Last week a visitor asked about the possibility of a thunderstorm arising
during his visit to the Oregon Coast.
While common in central and eastern Oregon, thunderstorms are relatively
rare in western Oregon. My guess is that the air doesn't often have
the chance to get warm enough or rise high enough to form thunderheads.
Clouds form as (relatively) warm, moist air
rises to a cooler level in the atmosphere, condensing invisible vapor into
visible water droplets. Thunderheads are very tall cumulonimbus clouds
that develop positively charged water droplets at the top of the cloud and
negatively charged water droplets at the bottom. These tall,
polar-charged clouds can create turbulent local winds and heavy rain.
Lightning is the sudden discharge that balances out the electricity.
Lightning may occur between clouds, within a cloud, or between the cloud
and land. The discharge may form a single line, branches, a sheet, or,
rarely, a ball.
The first stroke of lightning, invisible to our eyes, is the flow of
negative charge from the cloud to the ground (or another part of the cloud)
that sets up a channel for the return stroke. The return stroke from
land to cloud is the flash we see—too fast for us to see the actual
movement.
Lightning discharges considerable amounts of electricity, 30 million
volts—or more. The huge amount of electricity suddenly heats the air
to very high temperatures, perhaps millions of degrees, which produces the
thunder.
(Sound travels about a mile every five seconds; count the seconds between
the flash and the rumble, then divide that number by five to get the number
of miles between you and the thunderstorm. If you hear the thunder at
the same time you see lightning, take cover: the lightning is flashing
nearby. Avoid open or high places, tall or isolated trees or poles.)
The great heat of lightning may fuse the rock, soil, or sand as it makes
contact with the ground, forming a "fulgurite."
"Solidified lightning," fulgurites are usually long and slender,
often tubular. Fulgurites typically have a glassy core, where the heat
is apparently greatest, with a gritty skin of fused grains.
Though thunderstorms are rather rare on the Oregon coast, they are not
unheard of: There is a locally-formed, foot-long fulgurite available
to view at the front desk of the Reedsport office of the Oregon Dunes
National Recreation Area, offering concrete evidence of past Oregon coast
thunderstorms.
