Many people associate summer with certan smells: gardenias, honeysuckle, roses, barbeques, fresh cut grass, chlorine from swimming pools.
To me though it's a sound that tells me summer has arrived, the sound of cicadas. This year is supposed to be one of those 17 year cycles with a huge emergence of cicadas from Georgia to Connecticut. Well in our part of Georgia we have cicadas every year and I haven't noticed that we have any more than usual this year.
They're supposed to emerge when the soil temperature (8 inches underground) hits 64º. They come out in early spring here, but you don't really hear them till the overnight temperatures stay in the 70s. Their distinctive sound is made by a thin membrane on the abdomen of the male that they vibrate.
In some countries cicadas are eaten, I prefer to just listen to them.