Marfa weather is High Desert weather. The one thing that you can count on is that if the sun is up it will be much warmer than when the sun goes down.  Now that sounds obvious but a 30 to 40 degree swing each day is normal.  The main reason for this is our altitude.  Marfa is about 4848 feet high, near enough to a mile high to make you catch your breath, catch some sun, and catch a sky full of blue.  Enjoy