Jakke Makela

Attachment of Natural Lightning Flashes to Trees: Preliminary Statistical Characteristics

Lightning attachment to trees was studied based on 37 trees that were struck in Finland in summer 2007 and 2008. The type and severity of lightning damage was correlated with multiple parameters related to the flash, the meteorological characteristics of the strike time, and the surroundings of the tree. Damage was classified into three categories: bark-loss (minor), wood-loss (extensive), end explosive (complete loss of tree material). Four statistically significant parameters were found. The absolute value of the peak flash current is strongly correlated with damage; also, an indirect argument suggests that positive flashes cause more damage than negative flashes. The amount of damage is inversely correlated with the rainfall in the previous three hours, indicating that a wet ground and tree surface protect trees against damage by providing a conducting path to the ground. The ground type also has a weaker statistically significant effect, with poorly conducting ground leading to more extensive damage. Old and rotten trees are statistically most likely to experience explosive damage. Other parameters are inconclusive. The distribution of tree heights points to the possibility that the electrogeometric method does not necessarily predict the strike probability to a given tree. In many cases, the struck trees were clearly within the protective radius of a higher structure, although this is often difficult to determine from photographs. It is therefore suggested that models of lightning flashes to trees should include both the conductivity and height of the tree. The results may be significant for remote prediction of lightning damage, as well as for understanding lightning protection of structures which include trees.

Lightning location system accuracy determined from strikes to trees

Lightning strikes to 41 trees are used to estimate the accuracy of the Finnish Lightning Location System (LLS). With some limitations, the data set is large enough to estimate the validity of the uncertainty ellipse which is provided by the LLS. Such an estimation does not appear to have been done before for natural lightning. The method may give slightly overoptimistic results because extreme outliers have been removed. However, using a two-part quality control system, the flash causing the damage could be identified with high reliability. It is shown that the uncertainty ellipse given by the LLS is statistically consistent with the distribution of lightning location errors inferred from tree strikes.

Comparative study on preliminary breakdown pulse trains observed in Malaysia and Florida

In this paper, the preliminary breakdown pulse (PBP) trains preceding the negative first return strokes recorded using a broad band antenna system in Johor Bahru, Malaysia and Florida, United States were analyzed. In lightning flashes observed in Malaysia, the ratio between the amplitude of the largest preliminary breakdown pulse (PBP) and the peak return stroke electric field (RS) was 27.8% and the corresponding value in Florida was 29.4%. The time interval between the beginning of the preliminary breakdown sequence and the initiation of the return stroke was 57.6 ms in Malaysia and 22 ms in Florida. The results of the study support the hypothesis that the PBP/RS ratio increases with increasing latitude.

Estimation of lightning hazard of an approaching thunderstorm

Lightning location data from Finland has been analysed to provide information about the usability of the 30/30 rule, widely used in the personal lightning safety. The criteria are formalized so that a flash within 10 km of a given location launches the alarm. The storm is defined to produce a critical danger situation when a flash hits within 1 km of the location. With these definitions, the probability of false alarms is relatively high, up to 97%. On the other hand, in about 10% of the cases the 30–30 rule does not provide any pre-warning (i.e. the very first strike in the storm is 1 km away). In the cases where a prewarning is given, average lead times from first warning to danger are as much as 20 minutes. Although these results are preliminary, they suggest that the 30–30 rule may not be optimal in the case of a country with very low flash rates and densities, such as Finland.

Local detectors in preventive lightning protection

In lightning protection theory the preventive lightning protection (PLP) deals with using the forecasting tools for protection, combining them with special preventive actions. In zonal preventive lightning protection (ZPLP) location inaccuracies in the forecasting tools are assumed to be negligible. This approach is justified in the case of high-end measurement devices such as lightning detection networks. However, as the electronics industry is able to miniaturize sensors more and more, a new class of warning devices, is emerging aimed at very low-cost real-time warnings of lightning activity near individual structures. These local sensors in general have the characteristic that their inaccuracy grows as the distance to the flash grows. Also some sensors are capable of signaling the development of a thunderstorm cell before lightning activity starts. Such inhomogenity is not covered in traditional PLP theory. This paper deals with the theoretical introduction of such local detectors into the theory of preventive lightning protection.

Simulation of a narrow band HF lightning location system

A lightning detection network built from numerous small sized low accuracy devices with random antenna orientation has been simulated. Effects of sensor distribution, antenna directionality and sensitivity on location accuracy are studied. Effect of sensor density on accuracy is explored and it is estimated that a few sensors per 100 km2 results in fair accuracy.

Improved infrared temperature sensing system for mobile devices

An infrared (IR) temperature measurement system consists of not only a sensor module and electronics, but also an optomechanical system that guides IR radiation onto the sensor. The geometry and emissivity of the parts affects the reading, if the detector sees not only the target but parts of the measuring system itself. In normal industrial applications, the optics is designed so that the surfaces stabilize to the same temperature as the sensor. This allows the error caused by the device temperature to be easily calibrated away. The correction is valid for stationary conditions and usually near the calibration temperature, which is typically at room temperature. However, we show that if the sensor is embedded into a mobile (hand-held) device which has heat sources, such as power electronics, the normal conditions are no longer valid and the calibration fails. In order to improve infrared temperature sensing for mobile devices, the optics concept was studied and detailed design was performed. In addition, the optics performance was modelled and verified by measurement sensor prototyping. A calibration procedure noticing operational temperature variations was applied. The repeatability of the implemented IR temperature sensor using on a correct transferred calibration curve was better than plusmn0.5degC in an operational temperature range from +12.6 to +49.3degC and target range from +10 to +90degC.

Optical information storage in cellular mobile terminals

The trend towards so-called digital convergence (multiple functionality within a single terminal) is opening up a need for high-capacity storage within the cellular mobile terminals (CMT). Solid-state memories and magnetic microdrives are the most commercially mature options. Optical disk technology in this size range is immature, but has a unique potential: no other medium at present has the capability to be simultaneously low-cost, high-capacity, and exchangeable. In this paper, we explore the requirements for the implementation of optical disk storage in a CMT environment. From the technical point of view, these requirements include small form factor, high-enough data density and throughput, low power consumption, robustness, low cost, mass productability, and modularity. Although current technologies may satisfy some of these requirements individually, there is a need for combined optimization of all of these parameters. From the commercial point of view, the most crucial requirement is global standardization. Such standardization is crucial if wide interoperability is wanted (between CMT manufacturers, and even more crucially between CMTs and other appliances). Current optical storage standards are industry-driven and tend to be proprietary and/or incompatible. Even if the technical challenges can be met, optical data storage is not likely to be accepted in CMT applications unless global standardization proceeds more quickly than it is doing at present.