It was called new type bomb.
The New Type Bomb Control Committee was launched by the Japanese government on August 9, 1945.
At the same time, the government summarized basic guidance how to cope with the new type bomb on the day. It was announced through the national papers on the next day. As far as I read several papers, general guidance was like this -
1. Evacuate into the shelter.
(Tunnel type is more effective than pit type)
2. Wear white clothes before evacuating to the shelter.
If not available, wear any clothes to hide your skin.
(Make sure to wear gloves, socks and hood too)
3. Don't expose your skin to the air during evacuation.
(Hide behind a building if no shelter nearby)
4. Pay attention to the government's future guidance as it will be revised situation by situation.
"Tunnel type is effective. Be careful about burn. etc."
"Don't expose skin. No need to change conventional air defense system. Make your shelter more sturdy.
"Wear white clothes and evacuate to the shelter. If you find a parachute bomb, that's it."
Docs for approval of the committee dated August 9, 1945
Source:
ミュージアム巡り 昭和二十年 新型爆弾対策委員会 - 塩哲の色不異空