I have found, for example, in my location (mid 40 North latitudes) that printing is not feasible from early November to early February both because the days are quite short, and because the sun is low in the sky and moves so rapidly that changing shadows create exposure problems. At the opposite extreme, near the time of the summer solstice, printing at mid-day may involve, with gum printing, exposures of less than one minute making it necessary to virtually hover over the print!
Usually, I make exposures in sets of threes. The first one using a particular negative is made using my best guess for the season, hour and weather. If it is too dark or too light based on first inspection, the next print's exposure will be longer or shorter as necessary, and if required, further adjustments will be made for the third one. Exposures are made in quick succession, so that lighting conditions will similar.
While conventional exposure meters are of little or no value, I have found that a very inexpensive device (a coin) provides valuable assistance. When I coat paper, I coat it beyond the boundaries of the image. After the paper and the negative have been put in the contact printing frame, I place a coin on top of the glass somewhere over the coated area, outside the print area. By moving the coin aside periodically, I determine the degree of development by the contrast between the area under the coin and the area surrounding it.
This very simple device (my 'exposure meter' costs 10 cents and is re-usable!) also serves two other purposes. When developing the print, the circle under the coin will have returned to the color of the paper (all coating will have dissolved) when development is complete.
Also, the shape of the coin's shadow indicates when movement of the sun during exposure is a problem or when the sun is too low in the sky to print effectively. When the sun is high, the shadow left on the paper is a near circle. When the sun is at a sharp angle. the coin shape becomes a noticeable oval. The negative does have a measurable thickness. Particularly in three color gum or other printing processes involving multiple printings on the same paper, the movement of the image cast by the image on the negative can cause what appear to be registration problems, even if the negative is perfectly aligned.
While the use of the sun as an Ultraviolet light source does present challenges, it does have its satisfactions. While it may take more attempts to achieve a satisfactory print, in my opinion the print that finally is obtained is superior when the sun is used as a UV light source.