anvil vs thunderhead

anvil

noun
  • A horizontal-topped mass of cloud, shaped like a blacksmith's anvil, that forms before a thunderstorm. 

  • A stone or other hard surface used by a bird for breaking the shells of snails. 

  • The non-moving surface of a micrometer against which the item to be measured is placed. 

  • A heavy iron block used in the blacksmithing trade as a surface upon which metal can be struck and shaped. 

  • The incus bone in the middle ear. 

verb
  • To fashion on, or as if on, an anvil. 

thunderhead

noun
  • The top portion of a cumulonimbus cloud, which tends to be flattened or fibery in appearance, and may be indicative of thunderstorm activity. 

How often have the words anvil and thunderhead occurred in a corpus of books? (source: Google Ngram Viewer )