Annual-cycle assessment of street orientation effects on outdoor thermal comfort in a hot-summer cold-winter district

Imagine your city as a giant, open-air living room where the furniture is buildings and the floor is the pavement. Just like how you might feel chilly sitting in a drafty corner or sweating under a direct lamp, the people walking down a street feel different temperatures depending on how the buildings around them are arranged. This is the world of "urban microclimates"—the tiny, local weather systems created by our streets and squares. A key player in this story is "outdoor thermal comfort," which is just a fancy way of asking: "Is it pleasant to stand outside right now?" If the sun is blasting you, it's uncomfortable; if a cool breeze is trapped by a wall, that's great. But here's the tricky part: the sun moves. It shines differently in the summer than in the winter, and the angle changes every hour of the day. Most scientists have only looked at how streets feel on a single, hot summer day, like checking the weather only on the hottest Tuesday of July. But real life happens all year round, and a street that feels perfect in July might feel like an ice box in January. Understanding how to design streets that feel good all year long is the big puzzle this paper tries to solve.

The researchers behind this study decided to stop guessing and start simulating the whole year for a historic area called Yuhou Street in Chenzhou, China. This city sits in a "Hot-Summer Cold-Winter" zone, meaning it gets scorching hot in the summer and freezing cold in the winter, making the design challenge extra tough. Usually, checking the weather for every single hour of a whole year (that's 8,760 hours!) would take a computer so long to crunch the numbers that city planners would give up and go home. To get around this, the team used a clever math trick called a "Fourier harmonic regression algorithm." Think of this like a music compressor: instead of listening to a whole symphony of 8,760 hours of weather data, the algorithm picks out the 12 most important "notes" (or representative days) that capture the entire song's rhythm.

Using these 12 special days, the team ran 48 computer simulations to see how four different street directions—North-South, East-West, Northeast-Southwest, and Northwest-Southeast—felt throughout the year. They didn't just look at the temperature; they mapped out "thermal perception" (how hot or cold it feels) and "autonomy" (how often the street is comfortable without needing help).

The results of these simulations revealed a clear winner and a clear loser. The Northeast-Southwest orientation turned out to be the "Goldilocks" direction. It wasn't too hot in the summer or too cold in the winter; it was just right. This layout managed to balance the need for shade during the sweltering summer months with the need for sunlight during the chilly winter days, making it the best compromise for the whole year. On the other hand, the East-West orientation was the troublemaker. In these simulations, it showed the highest risk of thermal discomfort and spent a lot of time in a state of "non-autonomy," meaning it was often too uncomfortable for people to enjoy without some intervention.

Ultimately, this study suggests that by using this new, faster way to simulate a whole year of weather, city designers can make smarter choices. Instead of just designing for a single summer day, they can use this method to find street layouts that keep people comfortable all year round, specifically in regions that swing between extreme heat and cold.