A cloudy is day often regarded as a great opportunity for getting nice portrait pictures! The well-established reason is soft quality of light. Yes! Clouds make light relatively soft but it is important to understand that how the same light coming from the sun becomes soft because of a clouds.
The distance of between Sun and the Earth is huge, as a result the light rays reaching a photographer’s subject are almost parallel. Such a direct beam which is spread within very small degrees results in well-defined shadows. This light is known as hard light. On a cloudy day the light source still remains same but the clouds act as a modifier. For the photographers subject the light source is now wide spread cloud cover. In the thick layer of clouds, the light bounces back and forth and gets spread to a greater width before reaching earth. This wide spectrum has light coming from many directions to the subject as a result the shadows get filled by light and are diffused greatly. The quality of light is quite soft as compared to the direct sunlight.
Alternatively one can also observe the variation in the size of the light source when compared to the size of the object. The larger light source which facilitates the spread and adds to the angles from which the subject receives light is considered soft. The same principle applies when we compare the light quality or nature of a Speedlight (flashgun) with the one of a soft-box. In the case of a portrait the size of the light source (speedlight) is far smaller than the soft-box. A soft-box can easily be one hundred times bigger than a speedlight thereby providing a considerably diffused light.
It is not always necessary to subscribe to the idea of soft light. Many a time for having dramatic effect, one can choose to go for harsh light also. Tilt and swivel heads of many speedlight models help once achieve bounce light in indoors. The bounced back light takes the advantage of vast surface from where the light is reflected. This feature of on camera flashes only helps in taking photographs in indoor situation. For the outdoor purposes, collapsible reflectors are of a great help. Quite obviously, more camera and lighting equipment can be helpful in achieving great photographs but needs more manpower to mobilise the whole setup thereby increase the production cost.