Depending where you are cycling, summer can mean different things to different people. In Ireland we class anything over 18 degrees as warm whilst in Spain they need to see 25 before they start to get excited. Shorts and short sleeves are fine in the good weather. In Ireland and The UK I tend to wear something that still has a little bit of warmth in it, whilst in Spain or Portugal i wear clothing that is as cool as possible.,
A light base layer
I always like to have something under my jersey, even if it is just to keep a bit of warmth when descending Mahon Falls or Tickincor.
A short sleeve jersey
These come in mainly 2 different types that can be described as normal and extra ventilated. I usually stick with the normal jersey but once the dial goes over 25 degrees I have been known to pull on an extra ventilated jersey to make the gang think that I am in really good shape.
A good pair of shorts
A really good pair of shorts with a top of the range chamois is probably the best thing a cyclist can spend their money on. All of the Flynx or Rogelli shorts that I have tested have 220g lycra which adds a level of compression that is supposed to help fight fatigue. That may or may not work for everyone but they definitely feel very comfortable on. I would have to say that they are the most comfortable shorts that I have ever worn, and I sure have worn many, many different types of cycling shorts.