In the regulation of building sunshine interval, why should we calculate the severe cold day and winter solstice day separately?

Because of the great latitude difference between north and south in China, the climatic zones are also different.

Basically, the simple statement is:

The winter solstice is the shortest sunshine day, but depending on the orientation of the building, it may not be the shortest sunshine day in your building.

Great cold weather is generally the coldest day, and it also needs sunshine time.

In two days, calculation methods are needed, such as limiting and adjusting the azimuth of building windows to reduce lighting and lead to darkness in the building.

Extended data:

Our country is going through a process of change. At the earliest, it was determined that the winter solstice should last no less than two hours. Later, the relevant departments found that this standard was difficult to achieve in some cities due to high latitude and dense urban buildings. Therefore, in the latest revised standard, it was changed to take cold days as the date to determine the sunshine time.

The winter solstice is the shortest day of the year, so the relevant departments regard it as a reference date to measure the sunshine standard. According to the observation, the noon sun height in the area of 40 north latitude is 27 degrees from winter to Sunday, and it rises to 30 degrees in the severe cold day.

According to astronomical theory, the higher the sun is, the shorter the shadow is, and the more sunlight shines on the ground. In other words, in cold days, the sun shines more sunlight on the ground than in winter solstice.