Rain and Lightning

September 26, 2007

Frequency of Lightning

• Factors effecting where lightning will strike:– Size of cumulonimbus cloud base– Height of cloud base– Intensity of precipitation– Presence of tall objects– Conductivity of soil

Cloud Charge

• Rapid updrafts and downdrafts generate a separation of electric charge within clouds

• Positive charge near top, negative charge near bottom creating electric cell(s)

• A single thundershower can have several cells within itself and several possibilities for discharge

• Differences in charge can be horizontal or vertical within a cloud

Cloud to Ground

• Flow of electrons moves from cloud base to surface

• Negative electrons at cloud base cause build up of positive electrons at surface

• A streamer of electrons carrying a negative charge begins to probe outward and downward from the cloud base – stepped leader

• Electrons are seeking path of least resistance to the ground

Stepped Leader

Cloud to Ground• When the stepped ladder is 330 to 660 ft from

point to be stricken, a flashover of positive charge takes place

• The flashover meets the downward moving leader a short distance from the cloud base to the ground

• Massive discharge follows beginning at surface and moving towards cloud

• Electrons travel down to ground, but discharge takes place in upward direction

• This creates the return stroke, which is responsible for the visible flash

Cloud to Ground

• The return stroke is only a few centimeters wide, but is as hot as the sun

• These often ignite fires

• Because an ionized, conducting path has been established by the first stroke, a second leader moves towards the surface much faster – dart leader

• A return stroke follows the dart leader

Cloud to Ground

• This can be repeated several times

• The greater the number of return strokes, the brighter the flash and the louder the thunder

• A multiple stroke presents a much greater fire hazard

Lightning-ignited fires

• Dry lightning (lightning not accompanied by rainfall) often causes fires

• Accepted contributor to fires in the West

• Nay-sayers in the East claim its too wet for lightning-ignited fires

Intracloud and Intercloud Lightning

• Zones of positive and negative charge arise at different altitudes within clouds

• This creates a multitude of possible discharge paths

• This type of lightning does not damage objects on the ground, except indirectly through an electromagnetic pulse

• Accounts for 80% of lightning

Ground to Cloud Lightning

• When the surface develops a negative charge and the cloud base a positive charge

• Electrons flow upwards instead of downwards

• Upside down lightning

• Less destructive than Cloud to Ground lightning

Superbolt

• The most intense form of lightning discharge

• When positive charge builds up at the top of the cloud and a flashover occurs from the surface to the cloud top

• This carries more current than ordinary lightning strikes

• Can strike the ground a considerable distance from the cloud

• Very loud thunder

Odd forms of lightning

• Ball lightning – enters building through utility wires– Are spheres a few centimeters in diameter

that roll through the air as balls of fire– Last for a few seconds and then disappear in

smoke

• St. Elmo’s fire – slow discharge of electric charge off pointed, metal objects– Gives lightning rods, masts, antennas halos– Common on airplane wings

Ball Lightning

St. Elmo’s Fire• Served as an omen of heavenly intervention for

sailors

Thunder• When return stroke occurs, the air is heated

instantaneously to several thousand degrees Celsius

• The rapid heating causes to air to expand generating a shock wave that we hear

• Thunder is usually audible for 5 miles• Length depends on local topography, wind

speed, and direction• Lightning is seen before thunder is heard• To determine the distance to a lightning flash:

count the number of seconds it takes for thunder to arrive, then divide by 5 to get distance in miles

Electromagnetic Pulse

• In a lightning stroke, electrons accelerate as they jump between poles of opposite charge

• When electrons accelerate EM fields are produced

• These create bursts of energy called electromagnetic pulses

• These run as currents through utility wires, telephone lines, antennas, fences, and metal

Electromagnetic Pulse

• This is why you should never use electrical appliances, telephones, or anything plugged into a socket during a lightning storm

• EM pulses can be produced by intercloud and intracloud lightning that reach the ground

Lightning Rods

• Lightning rod- metal conductor, usually steel, erected vertically atop a structure. The rod is electrically connected to the ground by heavy wire

• The wire should not run inside the structure or near flammable material

• The wire should connect to a ground rod, driven into the ground away from the structure

• The height of the rods is dependent upon the height of the structure

Lightning rod

• Even if you have a lightning rod, you can still sustain damage from EM pulses

• Unplug appliances

• Lightning can enter through windows

• If caught outside during a storm, move to a low place away from tall objects and crouch down

• Get into a car, not under one

• Boats are dangerous places during lightning storms

Lightning Isn’t all Bad

• Lightning strikes generate nitrogen compounds in the soil, which are fertilizers

• If we didn’t have lightning, the atmosphere would eventually explode

• Lightning may have initiated life on Earth