TABLE OF CONTENTS The average duration of the cloud flashes as measured from records of electric field changes can range from about 200 ms to about 500 ms. Typical slow electric fields generated by cloud flashes within about 10 km are shown in Figure 4.31. Note that the fields can be either unipolar or bipolar. The change in the dipole moment associated with the cloud discharges may be of the order of 100 Ckm. The charge transfers associated with the discharge as evaluated from the electric field measurements may range from 0.3 to 100 C. Several attempts were made to extract information concerning the average separation between the charge centres that take part in the discharge and values were found in the range of 0.5 km to about 3 km [154].
The most extensive study of the electric fields generated by cloud flashes was conducted by Kitagawa and Brook [132]. According to these authors, the electric field signature of cloud flashes consists of three portions which they classified as initial, very active and the final stage. The initial portion has a typical duration of 100 200 s and is characterised by small microsecond scale pulses. During the very active portion, pulse activity is accentuated and is characterised by the pulses with the highest amplitudes in the flash. The final stage is composed of relatively small step-like...
