Ayāt prayer becomes obligatory due the following four things: 1. Solar Eclips; 2. Lunar Eclips; 3. Earthquake; 4. Thunder and lightning, red and black cyclone and other similar celestial phenomena, which frightens most of the people.
Question: what makes ayāt prayer obligatory according to Islamic law?
Answer: It is comprised of two rak‘ahs, with five rukū‘s and two prostrations in each rak‘ah. According to Islamic law, it becomes obligatory due to solar and lunar eclipses, even partial ones; an earthquake; and any abnormal phenomenon which scares most people, such as an unusual black, red, or yellow storm, an intense darkness, landslide, a cry (from the heavens), and the fire which sometimes appears in the sky. Āyāt prayer is not made obligatory by that which does not frighten most people, excepting eclipses and earthquakes, or by things which scare exceptions among people.[1][2]
1. Ayatollah Seyyed Ali Sistani , Islamic Laws, āyāt prayer.
2. Ayatollah Seyyed Ali Khamenei, Practical Laws of Islam, Q707.