Lightning is essentially a huge spark between regions of different electrical charge. Warm air carrying water vapour rises up from through the atmosphere if it is 'unstable'. As it rises, tiny water droplets and ice crystals condense out and are carried up with the rising air. As they reach the top of the cloud, they grow and become too heavy to be supported by the rising air, and begin to fall back through the cloud. As the larger ice particles fall, they collide with smaller particles and droplets still rising through the cloud, and each collision leads to a tiny exchange of charge. The different particles then carry opposing charges in opposite directions. Once many particles build up in a region, all containing a small amount of the same charge, this charge will want to try and escape, and does so by connecting with a region of opposing charge via lightning. You can find out more about this process in the video on this page.

How do we detect lightning? We can tell if lightning has occurred by listening for the rumble of thunder or by looking for the flash of lightning. To do this in person requires a clear line of sight to the thunderstorm cloud, which isn't always possible, and a large number of observers across the country monitoring for thunderstorms all the time, which isn't very practical either. Modern lightning location systems tend to look for the electromagnetic pulse generated by lightning strokes. These can be detected using antennae that look for this distinctive pulse. These systems can then either measure the direction the lightning came from, or the exact time that the radio pulse arrived, and use this to determine where the lightning occurred. Alternatively, we can use satellites that orbit the Earth to look for the visible flash created by lightning. A little more information on how lightning is detected in the United States can be found here.

Where does lightning strike most often? Lightning is most common in the tropics, and least common around the poles. We can use satellite data to build up global maps of lightning density, in order to compare how often lightning occurs in any location compared to any other. Current global lightning maps have been created from data from the OTD (Optical Transient Detector, 1995 - 2000) and LIS (Lightning Imaging Sensor, 1998 – 2015). Neither system is operational anymore, but another LIS system is due to be sent up to the International Space Station in around 2016, and a new generation of geostationary weather satellites carrying lightning detections will soon be launched, beginning with the GLM (Global Lightning Mapper) on the US GOES-R satellites, first due for launch in 2016.