I'm one of those knitters who don't switch to lighter yarns made of plant fibres during the summer months. This May was historically warm here in Finland and I'll admit to picking smaller, lighter projects during the hottest days, but still wool and different kinds of wool blends are all I've worked with.
I wouldn't perhaps choose a long bulky cardigan to knit in the summer heat, at least for long periods at a time, but smaller woollen knits go well with sitting on the patio, sipping a cold drink and enjoying good company. I always have a sock project with me because they're easy to fit in any bag and it's easy to work a row or two here and there. At the moment I'm knitting Sachiko Burgin's Meerschaum socks from our issue 5 out of Tukuwool Sock and I'm sure I'll wear them a lot in the autumn.
Shawls and scarves are also something I tend to knit during the warmer months, but this summer I've mostly gone for lighter cardigans and sweaters. At the moment I'm also working on Rosa Pomar's Mungoche sweater with her Retrosaria Mungo yarn. Made entirely out of recycled materials, this 50/50 wool-cotton blend is summery and knits up beautifully, can't wait to finish this one!