data, are the primary sources for non-tropical system thunderstorms. 

Lift Index =
 
LI 6 or Greater, Very Stable Conditions.
LI Between 1 and 6: Stable Conditions.
LI Between 0 and -2: Slightly Unstable, Thunderstorms Possible, with lifting mechanism,
    (i.e. cold front, daytime heating).
LI Between -2 and -6: Unstable, thunderstorms likely.  Some severe weather with  lifting
LI less than -6: Very unstable, Severe Thunderstorms likely with lifting mechanism.
                
Adiabatic Lapse Rate =  The change in air temperature with changing height, resulting from pressure change. Heating is essential to produce severe weather of any type.  If a severe thunderstorm begins to lose heat, its potential to produce severe weather is lost.  This lapse rate is what occures when a thunderstorm is over head and lightning is popping all over and then all of a sudden the lightning stops and there is just a rain.  In other words, the system has lost its available heat and convection, thusly, the lightning's energy source has been reduced.

The above terms are necessary to understand when reading weather charts or hearing data via NOAA Weather Radio.   The CAPE's and Lift Indicies are used to explain atmospheric conditions and how the conditions relate to possible severe weater, rain or fair conditions.

Additional terms and definitions will be added on a continuing basis.


How to convert winds in knots to miles per hour (MPH) and vice versa.

There are times when forecast winds are in knots and one may wonder what is a knot in miles per hour. You will most likely need your calculator to figure the conversion from knots to MPH. You will be multiplying and you will use the X symbol on your calculator. Enter the value of knots you wish to convert to MPH, in your calculator and then push the X symbol and then enter 1.15077945 then push your = symbol. You have now found the MPH. Example: the National Hurricane Center issues a hurricane bulletin indicating a hurricane is at 100 knots. Enter the 100 knots in the calculator, then push the X symbol and enter 1.15077945 and then the = symbol. You should have come up with 115.07794. Now you know the winds are at 115 MPH. If you wish to convert MPH to knots, just reverse the process by dividing 115 by 1.15077945 and your result will be 100 knots.