One of the most frustrating things when traveling is to arrive somewhere wearing the wrong clothing because you didn't realize how hot or how cold it would be. While we know that temperatures in the 80-90-degree Fahrenheit range are hot and temperatures in the the 20-30-degree range are cold, sometimes it can get confusing for everything in between. If you're traveling somewhere where the temperature is going to be about 40 degrees Fahrenheit, expect to experience cool and cold weather. Dress based on tolerance for moderately cold weather.
Wear comfortable jeans or slacks. 40 degrees is a little cold for shorts or a dress unless you wear legging underneath the dress. You probably do not need to wear leggings or long johns underneath long pants unless you really can't handle slightly cold weather.
Wear a long-sleeved top with a high neckline. Any long-sleeved cotton top will work fine for casual outdoor outings or activities, but throw a sweater on top if you get cold easily. If you are going to be doing any running or hiking, it's a good idea to wear a specialized acrylic fabric or other sports shirts made specifically to draw moisture away from your body.
Wear an insulating jacket or a coat. You don't need a heavy coat, but you should have a jacket that is lined with wool, fleece or another insulating material. You may get hot in a long coat, so opt for a short one that reaches your waist.
Carry gloves and a hat in your pocket. You may not need them, but if the temperature drops later in the day, you will be glad you have them to keep your body warm. Add a scarf to a light jacket for more warmth.
Wear closed-toe shoes. Sneakers, boots or dress shoes are fine for 40-degree weather; flip flops or open-toe dress shoes are not. Open-toe shoes allow heat to escape your body through your feet, making your entire body colder.