July 01, 2006
Weather at the Cape
The currently estimated 40% chance of a Shuttle launch delay or scrub is not caused by the prediction of any specific storm,
but by the 40% chance that within a roughly 40 mile diameter circle centered at the launch pads at Complex 39,
one or more rainstorms may enter the zone.
These storms are part of the normal sea breeze "front" that tends to develop most days along many coastlines during the afternoon hours, as warm air rises over the land, pulling in the cooler air over the water.
The movement creates a certain percentage of showers and thunderstorms along it. In the upwind direction, the "no rain zone" may extend out to 30 miles.
The main risk of a thunderstorm or rain shower is lightning, which could strike the shuttle.
Airplanes are sometimes hit by lightning in mid-air, almost always with no damage,
but due to the many explosive components on the shuttle that could be triggered by electricity, this risk is too high to allow.
If the launch window is long enough so that they can wait until a storm exits the circle, the launch can then proceed.
Anvil Top Angst:
Large thunderstorms which had already moved west of the Cape toward Orlando have produced large
"anvil tops" near the top of the troposphere which can be electrically charged, but usually carry no rain.
These cloud streamers are being blown back toward and over the Cape by the high altitude winds from the west.
If the Shuttle with its exhaust plume were to plow through the relatively thin anvil-tops,
there is a chance that a lightning bolt could be triggered by the plume and reach the ground.
This would also directly endanger the shuttle.
The 40 mile circle around the pad only has to be free of dangerous clouds and rain for a short period before
and after the launch.
The shuttle would also not want to fly through a rainstorm directly. The high velocity of the Shuttle hitting
raindrops could damage the soft and fragile tiles.
Posted by johnstrickland at July 1, 2006 12:35 PM