It measures the time interval between a lightning bolt and its thunder; knowing the speed of sound, the distance from the thuderstorm is calculated. One have to click the start button as soon as a lightning is seen and click again the same button to let the widget know that the thunder is just heard. The calculation accounts for the temperature dependence of the sound speed: you can insert it on the back of the widget, as Celsius or Farenheit degrees or Kelvin scale; by changing the unit, the temperature already inserted is translated to the new unit. It is possible to show the distance in the International System units or with US units: also in this case, a change in the unit makes the present value to be converted. The display color tell you if it's time to turn off and disconnect your router to keep it safe from overloading. A button let you navigate to an online help on the web.From version 2, the movement of the thunderstorm is evaluated using the last 4 measures.
Works fine under Lion. Does what it says. Unlike the previous reviewer who hadn't even tried it, here in Australia we don't use miles, and the app nicely allows for kilometres instead, as well as other things. Everything works well. There is a help button on the flipside of the widget, and this works well. It takes you to a web site with good help and a way to email for support.
Cons
To make it go, you click on the button labelled "Lightning". It took me a while to figure that out. I was looking for a "start" button. The "Lightning" button changes to a "Thunder" button. Clicking that stops the timer and you have the result. There is nothing wrong with the app here. It was just a con of me.
Summary
Works well and free.
Count to 5
Defender 110
Pros
I cannot force myself to download software that replaces counting to 5. Every 5 seconds between the flash and hearing the thunder equates to one mile.
Job done.