For Calamity Tax if first understand meaning of Calamity:

It means a great misfortune or disaster, rievous affliction; adversity; misery: the calamity of war such as

Avalanches and landslides.
Earthquakes.
Sinkholes.
Volcanic eruptions.
Floods.
Limnic eruptions.
Tsunami.
Blizzards

So Calamity tax means goverment impose Tax for helping states that suffer with above disasters.

Calamity tax = Calamity + Tax to fight it

Calamity declared as a national calamity is qualified for this tax. Further goverment need to take a decision wether to impose a tax for that national calamity or not.

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