Weather from Sept. 1 thru 16 in Barcelona, Granada and Sevilla

If you sign into Wikipedia for each city, it'll give you the average high and lows for temperatures, the average rain volume and the average days of each month for those cities. The information is in a yellow chart for most larger cities in the world.

A light sweater for Granada evening perhaps.
I would not travel without pants - you may get arrested !!

Speaking for Barcelona, the weather is not straight forward, that is, the city is located in an area where the weather is not that easy to predict due to its orography. In principle it should be short sleeves but you might need long pants if the weather turns wet -which it sometimes does, April and Sept are the 'rainy' months so to speak. Simply pack both, for precaution.

average temps are pretty useless. I'd take some kind of jacket, just to be on the safe side - and it can be useful if the AC is too cold on planes, trains, etc.

All of the fleeces that I own would be too warm. Lighter layers are more versatile.

Thanks, and I don't want to get arrested. Good tip on Wikipedia.

Get the 10-day weather forecasts right before you pack, and adjust as needed. "Average" doesn't help if you're experiencing out-of-the-ordinary conditions. Sevilla was sweltering in mid-October, Madrid chilly -- but that was a few years ago.

Are long pants/skirts required in some places in Spain?

Our favorite travel month in spain is October and with 15 years experience we have survived with using a light windbreaker and an umbrella. Keep in mind when encountering some chilly weather we adjusted our schedule to accomodate more inside events. The point is for Sept and Oct we recommend not overpacking in an attempt to be prepared for weather.