Today s lecture. Charge separation in clouds Lightning and thunder

Transcription

1 Today s lecture Charge separation in clouds Lightning and thunder

2 Warming up knowledge Storm - convective precipitation (from cumulus and cumulonimbus (Cb) clouds) Prerequisite for a storm - cloud particles grow to precipitation size in a short time. Necessary prerequisite for a thunderstorm - coexistence of water and ice.

3 The types of thunderstorm Ordinary or isolated thunderstorm - one convective cell; lasts 40 min ~ 1 hour. Longer-lived multicell thunderstorms - consist of a number of cells in different stages of development. Supercell thunderstorm - longer-lived; one giant cell, less often than ordinary and longer-lived multicell storms, but more violent But they all need a buoyant air parcel and conditional or absolute instability for their formation!

4 Lightning (a transfer of charge) A B

5 Charge separation within clouds For lightning to occur, the charges inside the cloud need to be separated. The majority of thunderstorms occur in cold clouds 20 C 10 C 0 C Main charging zone Ground Fig Schematic showing the distribution of electric charges in a typical and relatively simple thunderstorm. The lower and smaller positive charge is not always present.

6 Precipitation hypothesis Requires the presence of graupel or hail particles. Collision of larger warmer grapple or hail particles with tiny colder ice crystals. In ice, the H ions are more mobile than the OH - ions This process is limited to temperatures below C

7 Is air conductor or insulator?

8 Capacitor Breakthrough field strength > 6000 kv m -1 ionize the air in a channel - strongly increases the conductivity of the air locally

9 Lightning Occurs in the presence of an ice and liquid phase inside clouds and requires that the cloud top is colder than -15 to C A transfer of charge Temperature can reach 30,000 K in a narrow channel, and the first stroke reaches up to 40,000A We only use 15 A in our everyday life

10 Lightning

11 Lightning Cloud flash Ground flash

12 Ground flash Lightning stokes towards the Earth s surface transport electrons from cloud to the surface Return stokes carry positive charges from objects at the Earth s surface to the cloud base. Ground flashes originate from the lower negatively charged part in the cloud. This discharge is called the stepped leader

13 Stepped leader (a) t = 0 (b) 1 ms (c) 1.1 ms (d) 1.2 ms (e) 19 ms (f) 20 ms (g) ms (h) 20.2 ms 10~100m from the ground (i) 40 ms (j) 60 ms (k) 61.5 ms (l) ms dart leader return stroke Discrete steps of 1 μs (~50m) each, why? return stroke

14 (a) (b)

15 Thunder Any sound you hear is made up of vibrations. The electricity passes through the air and causes air particles to vibrate. The sudden increase in pressure and temperature from lightning produces rapid expansion of the air, pushing apart the air particles with force and creating more vibrations. Can we see the lightning and hear the thunder at the same time?

16 The global electrical circuit fine weather current~ 1580A high electrical conductivity due to the increasing ionization by cosmic rays