Simple geometry explains why this is true on cloudy days.
Have you ever noticed that on an overcast day the sun will often break through the clouds close to midday?
Sometimes it may not actually break through, but it tries hard and the atmosphere brightens.
On such days, if you observe the cloud cover above, you will get a brief glimpse of the Sun's upper limb and then the lower limb as it crosses the meridian.
There are simple reasons for this phenomenon:
- At noon the Sun is 4,000 miles closer to an earth observer than at dawn. (Since the Sun is 93 million miles from earth, this might be considered a small point.)
- More importantly, at noon the sun penetrates less cloud cover and contends with less atmospheric pollutants, because it is directly above and not at an oblique angle.
- All observations were taken in the temperate zone, and may not apply to an observer in the extreme northern or southern latitudes where the Sun follows a different pattern.