Synoptic chart symbols?

Question

why is the symbol for thunderstorms shaped as it is?

http://www.bbc.co.uk/schools/gcsebitesize/geography/images/g_wc_ukw3.gif

Answer 1

Likely because the simple symbols implies that lightning may strike the ground from a cloud.  The usual symbol for a bolt of lightning is jagged because its path is as jagged as its pre-ionized path.

Answer 2

Up until the early 1970s, weather maps were hand plotted. Even then some of the old forecasters still preferred to hand plot and analyze several of their maps so they had intimate knowledge of the detail. There still are some that do this today but most do not.

The symbols were often chosen due to the speed that a person could make them.

A single quarter hemisphere map took a person about 3 hours to plot and analyze and with literally hundreds of stations to plot on a single map, simplicity and speed were important. The symbols also had to have a distinctive shape that stood out from other symbols. This is particularly true of thunderstorms and snow.  

To a practiced eye, those symbols seem to jump out from all the other data printed on a map.