Outdoor portraits are bit of a crap shoot if you don't have a plan. Even with a plan, you often don't know exactly what you will get with the environmental variables. With a little patience though, this type of photography is often spectacular!
Here are my thoughts of making great outdoor portraits:
Sunrise and early mornings: Usually this is great light with interesting skies. The dew in warmer months is a nice touch and the frost/snow in cooler months shimmers nicely. The downside is that you have to get up wicked early and the time is short.
Midday: Two scenarios which include: 1. Harsh sun that requires some artificial or natural shade, or 2. Overcast diffused light.
More people are more active at this time too so car and pedestrian traffic can make it a little tricky. We can use harsh light to make a mood for the portraits, but, that is often not the intent of the subject so diffusers and fill lights can be used to provide more even areas. An overcast sky diffuses the light for us, but, often makes the sky areas boring. I consider that when composing an image.
Sunset is generally going to be the best time to make an interesting and dramatic image, especially when the sky is going to be included. Don’t count out nighttime for great portraits either though. Really amazing things can happen after dark (especially in the winter) and it sets us up to makes very unique photos! This is one reason that I like to schedule these shoots later in the day. That way you get the benefit of strong light, sunset light, and a dark sky. Everything is there, and, we don’t have to get up at 5am!
At any time of day, a landmark that is itself interesting, or, holds significance to the subject(s) gives the portrait a positive boost. Sometimes these places require some permission and sometimes not. Reflectors and lights are generally must haves, at least for backup, so it’s tough to be stealthy in public places and permission (when needed) is essential for a successful shoot.
Photographing the world around us.