The colour of lightning flashes varies considerably. According to Trabert's observations at the mountain station on the Sonn blick, where the electric field during a thunderstorm is so strong that brilliant St. Elmo's fire can be observed, blue lightning is to be seen when negative electricity is being discharged by St. Elmo's fire, and red lightning is associated with the positive discharge. It may be noted that the difference between the types of St. Elmo's fire is striking. The negative fire is concentrated ; an ob ject like a flagstaff may be completely enveloped in fire. On the other hand, the positive fire takes the form of streamers, some as much as io cm. long.
The quantity of electricity which is transferred in a lightning flash can be estimated from the change which occurs in the electric field near the ground. The quantity is generally between io and 5o coulombs. As the discharge takes about one-thousandth of a second to pass, the maximum current is comparable with 5o,000 amperes. The difference of potential between the cloud and the earth before the discharge is of the order volts. The energy dissipated by the discharge of 20 coulombs through such a voltage is 'ow joules, or about 3,00o kilowatt hours: This energy would suffice to lift 500 tons through 2 kilometers.
When a lightning discharge occurs there is an immediate change in the electric force. The change is appreciable even at distances of ioo m. or more. In the case of nearby disturbances, there is in the interpretation of the observations some ambiguity. This disap pears, however, when the distance of the electric charges is great compared with their height. In a series of observations made at Khartum on April 23, 1924, and lasting four-and-a-quarter hours, the appearance of lightning estimated at between 3o and 30o km. away was noted at the same time as changes in the electric field. There were 37 discharges accompanied by a descent of negative electricity, and 444 accompanied by a descent of positive elec tricity. With regard to the discharges of the former type, it was observed: (I) That they appeared to reach the earth, whereas the others appeared to be in the clouds; (2) that they were of greater brilliance ; (3) that the illumination from the lightning lasted longer (as long as three seconds).
Some observations seem to imply that, throughout the storm a structure is being built up continuously in which there is a positive charge high in the cloud and a negative charge near the base.
According to this view, the majority of flashes pass between the upper and lower charged regions, the minority between the lower charged region and the earth. Simpson maintains, however, that the downward discharges of positive electricity do not originate very high in the cloud. He believes that many of these discharges terminate in the free air below the cloud. The most direct evi dence in favour of this view is the frequency with which branching discharges occur in photographs.
Measurements of the strength of the discharges from pointed objects have been undertaken. Wormell, working at Cambridge, used a single point 8.3 metres above the ground. Schonland, in South Africa, used a small tree, the top of which was 4 metres above the ground. The largest discharge recorded by Wormell in a single shower was 3o millicoulombs. Since the electricity dis placed in a single lightning flash is of the order 3o coulombs, we may say that the current from a thousand such points would provide enough electricity to make a flash.
Schonland gives the following table, showing the relation be tween the strength of the electric field over level ground and the current passing through his tree: Field (volts per metre) . . 3,500 5,500 11,000 16,000 Current (microamperes) . .07 •20 I.00 4.00 It will be seen that the current increases much more rapidly than the field. This is in accordance with other observations of the discharge from points.