It's usually around 20 Celsius here, daytime, during summer. I do think in terms of patterns and colors as well, but sometimes it's confusing.
E.g. this sport coat: http://cgi.ebay.com/NWT-795-POLO-Ral...item27b8c50f82
Its fabric composition (67% Linen, 20% Silk, 13% Wool) seems like spring/summer to me, the color too, yet it looks quite thick and heavy... (description says medium-weight).
As for lined/unlined, I would prefer fully lined.