This Morning’s Terra Modis satellite overpass (10.55am on Sat 2nd)
The Beaufort scale is used to estimate wind damage in the same way tornadoes winds are
mostly measured, by assessing the damage present.
Wind speed (km/h) Designation Description
< 2 calm smoke rises vertically, trees do not move
2-5 light air smoke drift indicates wind direction
6-11 light breeze weather vane moves, leaves rustle
12-19 gentle breeze leaves and twigs in constant motion
20-29 moderate breeze dust and loose paper raised, small branches move
30-38 fresh breeze small trees sway
39-50 strong breeze large branches move, wind whistles wires
51-61 moderate gale whole trees move, walking affected
62-74 fresh gale twigs brake of trees, walking difficult
75-86 strong gale slight structural damage occurs, branches break
87-100 whole gale trees uprooted, considerable structural damage
101-118 storm widespread damage
119+ hurricane severe and extensive damage
A bow echo is another possible tornado signiture which can sometimes form along a squall line. The bow echo moves with the center being the furthest forward section of the storm, like a triangle without the bottom line, or the end of an arrow.
CAPE stands for Convective available potential energy, and in laymans terms it is a measure of instability in the air. The higher the CAPE, when in a high Dew Point, high temperature situation the more likely it is for thunderstorms to develop. CAPE is one of the key elements chasers look for when deciding on an area to chase.
A cut-off low is a low pressure system which has become displaced from the normal flow of weather systems. Where normal weather systems are carried in a westerly flow by the jet stream, a cut-off low becomes detatched from the jet stream and is essentially left behind. These systems can remain for a long longer period than normal weather systems because they have to wait for the jet stream to return to that latitude and move it along. The cut-off low, in South Africa is often the cause of flooding, thunderstorms and even tornadoes at times.
dBZ is the way of measuring reflectivity in clouds, the higher the reflection the heavier the rain or possible hail. During a severe storm it is likely to see higher dBZ values.
Generally speaking dBZ reflectivity indicates the following.
dBZ – 10
Rainfall Rate – ~0.2
Significant but mostly non-precipitating clouds
dBZ – 20
Rainfall Rate – ~1
Drizzle, very light rain
dBZ – 30
Rainfall Rate – ~3
dBZ – 40
Rainfall Rate – ~10
Moderate rain, showers
dBZ – 50
Rainfall Rate – ~50
Heavy rain, thundershowers, some hail possible
dBZ – 60
Rainfall Rate – ~200
Extremely heavy rain, severe thunderstorm, hail likely
Dew point is the temperature at which saturation of water-vapor takes place. High dew points combined with high temperatures are often a helping factor in the development of thunderstorms. As the higher the dew point, the more moisture is present in the air.
The Dopplar Radar was developed in the late 1980s and since has become the leading weather radar, with the ability to supply information about wind velocity and reflectivity echoes within a storm. Recently the South African Weather Service installed Doppler Radar in South Africa for the first time.
A dryline is a thin area of extreme gradient in the moisture present in the air. Storm chasers in South Africa will place themselves just to the east of a dryline, where thunderstorms are most likely to form. Though a dryline is a global weather phenomenon and commonly sought after by chasers in Tornado Alley.
A dust devil is a large whirlwind, which can reach sizes equal to tornadoes, though do not need thunderstorms to develop.
Enhanced Fujita Scale
The Fujita Scale was replaced by the Enhanced Fujita Scale recently, this is the means of rating a tornadoes strength. Though the EF number is in most cases obtained through damage surveys, and through that the wind speed is calculated.
EF 0 – 65-85 mph
EF 1 – 86-110 mph
EF 2 – 111-135 mph
EF 3 – 136-165 mph
EF 4 – 166-200 mph
EF 5 – Over 200mph
Fetch is the area of rough seas created by a tight pressure gradient of a low pressure system. When low pressure systems have a tight gradient which extends a large distance, the wind winds increase and so do the seas. Though this is only really effectively ‘fetch’ when the direction of the winds are constant.
A hook echo is the radar signature of the area of a supercell where a tornado is most likely to form. It is caused by the rotation of the supercell, a hook echo will appear like a hook on the edge of a severe storm. Hook echos are often what are used to issue tornado warnings.
A hurricane or tropical cyclone is formed when an area of thunderstorms organizes and forms a low level circulation, once this area of rotating convection strengthens it becomes a tropical storm and then once it’s winds increase past 64kt it is officially a hurricane (when in the EPAC or Atlantic). Hurricanes a large scale weather events and can have outer bands stretching thousands of kilometers out. Hurricanes are extremely dangerous and can have winds in excess of 160 knots. Hurricanes are rated by their wind speeds, with 75 mph being a category 1 hurricane and storms with winds in excess of 155mph are rated category 5.
Instability is when the hair has enough heat and energy to rise at a quick pace, allowing for convection to take place. Generally the more unstable the air the more chance for a severe thunderstorm. Instability is related to CAPE.
The jet stream is a high altitude band of winds which fluctuate in latitude depending on the time of year, it brings with it weather systems such as cold fronts.They are located between 30 000 and 40 000 feet above the surface and have far stronger winds in general than surface winds. It is accurate to think of it as the meteorological conveyer belt.
For lack of decent words of my own I’ll quote wikipedia- “The lifted index (LI) is the temperature difference between an air parcel lifted adiabatically Tp(p) and the temperature of the environment Te(p) at a given pressure height in the troposphere (lowest layer where most weather occurs) of the atmosphere, usually 500 hPa (mb). When the value is positive, the atmosphere (at the respective height) is stable and when the value is negative, the atmosphere is unstable.” Lifted index is another important factor to storm chasers and an ingredient of storm formation, the lower the LI the better.
This is a layer of atmosphere which stretches from around 5 kilometers to 2000+ kilometers.
A mesocyclone is an area in selective supercells which is rotating. This rotation is caused by the updrafts and the wind shear present. It is often a precurser to tornadoes, as they form under similar conditions. Though a mesocyclone is far larger and is not a ground feature.
A ridge is another name for a large area of high pressure.
A roll cloud is quite a rare formation which can occur along the leading edge of a thunderstorm where updrafts and downdrafts interact, causing a rolling of air.
A thunderstorm is classified as a severe storm when there is either winds in excess of 93km/h, hail 1.9 centimeters or larger or tornadoes.
Shear is when you have various winds moving in different directions, which creates rotation in severe thunderstorms. The updrafts, downdrafts as well as the left to right or right to left winds create a turning motion. Shear is a key ingredient to the formation of tornadoes, though shear can destroy a hurricane by decoupling the various levels of circulation.
A squall line is a line of thunderstorms, usually a squall line will only last a few minutes in any given location. They can extend hundreds of kilometers long and can produce very strong gusts along the leading edge as well as possible tornadoes.
Surge is an event caused by both strong cold fronts on rare occasions, but more commonly with hurricanes. Storm surge consists of a high increase in the height of water levels due to the amount of fetch present. Large storms which move slowly are the most common cause for storm surge.
Tornadoes are still quite a mystery to man, we are not completely sure what causes one supercell thunderstorm to drop a tornado and another not to. Though research is being done each year to try figure out the workings of these weather phenomenons and to try find a way of giving accurate warnings ahead of formation. Physically a tornado is a rapidly rotating column of of air which descends from supercells, and are one of the most destructive weather events on earth.
A tropical depression is an organized area of convection, where there is a low-level circulation present. Usually convection which sustains itself for a period of days develops a low level circulation. This feature is a tropical feature and in many cases the beginning stages of a hurricane or tropical storm.
A tropical storm is a developed tropical depression once winds become stronger than 34 knots. It also needs to have a low level circulation present.
A trough is an elongated area of low pressure, which can bring thunderstorms to parts of South Africa, especially in the summer months when they pull down sub-tropical moisture. They are also commonly associated with cold fronts in the winter months.
A waterspout is somewhat like a tornado over the sea, though you do not need a severe thunderstorm for a waterspout to form and they can develop when the air temperature is a lot colder than the water temperature and form from regular cumulus or towering cumulus clouds.
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