The weather if its normal then try full sleeves and if it gets warmer or you like to wear short sleeves than wear short sleeve shirts. (stylesgap.com)
The outfit below is a great example of how a white short sleeve shirt works well in the summer. (theidleman.com)
I could keep my big hat in my pack and just wear the lightest short sleeve shirt without having to care about the sun at 10,000 - 12,000 ft! (eaglecreek.com)