It honestly depends on how the weather is on the day of the tour. It’s summer, so it shouldn’t be too cold outside. If it’s raining, be sure to wear a raincoat or something similar. But even if it’s sunny and warm, we recommend that you bring a sweater with you. The temperature inside the volcano is always the same – around 6°C (43°F). That´s NOT t-shirt temperature!
Also, we recommend that you wear hiking shoes – if you have them. Sneakers will work, but they will definitely get dirty and may get some punctures.