The Anvil Rule: How NASA Keeps Its Shuttles Safe form Thunderstorms

Cumulonimbus Cloud
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The National Aeronautics and Space Administration (NASA's) Anvil Cloud Rule is a set of rules that keep space shuttles weather safe during severe thunderstorms. It is one part of the Weather Launch Commit Criteria -- a set of rules created by NASA that defines weather conditions during which shuttle launch and landing are prohibited.

Rules Regarding Anvil Clouds

Do not launch through an attached anvil cloud.

If lightning occurs in the anvil or the associated main cloud, do not launch within 10 nautical miles for the first 30 minutes after lightning is observed, or within 5 nautical miles from 30 minutes to 3 hours after lightning is observed.

Do not launch if the flight path will carry the vehicle...

  • through non-transparent parts of a detached anvil for the first three hours after the anvil detaches from the parent cloud, or the first four hours after the last lightning occurs in the detached anvil.
  • within 10 nautical miles of non-transparent parts of a detached anvil for the first thirty minutes after the time of the last lightning in the parent or anvil cloud before detachment, or the detached anvil after its detachment.
  • within 5 nautical miles of non-transparent parts of a detached anvil for the first three hours after the time of the last lightning in the parent or anvil cloud before detachment, or the detached anvil after detachment, unless there is a field mill within 5 nautical miles of the detached anvil reading less than 1,000 volts per meter for the last 15 minutes and a maximum radar returns from any part of the detached anvil within 5 nautical miles of the flight path have been less than 10 dBZ on radar (light rain) for 15 minutes.

    What's an Anvil Cloud?

    Named for their likeness to an iron anvil, anvil clouds are the icy upper portions of cumulonimbus thunderstorm clouds that are caused by a rising of air in the lower portions of the atmosphere. When the rising air reaches 40,000-60,000 or more feet, it tends to spread out in a characteristic anvil shape.

    Generally, the taller the cumulonimbus cloud, the more severe the storm will be.

    The anvil top of a cumulonimbus cloud is actually caused by it hitting the top of the stratosphere—the second layer of the atmosphere. Since this layer acts as a "cap" to convection (the cooler temperatures at its top discourage thunderstorms (convection), the tops of storm clouds have nowhere to go but spread outward.

    Why are Anvil Clouds So Dangerous?

    The anvil rule is meant to protect space shuttles and the sensitive electronic equipment aboard them from three main dangers associated with cumulonimbus clouds: lightning, high winds, and ice crystals.

    In fact, shuttle​s are not only at risk from any lightning occurring within the anvil cloud itself, but it can also trigger more lightning to occur. When the space shuttle goes high into the atmosphere, the long plume from the exhaust gives a pathway through which lightning can flow. In addition, the plume will reduce the electrical field necessary to trigger natural lightning.

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      Oblack, Rachelle. "The Anvil Rule: How NASA Keeps Its Shuttles Safe form Thunderstorms." ThoughtCo, Mar. 5, 2018, thoughtco.com/anvil-cloud-rule-3444263. Oblack, Rachelle. (2018, March 5). The Anvil Rule: How NASA Keeps Its Shuttles Safe form Thunderstorms. Retrieved from https://www.thoughtco.com/anvil-cloud-rule-3444263 Oblack, Rachelle. "The Anvil Rule: How NASA Keeps Its Shuttles Safe form Thunderstorms." ThoughtCo. https://www.thoughtco.com/anvil-cloud-rule-3444263 (accessed May 26, 2018).