Natsu-jawan are Summer Bowls - vessels for drinking tea when the weather is warm. It is a traditional chawan (tea bowl), the wider shape allows tea cool more quickly.
There are many types of tea ceremony in Japan depending on the season, the time of day or a particular occasion. The first tea ceremony of the year held in January is called Hatsugama, which means “first kettle.” Asa-cha is an early-morning summer tea ceremony. Yûzari-no-chaji is the early-evening tea ceremony held in warmer months.