To an architectural photographer the sky in an evening exterior shot is like a pair of heels to a woman. It can make or beak the outfit. And just like a woman has a closet full of shoes you should have a database full of stock sky's so you can pick the right one for the right occasion. There are many times in which the sky you captured during the shoot will be the right sky for the photo. But for those times when its lacking a little something extra its nice to have a back up.
Our current archive of sky's is over 400 images and growing. They span the gamut from early morning to just after sunset. They range from cirrus to stratus to cumulonimbus and even no clouds for when a nice smooth gradient will do the trick. And even though most of our exterior exposures are horizontal we make sure to have a good amount of vertical sky shots for those vertical needs. So next time you anticipate a good sunset head to a good open area and add to your sky database. If you can't get to an open area with an unobstructed view then I find driving to the upper deck of a parking garage will do the trick.