Weather Explanations page 3

Air-mass Thunderstorm

Air-mass thunderstorm is generally a thunderstorm not associated with a front or other type of synoptic-scale forcing mechanism. Rather air-mass thunderstorms are associated with warm, moist and thus unstable air-masses in the summer months.Therefore the name! They develop locally during the afternoon or late in the day in response to insolation through convective heating from the surface. Thus they are also known as single-cell thunderstorms. Typically they do not persist very long - usually an hour or so - and dissipate rather quickly after sunset. 

Air-mass thunderstorms have a distinct life-cycle described by three stages: (1)cumulus stage, (2)mature stage and (3)dissipating. They generally are less likely to be severe t han other types of thunderstorms associated with widespread lifting or vertical motion at a front, but they still are capable of producing downbursts, brief heavy rain, and (in extreme cases) hail over 3/4 inch in diameter.

 

Since all thunderstorms are associated with some type of forcing mechanism, synoptic-scale or otherwise, the existence of true air-mass thunderstorms is debatable. Therefore the term is somewhat controversial and should be used with discretion.

Thunderstorm Probability

The Lifted Index is a measure of atmosphere's stability (or instability) and Meteorologists use it to determine the thunderstorm potential. It doesn't accurately predict the intensity of every single storm, but it is a useful tool to estimate the atmosphere's potential to produce severe thunderstorms.  "Parcels" (or bubbles) of air start to rise on their own if they are warmer than the surrounding air. This process is calledconvection. Consider an air parcel as it begins to rise through the atmosphere after being heated by the sun and the warming ground. The Lifted Index is defined as a rising parcel's temperature when it reaches the 500 millibars level(at about 5,500m or 18,000 feet asl), subtracted from the actual temperature of the environmental air at 500 millibars. If the Lifted Iindex is a large negative number, then the parcel will be much warmer than its surroundings, and will continue to rise. Thunderstorms are fueled by strong rising air, thus the Lifted Index is a good measurement of the atmosphere's potential to produce severe thunderstorms

 

The Lifted Index (LI)
RANGE IN K
COLOR
AMOUNT OF INSTABILITY
THUNDERSTORM PROBABILITY
more than 11
B LUE
Extremely stable conditions
Thunderstorms unlikely
8 to 11
LIGHT BLUE
Very stable conditions
Thunderstorms unlikely
4 to 7
GREEN
Stable conditions
Thunderstorms unlikely
0 to 3
LIGHT GREEN
Mostly stable conditions
Thunderstorm unlikely
-3 to -1
YELLOW
Slightly unstable
Thunderstorms possible
-5 to -4
ORANGE
Unstable
Thunderstorms probable
-7 to -6
RED
Highly unstable
Severe thunderstorms possible
less than -7
VIOLET
Extremely unstable
Violent thunderstorms, tornadoes possible

 

 

For example, if the rising parcel has an temperature of -5°C when it reaches 500 millibars, but the actual temperature at 500 millibars is -11°C, then the lifted index is -6 Kelvin (or K) indicating the potential for strong thunderstorms. However, there are no specific threshold values that correlate lifted index to thunderstorm severity. In general a negative Lifted Index indicates an unstable atmosphere, so the larger the negative number, the more unstable the atmosphere is, the stronger a thunderstorm could be. Lifted index values rarely go below -7.

 

Now, find out about the convection and thunderstorm probablility across the British Isles yourself. Have a look at WeatherOnline's new Lifted Index maps. The table above will help you to 'read' the map and to estimate the thunderstorm risk.
Important ! The Lifted Index is not a measured quantity, it is only a parameter that is theoretically derived. If the Lifted Index is favorable for severe storms but other conditions are not met, then no storms may form at all.

 

Convection

Convection is one of the major processes creating our weather. Convection is one cause of rising air in our atmosphere, usually warm air rises above cold air. Convection is the principle motor of cloud formation and circulation on all scales - including the atmosphere's general circulation as warm moist air is going upwards and colder, drier and denser air will be sinking downwards.  In general, convection is the vertical transport and mixing of heat and other properties of a fluid through mass motion. It is generally taken to imply vertical motion, being produced by differences in bouyancy, arising from variations in density. Evidence of convection happening in our atmosphere is seen with the formation and growth of cumulus clouds, for example. The horizontal transport of the properties of an air mass or fluid is called advection

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