What metering mode were you using? Your camera likely offers at least 3 different metering modes.
Okay, I see you use a Canon XSi. It has 4 metering modes:
- spot - which meters just 4% of the viewfinder at the center of teh viewfinder (for spot mode 4% is almost to much)
- partial - which meters 9% of the viewfinder at the center of the viewfinder
- center-weighted - which average the entire scene, but puts about 70% of the averaging emphasis on the central 20% or so of the scene in the viewfinder
- evaluative - which averages the entire scene.
Why is the light on cloudy days soft and shadow edges less sharp?
Because the light source is apparently a lot larger. Instead of a small really bright light source (the sun), the light comes from really big, but dimmer, clouds.
Bigger apparent light sources make softer light that 'wraps around objects' which means the shadow edges are much more diffuse. That is why pro portrait shooters use umbrellas and/or softboxes to modify their small lights. to make them apparently very much larger lights.
On cloudy days we get that soft light, but we have no control over it's direction, and of course the modified sunlight light is coming from overhead. Overhead is ok for use as a hair light, but not for a face.
So even on a cloudy day, some supplimental light is very desirable if you want to get pro quality results. Because there is no direct sunlight, reflectors don't work very good.
So your supplimental light needs to come from a flash unit. A flash unit on the camera hot shoe can be used for fill, but it can't be used for accent lighting. For accent lighting the flash unit has to be off of the camera, which is known as off camera flash or OCF.
To learn more about OCF visit
http://strobist.blogspot.com/2006/03/lighting-101.html
http://neilvn.com/tangents/flash-photography-techniques/
Get the books -
Light Science and Magic, Fourth Edition: An Introduction to Photographic Lighting
Speedliter's Handbook: Learning to Craft Light with Canon Speedlites
Off-Camera Flash Techniques for Digital Photographers
If there are no clouds, you can put your subject in open shade to again get an apparently large light source, the bright blue sky, without the hard sharp shadow producing sun.