To start, I'm not a UV specialist. From what I gathered from the sites, the UV's full strength is during noon, not any other times. However, ultraviolent rays WILL increase if you're in a place with reflections: snow, water, sand, etc.

I believe the reason why your glasses darken during mornings and evenings is because of the sun's angle and your glasses' surface area. When the sun is low to surface during mornings and evenings, your glasses are most exposed to rays because it's direct rays.

From above, it can't reach your glasses' surface (that would require you looking up often), so it doesn't darken.

Ultraviolet light is most intense at noon. The closer you are to the equator, the more UV light gets through the atmosphere to the surface, so you get more UV in Hawaii than anywhere else in the US.

The UV intensity varies with only one parameter, the atmospheric depth crossed by the rays, and that is because the rays are attenuated by the atmosphere. If you start with a 1000 UV rays, . The minimum depth, approximately 6 miles if the atmosphere were all the same thickness as at sea level, is when the Sun is directly overhead. When the Sun is at 45 degrees from the vertical, you get 8.5 miles of atmospheric attenuation and very little UV (it depends on the wavelength, but 5-100 times less). For example, 320 nm UV has a half length of a little over one mile, and the 2.5 extra miles will reduce the intensity by about a factor of seven.

When the Sun is at more than 57 degrees from the vertical (say, Michigan in winter), our government says that you can not get enough Vitamin D from the Sun even if you spend the day outside naked. So all that time spent in the garden at sunset does nothing for your Vitamin D, no matter the latitude. Being near water may double your exposure, and cloud cover matters. Pollution matters the most, so that, say, in Bangkok you will get ten times less UV than in Hawaii, even though you are at the same latitude.

Simple. The angle of incidence is equal to the angle of reflection. When the sun is overhead the sand/water, the rays are reflected directly back upward. When the sun is low in the sky, the reflected rays emerge with a low angle and strike you. Despite being lower in UV, the combination of direct and reflected sunshine is enough to cause a burn.